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Here’s a question for the electrical experts reading this: What is the best way to judge a boat battery’s level of charge on a day-to-day basis… voltage or amp hours consumed? I have heard arguments for both.

While voltage does show the true charge, it appears to be quite load dependent and I have heard that a truly accurate reading can only be determined after the batteries have been left without anything drawing on them for 24 hours*, obviously not practical on a live-aboard vessel. Amp hours consumed on the other hand, is based completely on whatever data you program into your battery monitor. This could be accurate or it could be way off.

Since we replaced our entire battery bank back in St. Martin, for no small chunk of change I might add, we have been extremely careful about their level of charge. For those who have not been tasked with babysitting batteries, it is a big responsibility. Deep-cycle golf cart batteries, the type we are using, should never be depleted to less than 50% of their fully charged state. If we base the charge state on voltage, a reading of 12.2v, we go to this level almost daily. If we base their state of charge on amp hours consumed, we never go below 80-85%. That is a huge difference and it determines how frequently we feel compelled to run our Honda generator (which uses gas, which costs $$$).

So electrical experts, what say you?

*“If voltage readings are to be a meaningful reflection of the state of charge, the battery must sit for at least 10 minutes; an hour or two is better, and overnight is best.” Boatowner’s Mechanical and Electrical ManualNigel Calder

ZTC Specifics:

We have 8 – Deka GC15 6V Golf Cart batteries on board, wired in series-parallel. Each battery is rated at 230AH. By my math this gives our battery bank a total capacity of 920 AH, in other words, pretty big!

This morning’s readings:

This indicates our batteries are at less than 50% charge…

…but this shows that only 7% of the battery bank’s capacity has been used.


  1. Have you equalized them? Do you have hydrocaps on them? Can you measure specific gravity?

    Equalizing is an overcharge performed on flooded
    lead-acid batteries after they have been fully
    charged. It helps eliminate stratification and
    sulfation, two conditions that can reduce the overall
    performance of a battery.
    Trojan recommends equalizing only when low or
    wide ranging specific gravity (+/-.015) is detected
    after fully charging a battery.
    1. Verify that batteries are the flooded type.
    2. Remove all loads from the batteries.
    3. Connect battery charger.
    4. Set charger to equalizing mode.
    5. Start charging batteries.
    6. Batteries will begin gassing and bubbling
    7. Take specific gravity readings every hour.
    8. Equalization is complete when specific
    gravity values no longer rise during the
    gassing stage.

    • Sorry Conchy Joe, that in no way answers the question. Checking the specific gravity of each cell of an 8 battery bank is not a practical day-to-day solution.

      To repeat, the question was:

      “What is the best way to judge a boat battery’s level of charge on a day-to-day basis…”

      • I guess what I was getting at is both numbers could be accurate, the problem is the batteries need to be equalized, and possibly have water added to get them back to the specific. It could be ghost load like others suggested as well.

        Equalizing is something you can do without ripping everything apart. Most folks rarely equalize, and then wonder they their batteries fall off so quickly. Companies recommend as often as once a month, but most the schedule it in their long say quarterly is plenty. Checking Specific gravity at that time is a best practice. Hydrocaps will also extend the life by returning water as the batteries gas, especially during equalization.

        So to answer your question: If the batteries are maintained as outlined above, I would recommend amp hours.

  2. Hi Mike, I had to replace our house bank as well about 6 months ago. $4,000 for 900 Amp hours or Mastervolt Gel batteries! With that kind of investment I was very careful about our charge control strategy and measurement. My research was pretty conclusive that you care about amp hours consumed during operation and you care about voltage during charging and when you are doing diagnostics (under no load after settling time). Using voltage as a guide during operation is just not accurate enough, as you have observed. There are too many things that influence it, and it’s not practical to be disconnecting loads during a voyage to figure out your charge state. We use a Mastervolt AH monitoring system and it seems to be doing a good job. Your Zantrex AH meter should work well also, but you should check to see that it’s reading correctly. It’s easy to test with a clamp ammeter and a known constant load measured over time.


    • “It’s easy to test with a clamp ammeter and a known constant load measured over time.”

      Can you be more specific about this? I do have a quality clamp ammeter.

      • 1. Shut off all your loads
        2. Take an AH reading on the Xantrex
        3. Put a DC clamp ammeter on the main feed out of the battery bank (can be the positive or negative cable, but be sure it’s before any loads).
        4. Turn on some resistive loads (lights are best) try and get it around 10 amps of draw shown on your meter
        5. Let it draw for a few hours, keep an eye on the clamp meter to make sure the load is constant.
        6. Calculate your measured AH consumed = Meter amps x time
        7. Read the Xantrex again and subtract the beginning AH from the AH at the end of the test.

        The reading should be very close to your calculated AH in step 6. If it isn’t then your Xantrex is not reading the correct amperage, check the location of the shunt (may not be picking up all the loads) and take a measurement there. Let me know if this is a problem and I can tell you how to troubleshoot further.

        If it is reading the same, then you need to make sure it is calibrated correctly to read 0 AH at full charge and that it is measuring the charge current correctly. Let me know if you need help with either of those tests.


  3. Good morning you two,

    Are you saying that your only charge method is your Honda?

    We use two, group 34, deep cycle 12 v batteries in series. We use a 100w solar panel (and the 50a alternator on our diesel when it runs which isn’t often).

    Our load is an Adler Barbour cold plate in the ice box, and the normal stuff. We just don’t have an issue with our charge state.

    12.1 volts on your system indicates approximately 40% state of charge. A much more reliable method to determine SOC is with specific gravity.

    Is it possible you have a bad battery? I know they are fairly new, but, perhaps it’s worthwhile checking them individually.



    • No, that is not the only charging method. We have 285W of solar too although obviously overnight, they were not producing anything. The pic was taken this morning before the sun was up. I am 99% sure the batteries are fine although I have not recently checked the Specific Gravity.

  4. Mike,

    Was the 12.1 voltage reading after resting the batteries for 24 hours? As you suggested, measuring the battery capacity by voltage is not practical when you are living aboard.

    Another option to cross check your Xantrex monitor would be to use a hydrometer (common in US auto parts stores, in the Caribbean maybe not so common)

    Go with the battery monitor.


    • Hi Eric

      Yeah, I’m not going to pull out the hydrometer ever day. 🙂

      So, I assume your vote is for amp hours, not voltage.

      • Mike,

        Yes my vote is for amp hours. I’ve been using the predecessor to yours (Link 10) for years. I’m assuming (warning! warning!) this picture was taken in the morning after the refer was on all night. Assuming it draws around 4 amps at a 50% duty cycle, you would be down (12 hrs * 4 amps * 1/2) 24 amp hours over the night from the freezer, add in dvd/computer use, anchor light, etc, and 66 amp hours seems in the ball park (Given he batteries were close to being charged from the previous day), but the 12.1 v reading doesn’t pass the smell test.


    • By the way, that link discounts voltage as a good measure but as far as I could see, didn’t really provide a practical alternative for those of us on boats.

  5. Mike,

    I’m no engineer, wait, I am an engineer, but not that kind. But we have some really smart people in our company that are that kind of engineers. We do a lot with batteries and solar power, so I am going to e-mail you a couple of pages from one of our manuals later today. Feel free to digest the info and pass it along if you feel it has credence to your situation.

    Brief synopsis – Use voltage, wait 3 hours under no load – but the e-mail contains other options.


    • Thanks Burt. I look forward to reading the info. As I said in the post though, shutting off all load is not a practical day-to-day solution.

      • There is an alternative in the document. see the part :

        “The Vuc is a simple method to measure
        voltage without disabling the load or disabling the system from
        charging. Use Voc when the battery end of life is in question and a more
        accurate means of measurement is required”

        Hope this helps

  6. Mike,

    You can use the Honda for the bulk charging phase, but the acceptance phase of the battery charging takes a while and doesn’t make sense to use a generator for (i.e. long charge times at a low current generation, who wants to run the generator for 3 hours at 5 amps?). The acceptance phase is best left to solar panels. I can’t remember, do you have any? If not, they would be a great addition, especially since the prices have dropped to about $1 watt.


  7. Voltage readings, with a load on the batteries, can be misleading. Can you switch some batteries out of service for a few hours? Check the V on the unloaded batteries, and specific gravity, then put back in service and see what the system V is with a “normal” load. A few times, and you should know, e.g., the batteries are still 90% reading 12.1V with 3A of load.

    My S.G. readings are typically higher than in Calder’s book, 1.3 on a fully charged battery. Not sure if that’s because I have a cheap $4 auto store tester, but just a warning that YMMV.

    • I have an equally cheap tester. 🙂

      I have no doubts that the batteries are OK. I “could” switch some of the batteries out of the circuit but they then wouldn’t be read by the monitor (I don’t think). That again makes this less than practical for day to day use.

  8. Mike –

    Batteries produce electricity by means of a chemical reaction – this reaction occurs at the interface between the electrolyte and the solid battery plates. When you begin to draw power from a battery, the chemical species dissolved in the electrolyte right next to the plates become depleted (thus lowering the voltage). For the battery to continue to deliver power, these chemical species must then diffuse from the bulk electrolyte, thru the depleted zone and to the plates. This is why measuring the battery voltage while the batteries are under load, or recently under load does not give a true measure of the electrolyte condition.

    Your amp-hour meter, on the other hand, does an accurate job of accounting for power consumed. Unless you are prepared to wait for the electrolyte concentration inside the batteries to become uniform, it is your best measure of battery state.

    s/v Eolian

    • This seems to be the consensus.

      So, if we agree that voltage is NOT an accurate reflection of the SOC of the batteries while under load, how low would you allow the voltage reading to go before you got concerned, assuming that there is still plenty of capacity left in the bank (according to our other readings)?

      • Frankly, I don’t pay too much attention to the voltage on my battery banks… unless I see it when there is no load on the bank, and it has been unloaded for some time.

        This might be a way for you to get a valid voltage reading – put all your loads on one bank, and take the other bank out of service. Check it in the morning…


  9. What’s the load on it when you read the meter? LOTS of ghost loads on most boats – indicator lights, background info (like on our helm VHF), memory-keepers/instant-on stuff and so on.

    In a home, just pulling the plug (or switching, of course) on all of your electronic stuff (TV, Tivo, radio, laptop, yada, yada) when it’s not actually being used, due to the “ghost loads,” can actually make a notable difference in your electric bill.

    So, for true voltage, you’d have to not only do what Nigel suggests, you’d have to either unbolt one end, or (assuming the battery switch is directly in line, no exceptions) turn off the master.

    I’d bet that what you’ve got happening is that your meter is showing the under-load (even small) voltage.

    Meanwhile, your number of times and depth of discharge will determine the life of a wet cell. Keep it topped up and the battery will last forever. Do as the floor sweeper/drive-along-polisher folks do (L16HC batteries), and run them down to 20% every day, recharging overnight, and they’ll last briefly (by comparison). The number of AH removed over a lifetime has a direct correlation to the lifespan of a wet cell, and depth of discharge isn’t the whole story…

    Meanwhile, do you use a battery pulser (something which injects small spikes of high voltage), a mini-equalizer, in effect? This has just been discussed in either liveaboard or cruisers-online-network’s mailing list.

    And, last, as I assume you know, there’s nothing which beats measuring specific gravity with a temperature compensated instrument for knowing the true state of your batteries…


    Skip, still jealous, but creeping toward getting back on the water

    • Hi Skip

      That 12.1 reading this morning was NOT a load-free measurement. No doubt there were several things drawing on it and our fridge was running all night.

      I am not familiar with the battery pulser thing you mentioned. I’ll look it up.

    • Haven’t heard from you since the old days Skip!

      Do you have blog going somewhere?

      s/v Eolian

  10. The disparity between total bank voltage, which indicates a fairly deep discharge, and battery monitor tally, which doesn’t, might be explained by a couple of things:

    1. A bad cell. If one or more of the cells in your battery is already failing, either from inadequate equalization on a regular basis or as a result of manufacturing defect, it can bring down the overall voltage of the whole bank even though you have charged it as much as it will now take. You should try to measure the voltage of each individual cell after a full charge followed by an equalization cycle, and then a rest period to determine if one is going bad. Specific gravity of the electrolyte should also help you identify a bad cell.

    2. Your battery monitor is off. This can occur if it reset to “full” before each cell was fully charged. It could also happen if it isn’t using a proper value for Peukert’s constant, which compensates for inefficiencies in charging.

    3. You are measuring voltage while there is still a load on the battery. This would be dependent on how much load was on the battery.

    4. Your voltage monitor is not accurate. Use another high quality digital voltmeter to measure battery bank voltage at the terminals to confirm your monitor is accurate.

    I’m sure I’ve missed some ideas. My best guess is that one or more cells is already failing. Have you equalized them regularly?


    • On further thought, measuring individual cell voltage after a full charge/equalization might not tell you which cell(s) are bad. If you measure after bank voltage has sagged, you should find the bad cells with voltages well below the rest of them, i.e. well below 2.10 volts. Electrolyte specific gravity should still give you an answer after a full charge/equalization. If a cell has become badly sulfated, the specific gravity will not rise to the expected level even after a full charge.

    • I suspect the discrepancy is due to #3. That was not a load-free measurement.

      No, I have not equalized them lately. This is much easier to do when on shore power and we don’t go to marinas.

      • It would have to be a pretty substantial load, or a battery with serious problems like a weak and sulfated cell or two or more, for the voltmeter to show 12.1 volts when you have only used less than 10% of available capacity. Failing to regularly equalize a flooded cell battery that sees frequent charge/discharge cycles to between the “ideal” 50% to 80% is going to seriously shorten its life, and will quickly unmask a cell that might have been weak when new.

        • Are you on a boat Chip and if so, how do you equalize your batteries? Shore power?

          • The engine alternator with a smart/programmable regulator like a Balmar MC-612 will do it when shore power isn’t available. That does mean running the engine/generator once every few months long enough to get to full charge, and then for an extra 4 hours or so to get in an equalization phase. How often you actually have to do that will depend on how hard and how often the battery is cycled. It sounds to me like you need to do it now for sure.

            If your solar panels+wind generator could put on the order of 45 amps at around 15 volts into your battery (ballpark given the size of your battery bank) for about 4 hours they could do it too, but that is a tall order and you would need to have a smart controller for them to do that too.

            In any event, somehow getting in a good equalization now may rejuvenate your battery to some degree.

            • Yes, an big engine alternator would be helpful. Sadly our Yamaha 9.9s won’t cut it, especially given that neither of them are charging the batteries at the moment (a separate issue altogether). Perhaps this would be a good reason to go have some nice showers at Port Louis Marina. 🙂

              • I understand you have a Honda EU2000i portable generator. If you can wire it to your shore power receptacle, or directly to your shore power battery charger, it can supply enough current to handle about 70 amps of charger output (depending on the charger). This should be enough to provide an equalization charge for your 950 Ah battery.

          • Solar panel charge controllers like some of the Bluesky products and others have equalization built in. Generally this is because solar panels put out 17 to 30 volts, which makes it easy for them to provide the higher voltage, lower current required for equalization. If you don;t have some sort of charge controller, preferably MPPT with an equalization feature, you could be boiling your batteries. Keep in mind it is 8:30 and I have had numerous Caribe/Kalik/Red Stripe/Fat Tires.. 🙂

  11. I’m no expert, but what I’ve noticed is that when you remove the load from a battery, the battery very quickly shows an increase in voltage, i.e. it’s load dependent. Thus, were you to remove the charge from your batteries when they show 12.1 as in the photo, you might indeed find they settle at 12.3, 12.4.
    I’m going to email Nigel (Calder) about this and ask his opinion – I’ll post here when I hear back from him.


  12. Mike, this is argued endlessly everywhere and on all the forums. Experts abound!!!!!!

    Have you got the Xantrex manual, or is it on their site? Otherwise I suggest you email their tech-support people and ask. They should know how to interpret their data. I hope!

    Good luck


  13. Mike,

    I’ve enjoyed your adventures for almost 2 years now, keep up the excellent blog!
    As a Electrical Engineer, I am not a battery expert, but here is my vote, I would use Amp Hours over Voltage. It is a better measurement of the actual depletion and hopefully capacity of the array. Voltage is more dependent on the present demand on the cells, and may drop during heavy loads.

    How did you get 50% charge remaining from the first meter picture? Looks like it is just down 1 bar, like the second one?

    Batteries are fairly fickle devices, and as you are well aware, not always dependable. They don’t like heat, vibration, or salt… I’m sure that you don’t have any of that on board? How has the ice maker been working for you? Hopefully, it is delivering vast quantities of cool cubes for your drinks! It is a fairly large current demand on the batteries.

    Take care,


    • Hi Kurt

      12.2 Volts is considered to be a 50% charge level on a lead acid battery, that is where I got that reading, not from the bars showing. Using some form of amp hours measurement seems to be what most people are recommending.

      Thanks for chiming in!

      Oh, as for the ice maker, running that really makes the voltage drop! 🙂

  14. Just FYI, this is a copy of my reply to you on FB… I thought I’d cross-post it here for the readers of your blog who might not be in the FB group.


    First of all, did you recalibrate the battery monitor when you put the new battery bank in? Generally, IIRC, you do have to recalibrate them when you switch out the batteries for new.

    Second, amp-hours are probably far more reflective of the actual use and state of charge, but that can be thrown off if you have a very high load for a period of time.

    This is because the amp-hour rating of a given battery bank is for a given load. The greater the load, the lower the amp-hour rating for the bank. That’s why batteries have different amp-hour ratings at the 5-hour and 20-hour rates, with the 20 hour amp-hours being a larger number. This is due to the Peukert factor–which basically states that the larger a battery bank is relative to the loads placed on it, the larger its effective capacity.

    The voltage is going to be affected significantly by what loads are on the electrical system at any time and will appear much lower than the actual state-of-charge-voltage would read at any given time because of the loads on the system. It really isn’t an accurate indicator of state-of-charge unless the batteries have rested with no load or charging for a few hours.

    I would also point out that regardless of how little you draw a battery bank, there is a finite number of charging cycles that the battery bank can handle. By only drawing the battery bank down to the 80-85% state-of-charge level, you’re wasting a lot of battery life capacity in many ways.

    By doing that you’re also forcing the generator to run when it is going to be the least efficient at charging the batteries, since the battery charge acceptance rate generally drops off drastically at around 85% and your running the generator to absorption charge the batteries at a much lower rate than would happen if you ran the batteries down to the 50% capacity level.

    From 50-85% or so, the batteries can accept the maximum charging current or bulk charging. By not dropping down below 80% you’re wasting a lot of fuel and wear/tear on your generator.

    For example: with your battery bank of 920 AH… the 50% charge level is 460 AH. The 85% charge level is 780 AH. If you allowed yourself to drop down to the 50% charge level on a regular basis, you’d get 320 more amp-hours out of the battery bank between charging instead of just 140 amp-hours or over THREE TIMES AS LONG BETWEEN CHARGING.

    AND when you did run the generator to charge the batteries, you’d charge replace that 320 amp-hours at a much faster rate than you would the last 140 amp hours.

    Say the batteries can accept a maximum bulk charging rate of 150 amps and charge at a rate of 30 amps in absorption mode (please note, that this is a simplified model as the absorption charging current acceptance rate will drop as the state-of-charge increases).

    So for the 50-85% recharge of 320 amp-hours, you’re going to take about 2:10. For the remaining 85-100% recharge, you’re going to take 4:40 or so… See the problem. You’re wasting a lot of unnecessary fuel IMHO.

    In fact, many long-term cruisers run their batteries down to the 45% charge level and only charge back up to 100% on a occasional basis, say once every two weeks. This is enough to prevent problems from occurring but maximizes the used capacity of the battery bank and minimizes fuel used/wasted recharging the battery bank.

    Using my hypothetical numbers, if you use 150 amp-hours per day…

    It would take you about three days to get down to the 50% SOC level and you could run the generator for about TWO HOURS every TWO DAYS and get back up to the 85% SOC level or so…

    But if you only run the batteries down to the 85% SOC level, you’d have to run the generator for over FOUR HOURS to get back to the 100% SOC level EVERY DAY, using EIGHT TIMES AS MUCH FUEL roughly.

    Ideally, you’d run the batteries down to the 50% SOC level and run the generator every TWO DAYS for TWO HOURS, and then once every TWO WEEKS OR SO, run the generator for the SIX plus hours it takes to recharge to 100% SOC, and run for three days instead of two before recharging at that point…starting the cycle of running down to 50% SOC –> 85% SOC for another two weeks or so.

    I hope this helps. 😀 Any questions, let me know.

    • Thanks for posting that here and on FB, Daniel.

      • Well, except for the fact that it’s well proven that with flooded lead/acid batteries, the deeper you discharge them the fewer cycles you will get out of them. So taking them down to 50% all the time will kill them quicker than taking them only to 75% full. Taking them below 50% regularly is really getting into a bad spot. Having a monster bank makes it harder to recharge them properly and equalize them especially in the absence of a diesel with a big alternator and a smart regulator. Do you really need that many amp/hours in your bank? We live aboard, have 450 amp bank, rarely take it down below 75% in 24 hours. Fridge/freezer is the biggest load draws 6 amps when running, cycles 15 mins per hour, very well insulated box. A 220 watt solar panel with MPPT controller pretty much fills the bank on a sunny day. Controller does an mini-equalize every 7 days and if I need to I can do a full equalize off the alternator, but so far in 3 years have not had to do that yet. Batteries still act like new.

        • My understanding of the deep-discharge of batteries is the same as yours although that may open up an entirely different argument.

          As for our battery bank, it is what it is. I’m not sure how having a larger bank makes it any harder to charge them properly. We only need to replace what is taken out.

          You said that you only take your batteries down to 75%, Robert. I assume this means that you reply upon the amp hours reading on a battery monitor to get this figure? That was the original question after all.

  15. Go have a “bad day” fishing, you’ll feel better! lol

  16. Mike,

    Battery monitors are often off, and continue to get less accurate as time progresses between calibration(s). Voltage or specific gravity are accurate but as you and others already say somewhat impractical because of the load/no load differential. But, for certain if there is a load or even light ghost load on the bank while taking a voltage reading and that causes you to charge early….you are doing no harm. If on the other hand the monitor is off because bank capacity is slightly reduced (as always occurs between calibrations-ezqualizations) and you’re consuming down to the 50% as indicated by the monitor, you may be below, sometimes well below, the 50% level and are doing harm. So I’m not saying one is better. Both deserve credit if used carefully. Monitors do fine…deserve watching which many cruisers don’t do. They also cost $$$ and add complexity. We don’t have one on Celebration, use voltage, sometimes end up charging early, bank is all AGMs in year five and going strong.

      • Many here are argueing the accuracy of the available amp hours remaining. I ask to what end? It really doesn’t matter to me if I have 110 or 45 amp hours left. It’s OK to charge at either point and again, do no harm. Perhaps I view the issue from a simple non-technical perspective. On the other hand I tend to get a more years from batteries than other cruisers.

        • To me it matters if you are interested in how long you can go without running an engine/generator to charge them, assuming your solar/wind can’t keep up.

  17. Mike

    Look into desulfator, not to do with your problem, I vote amps. Volts just once no load for an hour would be a good indicator.

  18. You are trying to get the most time out of your bat. How long does a set of bat last, I looked after about 200 bat. smaller ones . We used a good load meter once a year ,7 to 12 AH. and we would get 3 year use 16 to 32 AH. 5 years use about 95% of the time and thats all she wrote.

  19. If you have a 920 ah bank and the voltage is reading 12.1 with only 7% used you either have a HUGE load on the batteries when checking the voltage or a problem. I would guess you either have a dead cell, bad voltage meter or need to recalibrate your amp hour meter. Also golf cart batteries are meant to be used and abused. I would try to keep them above 50% but if they fall below occasionally it’s not a bad thing. I could be wrong but something in your readings does not ad up.

  20. On a day-to-day basis using the consumed Ah figure shown on the Xantrex monitor is the best way to derive overall state of charge, with the following caveats:

    1.1. All loads taking power from the battery must be wired is series with the current shunt from which the monitor calculates load. This can be validated by checking the wiring.

    1.2. Maximum capacity of the battery, which reduces over time, is known. See item 3.2 for more info.

    The reason voltage measurements are not a reliable guide to charge state is:

    2.1. The battery has an internal impedance. This creates a voltage drop between the cell open circuit voltage and that measured at the teminals if the battery is loaded. This voltage drop is not constant, but varies proportionally with load current. The internal impedance also is not constant but increases with discharge.
    2.2. Battery voltage is temperature dependant.

    Another reason why voltage measurements are not reliable, but also renders the remaining Ah calculation unreliable, is

    3.1. Sulphation of the lead plates, which contributes to increased internal impedance, exacerbates the difference between open and on-load voltage. Sulphation has a more profound effect as it leads to misinformation when interpreting open circuit voltages as representative of state-of-charge, or consumed Ah to derive available remaining capacity. The issue here is that sulphation decreases the amount of lead available to take place in the batteries electrochemical reaction. As an simple example, if 20% of the batteries lead was subject to sulphation, the maximum charge available would be reduced to 80% of the original capacity. The open circuit voltage and Ah monitor however would indicate that the battery capacity was 100%.

    To establish the actual maximum charge available from a battery a load test is required. But that’s a different topic!

  21. Mike,
    This might sound stupid but try this; calculate the amp hours per day your equipment would use, add 30% for that extra DVD on the laptop… and use that is a base to decide AMP or voltage as ground truth. 920ah is a lot of battery and I find it hard to believe you are depleting more than 50% overnight especially with the 285 amp hr solar panel. I have the same Xantrex indicator and have tested against my military grade multimeter and it was spot on for voltage, but that isn’t the whole story as you have already read. Either way, I would love to be pondering this question over a cup of coffee while anchored in paradise such as yourself.

  22. Mike,

    As you know, loaded voltage is a completely unreliable indicator of SoC. That said, I do agree with the couple of comments that 12.1 sounds unreasonably low for the putative SoC of the bank at the time. One question would be how reliable is that reading? IOTW, have you checked the voltage at the batteries (and at the point where the Xantrex meter connects) with a known-accurate voltmeter?

    Moving on to the SoC meter: This is going to be your best method for conveniently determining SoC while the bank is actually in use, BUT you must adjust the meter, and that can take a lot of tinkering and trial-and error. Specifically there are three things that need to be set properly:
    1. Peukert coefficient for your batteries
    2. Charge Efficiency Factor
    3. “Charged” parameters for your charging system and battery combination

    Even if you manage to nail all three dead-on, your meter will still accumulate error over time, and this is especially true if the meter does not have a temperature sensor attached to the batteries to enable temperature-compensation of the readings

    The Peukert coefficient is an inherent characteristic of the battery and can often be obtained from the manufacturer. Peukert’s formula states that the amount of energy removed from the battery increases as load increases. IOTW, drawing 10 amps for 1 hour removes more energy from the batteries than drawing 1 amp for 10 hours, even though it would seem on the surface that each of those should remove 10 amp-hours. This is also the reason why a battery’s capacity in AH is given at a certain hour rate. Your 230 figure is probably stated at the “20-hour rate” which is common for marine deep cycle batteries. If you can’t get the Peukert coefficient (aka Peukert exponent) for your batteries from the manufacturer, you can calculate it from other published specs, such as having multiple capacity ratings (e.g. a 20-hour, 10-hour, and 100-hour rate) or a capacity rating, a reserve rating, and a CCA rating. Formulas can be found on the web.

    While the Peukert coefficient impacts the accuracy of the measurement of energy coming out of the batteries, the Charge Efficiency Factor (CEF) does the opposite — it impacts the accuracy of the measurement of energy going in. Xantrex meters by default “calculate” a CEF every time the batteries are charged (more on this in a moment). My experience is that they are not very good at this, especially if the batteries don’t get topped off very often. I usually set this number myself based on direct measurement. That’s a trial-and-error process. Your Xantrex manual will explain this parameter.

    The “charged” parameters are the conditions under which the meter knows that the battery bank has been topped off to a full charge. These can be tricky to set, and if you are not careful the meter will never see the batteries as having been fully charged. Thus it will never reset to zero DoD (100% SoC) neither will it recalculate the CEF, and this, over time, leads to a highly inaccurate meter reading.

    All a long-winded way of saying that it’s not perfect, but the SoC meter is your best bet, and you’ll need to do some fiddling with it to get it to report reasonably accurate numbers. That, BTW, requires actually topping the batteries a couple of times, and so is best done at the dock with a shore power connection if possible.


    -Sean, aka “that electrical guy”

  23. Good Morning. Without reading through all of these comments (although several that I did read are very insightful, Celebration and Dan K in particular), I say:
    1. Amp hours consumed gets my vote for monitoring an a day to day basis.
    2. Not 100% sure about your model but at least some of the Xantrex monitors must be reset after 100% charging or their data readout becomes cumulatively wrong (misleading).
    3. All data provided by any battery monitor is reflective of the programmed parameters and proper installation.
    In light of the above, I would not draw any conclusions about your monitor readings without first confirming that the monitor was correctly shunted at the correct place in the circuit as I believe they only monitor downstream of the shunt; that the parameters are correctly programmed; and that the unit is reset when and if necessary. FWIW, I have had 2 “reputable” marine electricians work on my charging system; both provided contradicting information and the second “corrected” wiring that the first performed. The first provided and installed the Xantrex monitor and the second replaced it when we replaced out inverter/charger with a Magnum unit and its proprietary monitor, telling me that the Xantrex monitor had known problems. On a related note, they both informed me that my SSB installer (guru) incorrectly wired the radio into the system. Good luck.

    • I agree that the AH readings are completely dependent upon the settings that I inputted into it. GIGO. I’m pretty sure I did it correctly though. I did actually verify once again yesterday that the total AH amount of the bank (920) was input properly and that it hadn’t somehow reset itself to the default 400 AH.

  24. It is conversations like this that bring me back to reality where I know I could never own a boat and run it myself.

  25. I admit I didn’t read all the comments, but here’s my opinion on the subject. For reference, I have a Link 2000 and 4 golf cart batteries for a house bank (~440Ah). Charging is only genset/shore power (still, I know, it’s 2012!). We never go below 50% charge.

    Anyway, as a general rule I trust the Link 2000 and Ah consumed, but it has to be properly calibrated and synchronized to the batteries state of charge. If it is out of sync (for example, resets itself to 0Ah consumed before it should due to partial charging), then the Ah consumed doesn’t work. The Link can get out of sync if there is a small charge applied for awhile, such as idling one engine which only puts out a few amps for one of our alternators.

    Next up, is to measure the voltage of each, individual 6v battery when there is no charging present. All the batteries should be at nearly the same voltage. If any battery (or batteries) show lower voltage than the rest, then they are not being consumed or charged evenly, or one battery has started to go bad. If this is the case, check your battery cross connect cables very carefully, and equalize the bank.

    It is important to equalize flooded batteries every couple/few months. Especially a bank of golf cart batteries, as use and charging can be uneven, and equalization helps even things out.

  26. Not to add too much to this discussion, but here is a simple way that I feel works.

    Take the capacity of the battery and and divide it into a good guess of the Amps you draw on average. (example: 20 amps/hr is your guess at observed use/920 amps available in batteries = 0.022). Multiple the .022*10 and that is ‘working voltage load” for your battery bank (the effect of your house load on your battery voltage). I suggest you assume 12 volts is actually half way (not 12.1). Subtract .22 from 12 and you get 11.78 Volts DC. Which means that at 11.78 volts DC you should be at about 50% battery capacity. With solar cells you should check this at night obviously.

    This is the equation we used to remotely monitor the batteries of a few thousand cell phone towers. It breaks lots of rules, and makes lots of assumptions. But its easy and it works for modest, continuous loads on batteries.

    As a separate method, you could log your voltage observations and define a pattern. The period where you go the longest at the same voltage, is usually the 60-70% capacity curve. After that the decline in voltage will accelerate faster over time.

    Of course, cooler days, more sunny (charging) days, more use, age of batteries, etc. will affect the ‘feel’ you get for the batteries.

    Lastly, don’t stress over it. A lightly used battery bank will last only a year or so more than a heavily used one. When your 12 volt devices wont turn on, charge your batteries.

    We are enjoying following your travels. We hope to start south this November.

    All the best,
    Eric and Bonnie

  27. Sooooooooo… I’m not going to be any help, I just have questions. 🙂 Why did you pick the deep cycle golf cart battery to begin with? When you say you should never let the charge go below 50% is that the drop dead percentage or an ideal? Can’t AGM or gel batteries be completely depleted and be okay? I’m not implying that’s ideal for them either, but having a larger margin for error isn’t a bad thing, right?

    This whole thread brings new meaning to MGB’s constant reminders to “turn the lights off.” Gah! I have so much to learn….


    • Why choose golf cart batteries? They were in the boat when we bought it.

      Letting the battery go below 50% on a regular basis will damage it, reducing its lifespan and ability to hold a charge. I am pretty sure this applies to AGMs and Gels too although I have never really studied anything about them.

    • Tell MBG to install some LED lighting and then he won’t stress about it so much. 🙂

  28. We have a similar issue with our gel cells after sitting around. The amps are not too low, but the voltage is. Things we found out: computers running off the inverter kill the voltage, a good run of the motor while moving anchorages helps, we need to tie up to a dock soon and get a steady charge for 24 hrs at least (it isn’t always sunny in the tropics or windy). I am planning on putting on a wind generator to try and keep a light charge on (when it’s windy) during the night.

    Anyways I read this and I feel your pain, it is hard to get a good steady charge at anchor. I have come to the conclusion that shore power for a few days is key, even if it is a bit costly.

    • I’m not convinced that shore power in the answer FOR US. I think we do generate enough power to fulfill our needs, either by solar or by the additional use of our Honda. I’m still working on this.

  29. Hi Mike,

    It’s funny, I googled “calculating amp hour usage from voltage drop” and you were the 3rd entry !!! You can’t buy this kind of SEO ;~) ! Hope all is well, I am heading for the Exumas this week after an aborted attempt where the forward head tried to sink the boat (2 feet of sea in the boat !)

    fair winds,

  30. Hi Mike,

    Having been on your boat and knowing a little about how you, your better half and cat live, I can’t imagine you are actually “using” over 100 AH / day. I have a 315 AH bank with a 120 Watt panel, MPPT controlled and an Air Breeze wind gen. I get 7 AH out of the one panel x 5-6 hours a day (30-40 AH / day) , the wind gen of course is more variable giving from 0-15 A / Hr (I figure I average 25 AH / day ). I have about the same refer load if I turn it up to freeze 50% of the box (3cu ft).

    IF you are running out of amp hours, I suspect a bad cell/battery, problem in charging circuit, or an electrical leak. After ruling out bad battery, a clamp on amp meter and a couple hours of methodical diagnosis (inputs and outputs) should yield some sanity check answers.

    Food for thought for next time around, I replaced lead acid with AGMs a few years ago and I find they accept charge faster (as documented … good for wind generator/Honda input), and that the SEEM to handle discharge cycles better. I thought I had a unknown voltage draw with the lead acids, but my newer setup (6 years) with 120 solar, air breeze and 3 G31 AGMs requires other charging only about once a week or so, and I only do this IF/when house voltage drops below 12.2v. Never found any leaks, but you should check with clamp on at battery … a bilge pump wire with bad insulation in a tiny amount of water (or other sketchy hard to get at wiring) could be bleeding your amps away !

  31. This may be a dumb question, but wouldn’t eight 6V, 230 amp-hour batteries hooked up in series give you a combined 48 V and 230 amp-hours? That’s what a manager in a battery store told me. He said you add up the volts but not the amp-hours. Is this wrong?

    • Most 6V systems would be set up in series/parallel. Pairs of 6v batteries are connected in series to create 12V and then those pairs are connected in parallel.

      A typical connection of Eight 6V – 200AH batteries connected in series/parallel would give 800AH at 12V.

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