Reducing Generator Run Time and Selecting Battery Bank Size

I recently had lunch with a friend named Hans who owns a very nice 54-foot offshore sailboat. Hans does some pretty serious miles, with a bunch of trans-Atlantics and countless voyages from Nova Scotia to the Caribbean in the fall, and back home in the spring, to his name.

Hans shared that he was considering replacing the house bank on his boat with lithium batteries to dramatically increase the effective bank size from its already large capacity, about 800 amp hours at 24 volts (1600 amp hours at 12 volts).

When I asked why he would take this expensive step, he shared that his goal was to reduce generator run time from its current several hours a day when sailing offshore. (Hans has two AC generators, one of 3.8 kw and the other a monster 14 kw.)


My first thought was that increasing the bank size would not decrease generator run time since the amp hours used have to be put back eventually, regardless of bank size.

OK, I Get It

But then I started to see his logic for his typical four to six-day passages: Nova Scotia to Bermuda and Bermuda to the Caribbean, and return.

In this case a massive house bank (lithium or not) would increase the chances that he would not have to disrupt his sailing with long and frequent generator runs since:

  • He usually goes alongside at the beginning and end of a passage and so can be fully charged before leaving and then recharge from shore power on arrival.
  • These passages typically, or at least often, include some motoring with the attendant battery charging.

And, by going with lithium, the possibility of several days of noise-free sailing goes up substantially, since the house bank could be safely discharged to about 25% of capacity, rather than the 50% that is prudent with lead acid.

We didn’t get into the details of Hans’ usage at sea, but knowing that his boat is fully tricked out with all the gadgets—plus autopilot, watermaker and big freezer—and based on our own usage at sea, I would estimate about 200 amp hours at 24 volts each day (400 amp hours at 12 volts) and certainly no less than 150 amp hours at 24 volts.

So, for the sake of discussion, let’s assume the larger usage and that Hans installs a 1300 amp, 24 volt lithium bank. That would supply about five full days of sailing, based on the above, before he needed to run the generator or plug in—problem solved.

(A lead acid bank of any type—liquid filled, Gell, or AGM—would be impractical, since to have the same effective capacity it would have to be over 2500 amp hours at 24 volts or 5000 amp hours at 12 volts!)

We Gotta Think

Hans’ story is interesting  because it illustrates how each of us needs to really think about our usage profile before selecting battery type and capacity, not just:

  • Make a decision based on some rule of thumb,
  • or what someone else has done,
  • or just assume that a bigger bank is always better.

For example, this change would make little sense for a sailing cruiser trying to reduce generator (or main engine for charging) run time, who:

  • Spends a lot of time at anchor and rarely goes alongside.
  • Regularly makes longer passages of a week or more in the trade wind belts where motoring is much rarer.

In both these cases a massive bank (lithium or lead acid) wouldn’t help since the generator (or engine for charging) run time would remain the same, all that would change is the interval between run times.

On the other hand, massive banks can make good sense for motorboats (or sailboats that motor a lot) who wish to remain quiet when at anchor and then will have lots of charging time while moving to the next anchorage.

Big Money

I digress, back to Hans. Of course, all of this assumes that he is willing to spend the some US$10,000 to US$15,000 that such a lithium bank and associated modifications of the boat’s electrical system would cost, not to speak of the added complications and fragility of lithium. (See Further Reading.) None of which he was keen on.

A Simple and Cheap Solution

Given these drawbacks, we chatted a bit more about other ways to solve his problem, during which I asked:

How about if you could reduce generator run time to half or even a third of what it is now for less than 10% of the cost of lithium and with minimal complication and installation hassles?

Hans perked up and allowed how that would be a perfectly acceptable solution.

Comments—A Quiz

So here’s a quiz for you: What is my simple and relatively cheap solution? Leave a comment with your ideas to solve Hans’ problem, and in a few days I will share mine.

But if you want to debate the benefits and drawbacks of lithium batteries, please do that on one of the two chapters below, after reading them.

Further Reading

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Meet the Author


John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

29 comments… add one
  • Taras Jun 15, 2018, 9:12 am

    Putting bigger / smarter alternator?

  • Sam Shafer Jun 15, 2018, 10:54 am

    By installing multiple higher capacity/smarter AC charging systems?

  • Nigel Jun 15, 2018, 12:47 pm

    Get a good battery charger to load up the genset.

    With a 4kw battery charger he only needs to run small genset for about 1 hour a day. Just make sure batteries can take 160amp charge current without cooking.

    I know, I do it! I have 1000amp hours of 24v Lifeline gel cells in a 71 foot ketch. And use about 200ah a day at sea. I run genset for about 60/90 mins a day into 3 parallel 2kva Victron inverter/chargers. That charges at about 160amps peak. Batteries do warm slightly (especially when old – 5 years plus) – but the inverters are temp compensated and back off if necessary.

  • Bill Balme Jun 15, 2018, 1:10 pm

    I’d agree with Nigel – buy a bigger charger and use that 14kW generator effectively. At 800AH, he could use a 200A (or even 250A?) charger and still only be loading that generator to a third of it’s capacity!

  • RDE Jun 15, 2018, 2:24 pm

    Hi John,

    If Hans’ goal is “to reduce the necessary generator run time while underweigh at sea”, why not increase the input rather than trying to increase storage capacity? He already has a small generator that can be used for load matching.

    –Add Solar panels (for the second leg south from Bermuda and for the Caribbean.)
    –Add a Watt & Sea for all passagemaking. Since most of the time on his typical passagemaking routing he will be powered up by sail, the extra drag will have little effect on speed under sail, and the captured energy is energy not spent on diesel.

    Or for a completely different vessel, I still remember the fun of the two weeks in the Bahamas on a 44 diesel-electric ULDB design. On inter island passages we just set the joy stick to neutral— the result of which was that fluctuations in wind speed were evened out, with the large prop always turning to automatically produce electricity in the puffs and produce thrust in the lulls. We rarely sailed below 9.5 knots, with 13.5 on a beam reach surf the prize. And by the way all the sails were dacron, with no free flying or Codes to be seen.

  • Steven Schapera Jun 16, 2018, 4:37 am

    The REAL underlying issue seems to be noise – not fuel cost. The reason he wants to limit run time is really to limit “noise time”. On this basis, a small, super-super-silenced generator is the way to go!

  • Ernest Jun 16, 2018, 8:18 am

    So we may assume the average use is 400Ah per day (24V), or 800Ah on 12V, being 9.6kWh per day?

    Assuming that he can charge at indefinite power 9.6kWh would mean having the small genset running for 2.5 hours, or the large genset for approx. 35 minutes, to fully recharge 50% depleted batteries, not taking into account the reduced “amperage” when batteries near 100%.

    Thus the task is to charge at the maximum current the batteries will take without being damaged. Assuming that Hans had installed brand batteries, such as Lifelines, the max. current the batteries can take in absorption mode is approx. 100% of the rated capacity, i.e. 400A on 24V, or 800A on 12V.

    To handle this he would need an AC charger capable of delivering such an amperage, or simply use a couple of chargers in parallel. The strongest I found during a quick research deliver at most 100A, for 12V (Mastervolt ChargeMaster 12/100-3) as well as for 24V (Mastervolt ChargeMaster 24/100-3) systems.
    This would mean either 4 or 8 charges in parallel hooked to the monster genset. Not a cheap solution, but a lot cheaper than converting everything to Lithium. Of course all these chargers need to be “intelligent”, read IUoU, and temperature compensated.

    Costs? 1.200 p.Pc for 12V, and 2.330 p.Pc for 24V, so roughly 9.500,- for the system, and reducing the generator runtime to approx. 40 minutes per day. Not too bad for his demands I’d say.

    What I would do first however was to check if the fridge/freezer insulation could be improved, as this might substantially reduce power consumption. Then change all lighting to LED if not already done. Basically review all big consumers if they can be improved, before attacking the battery system at all.

    • Ted Jun 16, 2018, 12:09 pm

      We have 500 watts of solar and a Duo-Gen wind/water generator. We do not have a genset. Under way we make more electricity than we can use.

  • John Jun 16, 2018, 9:16 am

    Hi All,

    All great answers. I’m going to stay out of this and see where it goes. Keep them coming.

    • Ernest Jun 16, 2018, 9:19 am

      I see, this is a test situation. Will you grade us after that? *g*

  • Richard Jun 16, 2018, 9:17 am

    Install the correct amount of SOLAR panals and that keeps trickeling the batteries so they are mostly charged so you need to run the GenSet less.

  • Rick Gleason Jun 16, 2018, 9:44 am

    I would also look harder at the load side & changing habits, similar to increasing ref insulation (mentioned earlier), such as turning off equipment that is not needed and shutting down displays, etc., using simple stick autopilot on self-steer vane (only when necessary) to reduce energy use. — What is essential?

  • Mike hiscock Jun 16, 2018, 11:50 am

    Assuming his lights are LED, the biggest draws are probably the autopilot and freezer. Does he need the freezer for short trips or the second half of longer ones? Does he have a wind vane?

  • Art Watson Jun 16, 2018, 6:24 pm

    “Several hours a day?” Even when offshore? That seems really excessive. I agree, ways to reduce the load should be considered.

  • Bryce Jun 16, 2018, 6:25 pm

    I’ll take a stab at it. Hans has a very different profile to us, but seeing as he is normally only be away from the dock for 6 days at a time, why change anything apart from his routine? The chance of battery damage from not fully recharging the bank in this time-frame isn’t high, and I’d be comfortable cycling 50-75% daily.

    Summary (working in 12V for simplicity):
    Current bank: 1600AH
    Usage: 400AH/day
    Big generator 14kw = 1166A

    Assuming Hans currently has an AGM bank that can take 100% of it’s AH rating at 50% SOC, and that he doesn’t mind not fully recharging the batteries underway, he could get away with 30 minutes gennie run-time per day and cycling the batteries between 50-75%.

    Day 1: Start 1600AH, end 1200AH, evening charge nil
    Day 2: Start 1200AH, end 800AH, evening charge 1/2 hour putting in 583AH (round down to 400AH for simplicity), end charge approx 1200AH
    Day 3: Repeat day 2.

  • Matt Boney Jun 17, 2018, 11:14 am

    Install a Fischer Panda 280 amp DC generator which is much quieter than AC – as it runs at the speed needed to supply the charging load.
    Install a 1050Ah Lifeline AGM bank.
    Install as much solar as possible – but we only have 140 watts!
    Install a Duogen Wind and towing generator – we have twice blown the 35 amp fuse!

    All this has worked for us in the Med. We run the Genset every 10 days!

    • John Jun 18, 2018, 8:06 am

      Hi Matt,

      I agree there’s a lot to like about DC generators. That said the problem with them in the past, when compared to AC, has been reliability. Hopefully Fischer Panda have fixed that. Also, as far as quietness goes, there are AC generators that are very quite too, and variable speed, although that has reliability downsides too.

      That said, Hans already has a full working system, so our goal here is help him tweak it at reasonable cost to better meet his needs, not a major system change.

  • Johan Skylstad Jun 18, 2018, 7:23 am

    Compared to labeled capacity Lithium can be charged/discharged to 1C -3C up to around 80% with negligible wear, that means that a 100Ah bank can be charged with up to 300A up to 80% charge.
    In a 24Vx800ah bank,this means that it can be charged (or discharged) with 19-57KW. This way the “time” can be reduced as long as the charging electonics and generator can deliver that kind of power.
    This is the same as most Electric cars can supercharge to 80% in 20minutes.
    Also the efficiency with good electronics will be 99% vs around 90 for lead acid
    Volume is reduced by around 20% and Weight around 60% relative to same usable power.

    Whats left then is the cost difference for the batteries and additional electronics and inverters.
    On the other hand, it gives possibility to use domestic household stuff like coffeemakers, microwaves, washing machine e.t.c from inverter instead of running a generator (with appropriate inverters of course).

    • John Jun 18, 2018, 8:00 am

      Hi Johna,

      Yes, there’a a lot to like about lithium and also some very big down sides. I cover that in the linked article. That said, good AGM’s can take at least as much charging current, albeit not for as great a range since tail off starts at about 85% charge.

      That said, there’s no reason that a lithium battery bank is required to run household stuff through an inverter. Any lead acid battery can do that too. But the key issue is that a generator will still be required to recharge the batteries. As I said in the post above, all that changes with a larger capacity (lithium or lead acid) bank is frequency of generator run time. Total run time remains the same.

  • Dave Jun 18, 2018, 1:28 pm

    Recharge time is a function of the charge acceptance rate of the type of battery being used and the ability of the charging source to meet the demand.

  • Eric Klem Jun 18, 2018, 2:06 pm

    Hi John,

    My first reaction in a situation like this is to always focus on the loads side first as has been suggested by others. That type of load sounds like some combination of autopilot, freezer and watermaker. Balancing sails, improving insulation/deep freezing prior to leaving, and leaving with full tanks and entering with less than full tanks could help some.

    Another thought would be to try to play in the 50-85% charge area for the 4-6 days on passage and then immediately get to full and equalize once on shore power again. This way, the generator would be run really hard and the batteries would not be too badly damaged as the passages are comparatively short. This method could also include significantly increasing charger capabilities if the batteries and generator were capable so that the bulk phase could be shortened and since absorption is being skipped, that would make things much better. If he really doesn’t want to do the PSOC operation that this entails, he could try to use some form of renewable to go from 85%-100%. For example, running the generator at dawn every day and then using solar for the rest of the day would work well but the solar array would likely need to be quite large with those types of loads.

    Finally, switching to the Firefly batteries might be an option although I suspect at greater than the 10% cost target. This would allow deeper discharges and not doing time consuming absorption charging. Basically what is described above but with less battery damage.

    I am curious to see what the answer is.


  • John Jun 19, 2018, 10:16 am

    Hi All,

    Thanks for all the great suggestions. I will reveal mine in a post some time in the next week or so. By the way, several of you came up with the same idea and probably most all of you thought of it, but then discounted it as being way to simple and obvious—that’s me, just a simple guy.

    Also thanks for inspiring at least one new chapter for the battery online book. I should have that done soon, but right now I’m just putting the finishing touches to Part 4 of rig tuning…and I’m exhausted!

    Also, just moved back on the boat, so it’s all go around here.

  • JCFlander Jun 22, 2018, 3:51 am

    Hi, if I got a call like this, my first instinct would be to ask “how’s your battery monitoring system, and how it is adjusted”.
    If daily usage is 25% SOC and result is 3h of genset running daily, there is either some serious problems with AC/DC chargers or, more likely, battery monitor has SOC triggered genset autostart relay, and it is currently set to kick in on 75% SOC. So genset runs ~80% time against absorption phase.
    So, provided that AC/DC charging system is dimensioned ok and working ok, the cure would be to re-adjust BM from 75-100 SOC to 50-80 SOC mode. Cheers.

    • JCFlander Jun 22, 2018, 4:13 pm

      Oh, it could also be ‘the silly charger mode’ – no shunt on battery bank (there’s no law against leaving it out… or, it’s there but there’s no BM relay to call the shots) and house consumption keeps charge current up, so charger stops only when internal absorption timeout comes. On this case, genset start relay is on eg. charger unit and acts when house voltage drops sufficiently.

      • John Jun 23, 2018, 8:49 am

        Hi JC,

        Good ideas, but Hans does not have an auto start generator and he monitors battery charge with a properly installed monitor system.

        • JCFlander Jun 25, 2018, 7:14 pm

          Okay, back to that hinted ~1000-1500 usd upgrade.

          What makes 3-4h genset runtime with 25% of SOC consumption should be:
          1- Less than 0.3C AC>DC charger capacity (800Ah x0.3=240A)
          2- Running against absorption phase (I think you implied that this is not the case, and Hans doesn’t run gensets against absorption when underway )

          Solution1: more AC>DC charger capacity
          – Additional 100A 24V charger costs abt. 1500-2500USD, so that might fit into budget. If starting point is 140A and this gives 240A total, charging time should go down ~40%

          Solution2: DC charger upgrade for one genset
          – Prestolite 140A 24V fire truck alternator
          – Balmar 3-step charge controller
          – poly-v belt and pulleys
          Might fit to 1500USD budget, might not. Also requires some kind of frankenstein mod if genset is factory packed to silenced casing. (a case to roll your own DC gensets with used kubota and aforementioned parts… also doubles as scuba compressor…)

          Solution 3: Tamper with consumption side:
          – Autopilot: tiller pilot to hydrovane
          – Freezer: add salt phase change ‘cold battery’ to loop, charge when genset running
          – Watermaker: only run when genset runs (if DC model – if AC, nevermind)

          Solution 4: try to add more supply
          – 1500 USD gives some amount of solar power, but the custom mounting rack…

          • John Jun 28, 2018, 7:31 am

            Hi JC,

            Lots of good and innovative ideas. I will post soon on mine.

  • Thomas Nygaard Jul 1, 2018, 9:31 am

    I am surprised that except a couple of smart comments here nobody is talking about the two most important issues:

    1) the huge difference in charge acceptance rate of Li-ion vs lead/acid.
    2) the stupidity of using AC generators to charge batteries, since the required battery charger limits the whole thing.

    On my boat s/y Cheetah (see website) I’ve got roughly the same usage pattern as Hans (250Ah/day@24V) and I have no generator, no solar, no wind turbine, but two Prestolite 140A 24V brushless alternators on the engine, so my engine is my DC generator. On a passage or at anchor I run the engine one hour per day in two half hour sessions, that’s it. When at anchor we can run aircon at night (owners cabin only) and watemaker extensively together with occasional use of induction cooker, toaster, water kettle and Nespresso machine from our 2x2kW Victron MultiPlus in complete silence, but now maybe 1.5 hrs charging time per day. I’ve also got two fridges and a freezer included here. Batteries are 360Ah 24V Genasun.

    So forget about AC generators (99% of them are run at very low loads for excessive amount of time, extremely inefficient!), and go for DC charging of Li-ion! (Wind and solar can of course be useful as a supplement but my wife wants a clean boat without all the clutter!)

    • John Jul 3, 2018, 9:33 am

      Hi Thomas,

      First off, please do not use words like “silly” and “stupidity” in your comments. See our comment guide lines:

      Second, Matt and I have already written extensively about Lithium:

      Also, there are many good reasons for having an AC generator including, for example, air-conditioning (larger boats). And, as I wrote in the post, even when DC makes more sense, AC has, at least to date, been far more reliable.

      The point being that you have a good and well thought out system for your needs, and that’s great, but please don’t imply that anyone that does anything different is stupid. After all, some would say that regularly charging from the main engine is not a good idea. And also, if you lose the main engine, you have no way to charge.
      (I’m not saying your system is in any way bad, just that all systems have drawbacks and none are perfect.)

      There are a lot of ways to get the job done, and this post is primarily about boats that already have AC generators, not tearing a whole system out and starting again.

      By the way, I agree with your wife about clean decks:

      That said, I have seen some elegant solar installations too.

      Sorry to be so harsh, but once one person starts using denigrating words, things tend to get out of hand quickly.

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