The Offshore Voyaging Reference Site

Huge Battery Banks Generally Make No Sense

I’m working on the second part of the Adventure 40 electrical system specification.

One of the fun things about this project is that since we are starting with a blank page, rather than upgrading an existing boat, which I often write about, design fundamentals become more apparent.

Here’s one that just came to mind:

  1. If we want to use more power over the run of a day, adding charging capacity, not a bigger battery bank, is generally the answer.
  2. All making the battery bank bigger does, at least past a certain point, is increase the silent time (generator or main) between charging cycles, not decrease the total time we must charge.
  3. Whereas adding charging sources both reduces charging time—perhaps to zero, although that’s generally not good design (see edge cases below)—and increases silent time—a win-win if ever there was one.
  4. Of course there are exceptions where a whopping battery bank, in relation to charging sources, makes sense. For example, with a boat that motors much of the time and can therefore fully charge the bank and then stay silent for several days.

It could also be argued that if we tend to depend on solar, having a huge battery bank makes sense because, like with motoring, it would allow us to stay silent for a lot of cloudy days in a row, but that’s generally an edge case, particularly since most boats will move every few days and so harvest near-free power from a good alternator—designing for edge cases is generally expensive and inefficient.

And, in addition, if we have really huge battery capacity, now possible with lithium, in relationship to charging sources, it’s likely the bank will rarely be fully charged, so it won’t help on those cloudy days, anyway.

More from Eric Klem on balancing charging and battery bank size.

More on designing an optimal cruising boat electrical system:

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David Branyon

Absolutely right.

Matt Marsh

This is, generally, true. There is rarely any benefit to increasing battery bank size beyond what’s required to fit a day’s worth of consumption into a comfortable and safe cycle depth. In many (perhaps most) cases, that weight and volume could be put to better use for other things. And an efficient alternator/regulator setup to keep engine-running-for-charging time to a minimum is almost always a better use of a spare $2000 than adding an extra $2000 worth of batteries.

That said, there are some cases where excess capacity can be desirable. One that comes to mind is when you have a boat that relies on an electric autopilot, is usually short- or single-handed, and deploys movable solar panels at anchor but stows them below while on passage. Having sufficient excess capacity to keep the autopilot happy for a while if the engine is unusable for any reason might, in this scenario, be very desirable – as long as the boat can accommodate the weight without significantly compromising some other aspect.

(This is not the case with the A40, which uses a windvane while on passage and which, due to its size and its use of a lead fin keel rather than internal ballast, will be relatively sensitive to excessive battery weight.)

Some applications also just end up with a high house load and the owner/customer likes it that way. Refrigeration, lots of electronics, shipboard remote office with Starlink and computers, etc. can add up, and some people don’t want to be dependent on daily (or more) engine runtime. That can lead to a battery bank that, by other people’s standards, might be excessive. It’s only a problem if the boat can’t handle the weight (batteries under the sole make for decent internal ballast in a boat that’d want iron punchings there anyway) or if the charging systems aren’t sized accordingly.

On my own boat I went in quite the opposite direction – we keep our electrical loads down to the absolute bare minimum (radio, instruments, bilge pump, nav lights, a few cabin lights, and that’s it). Our battery bank weighs 80 pounds, is kept topped up by shorepower most of the time, and gets all the juice it needs from a half-hour of motoring per day when we aren’t docked.

The important things are that you start with an honest & accurate assessment of the actual loads and their daily runtimes, that the battery bank is appropriately matched to those loads, and that the charging sources are appropriately sized to work well with both the loads and the batteries.

Eric Klem

Hi John,

I think the use case for a “big” battery bank might be a bit wider than what you say from the boats that I see out there. I would say that battery banks that can last multiple days are a good idea for people who want to rely on charge sources that they don’t control the timing of but are somewhat regular. As you note, one example is a boat that wants to do a significant amount of charging by a renewable like solar but needs a battery bank to tide things over during normal cloudy periods. Another example might be people who go to a marina every couple of days where they can use shore power. I don’t like going to marinas but if I did every 2 or 3 days, I would totally try to make sure that any charging above normal engine running with a well set up alternator was covered by shore power. A similar example is for people who regularly weekend their boat, I would want to make sure that no “generator” time was required during the weekend and then have shore power or solar take care of it during the week. Even people who motor a lot but only move every 2 or 3 days fall into this category of wanting a battery bank that can get you to that next charge (in this case motor) opportunity as you noted. The example you gave with Andy’s boat is just an extreme example of this.

The flip side of this is that small battery banks make a lot of sense for people who use charge sources that you can control the timing of. Examples could be generators, engines, fuel cells, etc. For these people, a huge bank doesn’t make sense for the reasons you stated.

My own observation of long distance cruisers passing through our area is that the majority of those boats now have enough renewables to cover their generation needs with the exception being the super electrical heavy boats where adding enough solar simply isn’t feasible. I routinely hear people raving about how solar has changed everything and they don’t need to use a generator or have actually removed it. At the same time that many of these people are bragging about getting to full charge by noon on sunny days they are often discussing adding more solar to deal with cloudy days. What this tells me is that the boats actually don’t have big enough battery banks so they have made up for it with oversized solar arrays which still generate enough to get through cloudy days. I consider this a much worse solution and it is what inspired me to write the article you linked to. I have no stats on this and only see the cruisers coming through our area but my observation is that generator usage is way down although I think many people are not well optimized and hurt sailing performance as a result.

So I guess I see a lot of people in both camps, those who primarily charge by timing controlled charge sources, sometimes undersized, like gensets with oversized battery banks and those who primarily charge by non-timing controlled sources like solar with undersized banks and oversized charge sources.

By the way, I think a normalized unit of how big a battery bank is in relation to loads is needed and your unit of silent time is a good way of doing it. I suspect that much of the confusion around some of this stuff is what is the definition of big and small. To me, small would be 1 day of silent time while big would be 3 or more.

Eric

Paul Y

Having enough storage for one cloudy / rainy day is very useful.
Two nights, one day. That allows solar for 98% of the time. It means the generator / alternator only gets used in really rare circumstances.