Deciding on Lithiums
Batteries. Arguably the heart of any boat’s electrical system, or maybe it’s the lungs. Or the kidneys. Did I mention I’m not any kind of biologist?
A boat’s electrical system at its core consists of three things. Supply, demand, and storage.
Supply: your charging sources. Typically the engine, a generator, alternative power like solar or wind, shore power.
Demand: the things that will use that electricity. Lights, fans, fridge, inverter to power the computer charger, radio, stereo.
Storage. Batteries. These ought to be sized with both the demand and the supply in mind, and the whole system can be tweaked to make a harmonious whole.
The first couple are fairly straightforward. It’s likely your boat comes with an engine, and the rest will be a combination of where you are and the set up on your boat. As far as demand goes, you’ll look at what you want to power and how much energy it takes. (Whether you can actually MEET that demand is a totally different issue.)
It’s the last one I want to talk about here. Storage. Because a lot of our thinking and talking time over the past few months has been about batteries. Specifically, should we switch to lithiums even though they’re drastically more expensive?
Building a battery. No, it’s not a bomb.
Lead acid batteries (both sealed and unsealed) are fairly common on boats. These are batteries like the ones you have in your car. They’re heavy and not particularly expensive, and (as a gross over generalization) can put up with being discharged to about half their rated capacity before starting to suffer damage.
For years, we’ve had 200 amp hours of AGM batteries (a sealed lead acid type that costs more but doesn’t need constant maintenance), which we augmented with a separate start battery once we repowered. This means we’ve got approximately 100 amp hours of usable power to meet our demand; this worked well when we had a smaller diesel engine with an engine drive fridge. The fridge is our biggest power hog, so taking it out of the battery equation helped tremendously. We’ve since shifted both engine and fridge and had concluded that 100 amp hours was just barely adequate, a decision that jump started this entire “so what battery do we need?” conversation.
(To be clear, our batteries are about 6 years old at this point anyway and would need replacing before we head off cruising. There were a lot of things driving this conversation!)
100-amp hour batteries would run about $250 each, so $500 for two or $750 for 3, if we wanted to increase our capacity. Remember, we can get about half the rated capacity, so to get 150 amp hours of usable capacity we’d need to have 3 batteries. Fine. $750. These weigh about 65 lbs apiece and need a decent amount of space, both concerns we can deal with.
Lithiums. We’d heard a lot about lithium batteries. Near mythical in terms of charging/discharging rates and capabilities, with equally appealing advantages in terms of weight (31 lbs per battery!) and physical space required, these come with 2 major drawbacks. First of all, they can explode. With disastrous consequences. Secondly, they’re insanely expensive. Like, $1000 a battery expensive. Meaning, if we were going to get 3 of them, we’d be looking at $3000. Four times the cost.
Holy budget busting, Batman.
The idea of lithiums went on the back burner. Way back.
Then Jeremy started watching some Will Prouse videos on YouTube. Will is a solar energy guy who seems to have made it his life’s mission to test every conceivable aspect of an alternative electrical system known to humanity. He buys and tests solar panels, batteries, chargers and inverters and battery monitoring systems and more. Pretty much, if it goes into an alternative energy system he’s tested the component.
Well, okay, maybe not the main engine. That’s perhaps the aspect of the electrical system that you’re least likely to change in any case.
Will makes the case that if you’re handy and electrically inclined, you can craft your own lithium battery from cells. Yes, there are a number of other components to the battery (remember that “explodes” part?) but they’re not impossible nor that expensive to put together. And the cost differential is astounding. A 100 amp hour homemade lithium battery, just for the cells, costs $500. Hmm.
Conventional wisdom around lithiums is that you charge them to 90% of rated capacity and discharge to 10%, meaning you’ve got 80% of the battery capacity to use.
Which means that TWO 100 amp hour lithium batteries (weighing 62 lbs total) will give us 180 amp hours of usable battery. As opposed to THREE 100 amp hour AGMS where we would get 150 amp hours - for a weight of 195 lbs.
We’re getting closer. $1000 (plus extra some stuff, much of which we’d need even if we weren’t going with lithiums) vs $750. This math is starting to look more manageable. When you factor in the life of the different batteries, it becomes even more compelling, if not a “duh” factor. An AGM has a rough average lifespan of 500 cycles (charge/discharge); a lithium has an average lifespan of 3000 cycles. For every lithium replacement set, we’d be replacing the AGMs six times.
I think really, the kicker for Jeremy though, was the idea of building his own. Getting the finances to more reasonable was one important thing. I don’t think, however, that if he could have bought drop-in lithiums for $1000 he would have done it. He just likes projects - particularly the ones where most people say “no freaking way.”
Plus, he got to build what looks like a bomb in the garage. Wins all around.
Stay tuned for details on things like the cells, the build process, and all the “other” parts of the system - and all the testing he’s doing to make sure it works the way it needs to.