size of lithium battery bank - 3.2v cells | Boat Design Net

I'm building a 30' catamaran and so far don't have a windlass planned, nor refrigeration.. but could see that in the future. So basically I'll have radio, LED lights for nav and cabin, and some instrumentation which I haven't priced out yet. On a strict budget since I'm retired.

I'm searching for a place for batteries and, wanting to keep weight down, hoping I can afford lithium. I was thinking of a DIY system based on the marinehowto website, using 4 - 3.2V cells. Thinking for now perhaps 200 amp or up to 360 possibly.. 400 max but don't see the need yet.

My question would be how to fit these. I have a bench midship with space that is 10" wide or a touch more.. but see some packages come in something like 11" x 11" (w x l). Curious if these cells can be configured to fit - I've got a space 10" plus wide by 30" long. I'm having a hard time finding where to source these kinds of batteries in the U.S.A or wherever providing shipping is not too un-affordable. China is an option but not sure how the market is there with the trade war waging. I'm just too unaware of possibilities of fitting and getting. I do have an 11 x 11 plus area but is getting close to stern and want to keep weight out of those areas.

thanks, JD

You need to do more research on lithium batteries for marine use. Marine lithium batteries are usually 12v, and very expensive. I read the article you refer to and one paragraph stood out.
"DIY Builds:
This is a real cost saver but is not for the faint of heart or the limited skill DIY’er. In a DIY build you source the cells, confirm the cells are well matched for Ah capacity, choose all the components, choose the BMS, design the system schematic, choose the high voltage cut and low voltage cut relays, main contactor, wire and assemble everything, balance the pack and chose chargers, solar or alternator regulators that can be programmed to suit LFP. A DIY build is a very time consuming project but one that can save over 50% of the cost of a factory made bank."

LiFePO4 Batteries On Boats https://marinehowto.com/lifepo4-batteries-on-boats/

In other words if you don't know what you are doing, don't!

Lithium batteries are very sensitive to overcharging and heat. They require a Battery Management System. Some have a BMS built in, some don't. The charger for a Lithium battery bank must be made specifically for lithium batteries. In other words the system must be designed from the ground up for Lithium. They cannot be a drop in replacement for a Lead Acid battery.

Thanks for the reply BlueBell. Good to have data points. As for the expense of batteries themselves, I am seeing near parity in price. This is under the assumption you need nearly 400 ah of AGM to equal 200 ah of LifPo4 (over simplification perhaps). Also, lithium drop-in systems definitely are still rather expensive compared to AGM. As an example, I just looked at first battery offered by FisheriesSupply, a vendor I deal with regularly. Their Lifeline GPL4DL 200 ah AGM is nearly $700 and 130 lbs.. or $ and 260 lbs for two. Maybe the tech is changing there too so one doesn't need quite 400 ah to meet 200 ah lithium? I don't know. The lithium pack I was looking at was a GBS 200Ah for $ and 57 lbs. Shipping more than makes up BMS and other stuff. I said "was" because the vendor said that they now have an even better battery than that one they will quote me. Shopping around will no doubt turn up cheaper AGM batteries but I didn't put in the time.

The marinehowto site author is on the facebook page "Lithium on a boat".. so is ready to nay say any bad ideas. (I would be loathe to send anyone to facebook who is not already entrenched there..)

Technology is changing rapidly in this field as well.. I have some time yet before I jump.

A couple more points to consider for battery pack design - duty cycles (longevity) and discharge current (max amps).

Lithium is a winner for long life and high power applications. However, the incremental cost of lithium over FLA or AGM is disproportionately invested in these two features (putting aside weight for the moment). For the average cruiser, neither is economically justified.

Consider duty cycles. There are quality 12V 200aH+ AGM batteries, in 8D formats (or other) for less than $400. Northstar for one. You'll get 800- cycles to 50% depth of discharge, but you can run them down to 80% from time to time in a pinch. How many days a year are you on the water? For many us, probably 100 days is a good year, unless you're in the tropics or living on borrowed money Are you really gonna keep that cat for cycles? 25 years is a long time.

On the discharge side, for the size boat you've got, I can't imagine you'll draw more than 20A on any appliance, maybe twice that if you making ice at night while cruising with everything else running full-bore. What's the point in a battery pack that can pump out 200A if you'll never have anywhere for the current to go?

Neither of these points considers total battery capacity, but even that's a bit of a red herring. Unless you're on the hook for days on end, 100-150 Ah of usable capacity in an AGM may be all you need. In which case spending many thousands on a 400Ah Lithium pack is a pure waste of money. I think you said you're on a tight budget with all the other bits 'n pieces of the boat you're putting together? Lithium is not for people on a budget, full stop.

In the end, the choice of battery technology is more about what you actually need, vs simply choosing the lightest, smallest, most dense container to put your Kw into. Step back and think through 1) the total days you're on the water per annum, without shore power, 2) the total simultaneous draw of all appliances onboard, 3) How many Kw you need to store based on 1) and 2). Also consider, once you've fitted all the lithium bells and whistles, the weight differential is not a back-breaker, unless you've got a hog to begin with.

I believe that 80% of cruisers don't fit a profile that requires lithium. If you're one of these folks, and you choose lithium anyway, besides wasted money you've also added a bunch more points of failure (BMS, heating, charging @ sub-freezing, etc) into a dead simple, 12V DC system that should otherwise be bullet-proof.

FWIW, anyone of you in the 80% segment who want to beat FLA or AGM economics with DIY lithium, even quality prismatic components, its a pipe dream. Unless you are a EE, extremely anal, and comfortable with potential drama onboard, you're likely to be disappointed. There's nothing quite like a lithium battery fire to get the heart pumping! Forget an fire extinguisher, get a life-jacket, that sucker's gonna burn down to the waterline.

Once the real-world data is on the table, AGM usually comes out a better bet. Good luck with the cat, hope you're on the water soon!