T-Pain Is On A Boat: [Electric Boats] Re: Sailboat Props

Thursday, March 7, 2019

[Electric Boats] Re: Sailboat Props - Data Please

Excellent point, Fred.

My understanding of the advantage in USABLE capacity of LiFePO4 batteries over FLA or AGM batteries is the same as yours: 80% vs 50% respectively. Of course, that's the guideline to assume in order to achieve a reasonable number of cycles for each type. But LiFePO4 batteries provide a big advantage there, too, according to product literature, giving twice as many cycles as FLA or AGM when discharging only to 80% and 50% respectively.

So, to get the same usable capacity you'd have to use more FLA/AGM batteries. And to get the same number of cycles, you'd have to use twice as many sets of batteries. Looks to me as though, over the lifetime of the LiFePO4 batteries, they are actually more economical, even if the initial expenditure may be twice as high.

The fact that LiFePO4 batteries take up 2/3 the space and less than half the weight of FLA/AGM batteries seals the deal for me.

All that said, I'm told by the folks at ThunderStruck that most sailboaters implement their systems using FLA/AGM batteries owing to the lower initial cost, and likely also to the technical simplicity. I've read more than a few comments in various places expressing discomfort with needing battery management and battery balancing systems and all that, and/or a fear that making a charging/discharging mistake will spell doom for the lithium batteries, while FLA/AGM batteries can recover from one or two overcharging/overdrawing errors.

The diesel engine with accouterments that I'll be replacing weigh perhaps 375 lbs, but 8 - 6V 225Ah AGM batteries weigh 528 lbs! Add the weight of the drive motor and reduction gear, etc. and I'd be pushing 600 lbs of replacement equipment, while also taking up more of the engine compartment space that I do today.

Not very elegant! So for our situation, sailing on the Great Lakes where there are shore-power equipped docks in most transient ports, the advantages more than outweigh the disadvantages of having to take more care with the LiFePO4 batteries.

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Posted by: tvinypsi@gmail.com

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