20ah does not mean it can sustain a 20a draw for an hour. Doesn't work that way. And probably all you can draw from most packs is about 10a anyway. Most battery capacities are listed for a 20hr discharge rate. IOW, the rate of discharge that will completely empty the battery in 20 hours. So a nominal 20ah pack should provide 1a for 20 hours, yes, but at higher discharge rates the battery does not give up the juice as efficiently. If you could bypass fuses, circuit breakers, BMS, etc and pull 20a from a 20ah ebike battery pack you would likely only last 30 or 40 minutes. OTOH, if you only draw say 200ma, you would probably get about (guessing) 150 hours of discharge time. Google "Peukert Effect". I believe it is less an issue with Lithium batts than lead, but it is still significant. So as in almost all aspects of eboating, you should always automatically downsize your expectations or face disappointment.
The ebike battery is a very handy backup for short time operation at low power, or for a sailboat that needs power only for docking. It is compact and portable. But for what you get, it is rather expensive. A 20ah 48v pack costs around $450 or more. Parallel 10 of them and do the math. Then look at what 13 200ah LiFeP04 cells would cost, for a much more elegant setup. I do not advocate the use of multiple ebike batteries in parallel. Quality control is all over the board on these packs and they will not be matched well enough to work efficiently and safely in parallel, I think, though I will certainly entertain knowledgeable arguments to the contrary. Diodes are of course a voltage drop though they could indeed be used to prevent reverse current. I think better to go with bigger cells in series with a proper BMS, or to go cheap, lead acid golf cart batteries.
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