Had the same exact problem a couple months ago with one of two trolling motor batteries, same group, same age, same charger 2 bank MK. I tried numerous times to refill the battery, even individually charged it to no avail. Turns out it was bad battery plain and simple. Other is still holding charge and used as the cranking battery now. Put same size and group as well as age batteries in the trolling motor 24v lineup. Also switched to a Noco Genius smart charger. Funny how you can get a bad battery every now and then that I'd same brand, size and manufactured on the same date. Also, I agree with G, you need a separate cranking battery and a designated 24v system for your TM three batteries total.

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