It's been a while, but I managed to replace the two cells in my 4s pack that died. I put together a 12V battery pack/charge controller/motor controller/BMS in a dry bag. After bottom balancing to 3.0V and then charging for several days from the solar panel, yesterday was the first test on the water.
The cheap 40A PWM motor controller does control the trolling motor speed nicely, but it completely freaks out my power meter (which is between the battery and the PWM). I suspect the PWM is letting lots of noise leak back into the supply. At full speed the current shows almost 30A, which is less than it showed without the PWM (~35A). I haven't measured speed using a GPS to see if it's slower than before or not. At lower speed settings, the power meter just flickers or doesn't display at all. After running for 15 minutes at full speed, the PWM metal box feels warm through the dry bag, but not hot.
I decided to use a 4s BMS, mostly for its per-cell LVD function. My charge controller is set to stop just before the BMS will start balancing the cells, so it should function as a bottom-balanced system, until something upsets it. The BSM was cheaper than an adjustable 12V LVD, so I figured why not.
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