I have a 36v MinnKota Maxxum. 3 12v batteries ran in a series being charged by a MinnKota MK3150D. My first issue began when I noticed a drastic loss of power in the trolling motor.  I traced it back to one of the batteries. Found out it had a NEGATIVE charge. The previous owner was actually charging the battery that ended up reversing polarity on a separate single bank charger and had the cranking battery in a series with 2 of the TM batteries. I thought the single bank charger went bad(I was told by the guy at the battery store) and fried my battery. I replaced the battery and was fine for a couple months(probably one or two outings) until the battery I just replaced reversed polarity like the last one and also fried another battery in the series(it's reading 170v.) PLEASE HELP! I can't afford to pay for diagnostics. I'm thinking that I had a bad connection between these two batteries in the series. Would that cause them to do this? Does how I have them hooked up to the charger interfere with them running in a series? 

 

Also found this junk wired between the 2 batteries in question under shrink wrap. 

 

My current plan is to replace all 3 batteries and all cables/connections but my boat is basically useless until I can afford to do this. 

 

Thank you for your help. 

Just went back and checked with the multimeter the replacement battery is reading 30mv and the other battery in question is reading 1.853v. Thr 3rd battery in the series is reading 12.65v

You need to re wire your batteries. All 3 12v TM batteries must be the same age and condition, don't plug in a new battery in series with 2 older batteries. Think of each battery as a 12V with your charger charging each battery separely and each battery has it's independant 50 amp circuit breaker. The 36v power to the TM uses current in series from 3 batteries. 

Your cranking battery should be switched on/off with a 50 amp switch and on it's seperate 50 amp circuit breaker. You switch the cranking battery off when charging using the onboard charger with 4 banks.

Tom

I'm in agreement with the previous post - your batteries need to be the same age - not roughly the same age - but the same age.

Hey guys I found this very neat system of battery installation should come out handy. Check this video out!