Long story short....I'm an idiot.  Two weeks ago I crappie fished all day and forgot to turn the motor over on my brand new boat so my 12v starting battery died. All I had on board were jumper cables so in my infinite wisdom I jumped my 12v starting battery by connecting it to my 24v trolling motor system. And by that I mean I hooked the jumper cables to the positive terminal of one battery and negative to the other of the two battery 24v set and the other end of the cables to the single 12v battery. Essentially pumping 24v to a 12v battery.....I realize now that I'm an idiot. I'm a brand new boat owner and I panicked. It is what it is.

The reason I'm asking for help is that it actually seemed to work. The boat fired right up, 12v system maintains a charge, all gauges still work, didn't seem like anything was cooked. Fast forward to yesterday and after driving all the way across the lake to my crappie hole, throw the brand new minn Kota power drive with i-Pilot in the water and it's super slow and sluggish.  Like on power level 10 it feels like it's running at maybe level 2 or 3. If I try to turn the TM while the prop is spinning it almost comes to a stop. Battery meter shows a full charge, boat was plugged in for over a week, all fuses look good, the two battery connectors all look good. It's like my TM is only getting 12v instead of the 24v that it needs.

is it possible that by pumping 24v into the 12v battery somehow cooked one of the two TM batteries?  I would have thought the fuses would have popped first or worst case it would have cooked the 12v starting battery. Any help would be appreciated. I don't even know where to look next

Pull the batteries out and have  a load test done on both.

Sounds like you may have one bad battery since it is working, but not like it's getting full voltage/amperage. Test the batteries.

Charge batteries separately. And check how they are wired. Start at the batteries and work forward.

Thanks for the suggestions. I'm gonna get started but I foresee having to buy new batteries in the near future

You may have melted an internal connection in one of the batteries

giving it high resistance. That's why the volt meter says it's OK but

won't deliver much current.

Like mentioned above, have them both load tested and make

sure all your cabling is correct and clean when you re-install them.

ETA, I highly recommend separating the engines cranking battery from

any accessories. Cranking battery should only feed the engine and it's related

systems (gauges etc..). Keep all accessories separate. Nav lights, stereos etc..

should be on their own battery(s).

Well I checked everything over the best I could. I have 24v at the batteries, at the TM plug, and at the TM head. So right now the boat is at the shop getting it's annuals oil change/service and they are going to load test the batteries. Thanks for the replies.

The way my boat was set up from the factory, it has the motor, starter, and all the electronics (from graphs, live wells, to phone chargers and radio) operating off of one 12v battery and alternator. The TM has its own stand alone 24v two battery system.

I know a lot of boats are wired that way but I still recommend wiring it the way I described

even if it means adding a dedicated cranking battery. If you have a smaller motor (50ish HP or smaller)

a tiny group 51 battery won't take up much room or weigh very much and work just fine.

<----Boat mechanic