Went fishing this weekend and my MinnKota Maxxum 80# would only run at half power. It was variable speed but nothin like it should have been. My batteries are 3 years old (Interstate trolling batteries) and I think I have a bad one. Checked the voltage today and one was putting out 12.45 and the other was running around 10.20. Does this mean I have a bad battery????? Don't know if the trolling motor is having issues or if it is a battery issue.  

10v is a dead duck.  See if it will take a charge.

Have an onboard charger and keep it hooked up all the time. Just wanted another opinion before I bought new batteries...

New batteries will tell the tale

A fully discharged 12 volt battery will register 10 or so volts . Replace it . if you have a two or three bank charger that charges them all and charged them  they should register the same voltage. charge them put the prop in a bucket and run at low speed for a while then check the voltage. If the suspected battery is bad it wil have alot less charge left compared to the good one.

10v at zero draw is well done. Buy a replacement if she won't come back up on a wall charger after a couple days. I'm sure your TM will run fine when it has proper power. I'm surprised you got it to 1/2 power on 24v.

Thanks everybody... Sounds to me like a new set of batteries will cure the problem. XcoM274, I was being very generous when I said half power. It was probably more like 1/4 power. I'm just glad it got us around the lake that day even though it was extremely slow.

The good one will make a nice addition to my pond hopper 2 man bass rig instead of using an automotive 12 volt. I will have to take a buddy along to tote the thing for me though.  ;D