I was told recently by an auto supply house that you occasionally need to totally discharge your TM batteries to prevent them from acquiring a memory.  I've always heard the opposite, that you should never totally discharge a deep cycle battery.  Anyone have an answer?

www.batteryfaq.org

The idea that batteries have  "memories" seemed to originate with Ni-Cad batterys. It was common for the nicad folks to discharge their batterys to "equalize them" that is to bring all of the batterys in a string to the same state of discharge. Then when the striing was re-charged, all would be at maxumum capacity and there fore the string would provide maximim power. Of course you do not use ni-cads with your trolling motor.

Folks using flooded lead-acid batterys used a different approach. In order to "equalize " their batterys, or rather all of the cells in their battery, they overcharged them at a safe rate. This brings all of the cells up to the same voltage, and provides the user with the maximum capacity the battery is capable of delivering.

Completely discharging a deep cycle lead- acid battery is sternly discouraged by the lead-acid battery manufacturers. Deep discharge batterys should not be discharged below a 20% state of charge, and the cranking batterys should not be discharged below 80% state of charge. Doing so will dramatically shorten their life.

New battery technology now on the horizon promises batterys that can be discharged to very low levels for a thousand or more cycles. Both Lithium and nano-carbon lead batterys show consideral promise. However our wallets will take a significant beating for the priviledge of owning one. The last quote I got from a chinese firm was for $2200 dollards for a single 100 AH 12 v lithium battery. Of course you can get them at a discount if you need 1000s of them.

So for now we are still stuck with 90 year old technology, and I think it would be a good idea to pay attention to the manufacturers warnings about depth of discharge.

Thanks for the replies.  I found the answer on the batteryfaq website under "Battery Myths".