Testing Marine Batteries

I have (2) deep cycle batteries, 1 as a starter for my 40 hp & electronics and 1 for my 40 lb TM, that I have NOT been doing a good job of maintaining for various reasons (“Yes Dad, I know I need to take better care of my stuff”). I am open to replacing them if needed with a pair of Interstate batteries, but would like to have them checked to make sure they are junk before replacing. If I take them in for them to test, what numbers do I need them to show me that they should be replaced? I would like to have a little knowledge so I have confidence that they aren’t just telling me they need replacing to sell me new batteries.

jake47 wrote:

I would like to have a little knowledge so I have confidence that they aren’t just telling me they need replacing to sell me new batteries.

I bought my own tester just for that reason. I had never taken a battery in, tested and have them tell me it was ok.
I’ve tested batteries I thought were bad only to have them test ok and something else f’d up.
Money well spent. The directions tell you what to look for.

From what I recall, load testing deep cycle batteries doesn’t tell you much, but I could be wrong…

I have a good friend that sells batteries. He said it’s nearly impossible to accurately test a deep cycle battery. If you test a charged battery it will read good but you will have no idea what run time you will get.

He said the best way to tell if it’s time to upgrade batteries is to charge them and run them. If they die too quick, then replace them.

The tester as far as I know will only accurately work on starting batteries.

Please correct me if I’m wrong but I believe the above info is correct.

Let me start off by saying I know nothing about electricity and batteries.

My thought is that if you fully charge an old deep cycle battery, the fully charged voltage will be lower than a new battery. My theory is that a bad battery will no longer take a full charge.

Seems too simple to be true though.

I guess even if I’m totally wrong, it would help verify that you are getting a full charge from your charger. This I’m sure of.

… because I’ve been having issues with my charger and one bank failing. This has resulted in significant loss of battery life. Fixed my problem by fixing the charger.

just do this
http://www.marineengine.com/boat-forum/showthread.php?399693-Deep-cycle-battery-restored-with-Epsom-Salt

Thanks all. I will probably replace the one that I know is shot and see if I can start the motor with the other one yet.