I bought a new battey but I think that my old one is still good. How do I check it? I took it to Pep Boys but they told me that they couldn't check it because it is a marine battery. Something about a starting and deep cycle and the deep cycle is the problem as well as gel batteries they said. Said they would 'blow up if they hooked it to their load tester. Is this true? And how do you check a marine starting and deep cycle battery?

I use a multi meter for mine. I haven't heard the story about load testing blowing up a battery. I hope someone on the board could shed some light on that one.

It won't "blow up" but if you ooad test a battery that is near the end of it's life you may finish it off. The best way to check a batterys internal condition is by putting a charger on it, put it on 10 amp and see how much it draws and for how long. If it takes 10 amps for a few hours or even up to 24 hours then the charge indicator on the charger falls to 0-1 amps, the battery is good. If it won't take any current or doesn't stop taking current then it is no good anymore.

advance auto checked mine with no problem,
and yes, it was dead.

Might take to an Interstate Battery facility--they make and sell all kinds of batteries and have the test equipment ad documentation to use it.

You could test it yourself quite easily. Fully charge the battery noting how long the battery accepts a charge as Hillbilly suggested. Then use the battery to power a spare automobile headlight. A deep-cycle with a 120 minute reserve capacity in good condition should be able to run an average 55 Watt headlight for a minimum of 10 hours before falling below 10 Volts.

I had no problem telling mine was bad after charging and using a $15 harbor freight load tester. BTW, nothing blew up.
Gary

And how do you check a marine starting and deep cycle battery?

check the date on the battery. If it is over three years old, get 'er out of there.