We had a 2 year old group 31 gel cell start battery for our Perkins that read 12.6 on voltage when I metered it (couldn't use a hydrometer) but if I read the voltage on the terminals and pushed the start button the the meter read less than 6 volts. It will take 2 people to do it but it worked fine for me. If it won't hold at least 11.6 volts when you crank the starter it's wasted.

I purchased a new battery at NAPA. If you bring it in for the core exchange they'll test it for you under load also.