I have a battery that has kept going down with waht i suspect is a parasitic drain over the last several months, I charged it up with my battery charger on a 10 amp setting and it then read 12.99 volts. i left it set on my wooden work bench for several days and it only reads 10.50 volts. The battery is only 19 months old and i was told it was ok when i took it where i bought it and they performed a load test on it. what is wrong