[Engineer]

Reading through all these posts, Al and Don make some excellent points here. I want to add a little information for those that don’t go to the battery university website or want to spend hours researching battery characteristics.

If you discharge a lead acid battery too far, you hurt the battery and its amp hour capacity will be reduced.

Typical discharge to levels for batteries, or don’t go below this charge level.
Automotive batteries 80%
Marine batteries 50%
Deep cycle batteries 20%

Automotive batteries should not be discharged below an 80% charge level or you’ll hurt the battery. There is only 20% of the charge that is usable but the automotive battery will deliver more amps for the same physical size when compared to Marine or Deep cycle battery. In an automotive application the battery is discharge a small amount very shortly starting the engine and then is recharged by the alternator.

The FIRST battery seems to fall into the Marine category as the cycle life is greatly reduce if the charge drops below 50%.The information below is from the MK Battery website.
Life expectancy:
Cycle Use: 100% depth of discharge 250 cycles
80% depth of discharge 350 cycles
50% depth of discharge 550 cycles

What this means is that if you continually run your batteries down to low charge levels, you WILL significantly reduce the capacity of the battery.

On a side note, I don't like the battery charger FIRST supplies but they are better than the older non-smart chargers. I've seen these not sense properly and over charge batteries. This was probably due to a defective battery. I prefer the Black and Decker (used to be Vector) Smart Chargers. They use a three stage charging technology that helps extend battery life. I've given away or thrown away all my old battery charges for these new chargers, they are that much better.

FYI Some AGM batteries have platinum on the glass mat. This helps as a catalyst to recombine the oxygen and hydrogen bubbles if the battery is over charged or charged at too high of a current.