Hello...

I have a 12V 2.8Ah sealed lead acid battery... and a battery charger
that can be set from 50 mA to 300mA.

What should I set the charger at?

Not too sure if the battery is any good, but I thought I might try and
recharge it to see if I could get some use out of it. It had been a
backup battery in an alarm system.

Thanks