SLA batteries can be charged very simple, by applying a voltage of 14Vdc to them through a resistor to limit the current to prevent going over the max charge current limit.
Basically during the charging the current will gradually decrease in a linear manner and the voltage will go up until it reaches the 14Vdc.
If you don't go over 14Vdc there is no chance of overcharging at all, even without the resistor. The resistor is there to prevent monstrous current to flow in to the battery when it is fairly deeply discharged.
Bare in mind that deep discharges and too high charge currents are the two main things that will kill SLA batteries. Deep discharges can be prevented quite simple by just a hand full of components. A relay, with an NPN transistor in series (C-E) that will be powered as long as there is voltage. The NPN transistor base (B) is operated by the combination of a zener diode of the desired cut-off voltage plus a little (about 10%) 12V is a good safe value. Bare in mind that zeners are not too precise, so take care! The in series with the zener is a trimmer of about 50K, a 10K resistor. You can add a check light (led) if you like...
A way to measure the capacity is by measuring voltage, or better the current that an amperes meter will show when applying the 14V. As said with a full battery this will be zero, with an empty battery the max allowed current will flow.
You need to check the SLA battery specs for the max charge current, then do some ohms law to calculate the needed resistor to limit the current to that level and calculate the power dissipated in the resistor in order to choose one with sufficient power rating.
Ohms law will also tell you how long it will play, on average the 2X25W Tripath amplifiers draw 300mA at full output level in to a 4 ohm load. That is with musical signal and two channels. That means a 3A battery will roughly play for 10 hours at full output....
Pics can help a lot to diagnose something from any place on the globe...