I built a UPS from a wall charger, 12v 7.2AH SLA battery, and a small invertor. I'm getting 55 minutes of run time from this, which is far below the AH rating of the battery. I know that it is not going to deliver the full AH rating at more than the 20 hour rate (360mA). I don't want to build twenty batteries into the thing, but what is the point of optimal return as far as discharge rate is concerned? I already have a 2nd battery, will a ten hour discharge rate give me more than 2 hours of run time? I'll need to upgrade the charger in any case, even the one battery is pushing the poor wall wart to its limit.