A 12 Volt SLA battery will need a float charge of 13.8 volts (I think that's it) So while it may charge a 12 volt battery up from a very low charge state to a moderate one, it may never fully charge the battery, as it can't produce the required voltage. Test the wallwart voltage on a multimeter under no and very low load conditions. If it produces at least 13.8 volts no load then it will eventually fully charge a 12 volt lead acid pack, but figuring out exactly how long requires more information of the voltage/current graphs of the specific battery and wallwart.