500 mA, 0.5a might not be enough to get a battery to a high enough voltage that it can be fully charged with specific gravity maxed out . Many of these 0.5a wall warts simply cannot get a battery higher than 13.7 or 8, especially if it is a larger somewhat sulfated battery, and even when unloaded if the voltage is 18 volts.
AGM batteries tend to require less amperage to be held at higher voltages.
10 to 13% is a recommended charge rate, when one has all the time needed to complete the charge from a plug in charging source. When one is charging from a generator, then one does not want to run the generator for 9 hours just to meet this 10 to 13% recommendation.
One can indeed blast a battery with higher amperages. it is better to blast a battery with high amperage to a higher state of charge than it is for the battery to begin the next discharge cycle at a lesser state of charge and possibly fall below 50% because one was trying to slow charge it with the 'recommended' rate.
IF you get a Megawatt and an Ammeter, with your curiosity, you will accomplish so much more than the masses who want a plug and play automatic source to flash them a soothing green light and then add a year to their battery life in each praising of their green light purveying product.
I regularly hit my former 12v flooded battery with a 50% rate, and got close to 500 Cycles from it. Held to 10 to 13% rate maximum I would have come nowhere near that as there was not enough time to complete the charge before the next discharge began.
Amps and volts are not separate entities. Voltage is electrical pressure . Not enough pressure between source and battery and the amps can't flow . Once the battery reaches absorption voltage, the amps required to hold absorption voltage become less and less as the battery absorbs the top end of the charge. This takes about 4 hours once absorption voltage has been reached at the battery terminals, but the actual time varies according to many factors.
So get a real hydrometer and see how high the Specific gravity, and Voltage gets with your wall wart. It might not have enough juice to do the job, no matter how long it is left plugged in.
Or it might. You will have to tell us.
