Using power supply to charge smallish 12v seal lead acid battery?

A 12V charging system will put out anywhere between 13.8V and about 18V DC. Direct current is needed for charging. Depending on how big your "smallish" battery is will determine how long and if it can charge your battery. A 500mA supply is .5 A . So for a car sized battery this would only amount to a trickle or maintenance charge, and could be left on almost indefinitely.

As the battery size gets smaller you run the risk of an over-charge rate and over heating.

For a garden tractor size bat. I would recommend, say, 24hr charge then check voltage of the bat.

For smaller sized bats. try 10-16 hr charges and check voltage and the temp of the bat.

If it feels very warm after only a short period 1-2 hrs. you are charging at too high a rate,and either need a smaller charging unit or possibly charge more than one bat. at a time, to handle the extra amperage.

That is what it should read when it is without a load. A car alternator will put out 14.5 volts when charging a 12 volt battery, if it did not have a higher voltage, then it would not take a charge. The energy must reverse the chemical process in the battery so it can store the energy.

It's probably reading high without a load . It should drop to 12V when applied across the battery. Put the leads on the battery and take the reading again.

Any mathmatical formula will lead to the same result. Just be sure and let us know how long it takes to blow up the sealed lead acid battery. And that wont create near the load on the supplier as you're hoping. Not to mention, make it extemely hot on the circuit.You may wanna run an extension cord to the neighbors outlet so it wont damage yours, just his. Also stand right next to it for at least an hour, whatcha think?

Maybe, but I would recommend a charger that is designed to reduce the current to provide a trickle charge, when the battery is fully charged.