13 Volts is probabaly showing surface charge. It depends how precise the measurement is.
Immediately after charging a wet cell battery will show as much as 13.4 Volts but it doesn't represent the true charge state.
Wait a few hours or flip on the lights for 30 seconds and the energy level will drop to 12.8 Volts on a fresh battery. A bit lower on a used battery.
10 amps is still pretty high charge rate in my book. On a fixed setting charger I'd say 2 amps 6-10 hours or overnight. But I'm not an expert on batteries.
A shade tree way to load test the battery is to watch the voltage when starting. If it drops to 9.5 Volts the battery charge state was pretty low.