That'll work just leave it on for a few days and get a hydrometer if it's not a sealed battery. I have an 8amp 6/12 volt charger, probably 35 years old, doesn't even have an on/off switch but, it still works great! I'll charge my truck's battery overnight once a month or so just to top it off. I'm afraid the battery is getting weak though, volts drop down to ~8 when starting cold but, the date sticker on it is from 2015, not bad for being in the desert, ha ha.
Edit: my alternator is over 12 years old too!