Any battery charger that will switch to six volts is fine. One that is labeled "automatic" will automatically shut down when the battery is charged. Other ones (not automatic) must be timed since they will continue to send power to the battery. There are formulae for how much charge it has now (percent of charge) and what size the battery is. There may not be a formula for such a small battery.
If your charger is not automatic, just charge it for a short time (like 30 minutes). If your charger is not automatic and high amperage (above ten) make it less time. Then check to see if it's boiling inside. If so, take it off the charge and try it. When it's still charging, there should be small bubbles, but not big ones.
If you have a voltmeter, watch the voltage during charging. It should increase as the battery charges. When it stops increasing, the battery is fully opposing the voltage of the charger and should be fully charged.