Use a charger, with an
end voltage of 16 V, equipped with a voltmeter that is accurate to
within 1 percent.
The ambient temperature should be 15-38°C (60-100°F).
A battery that is extremely cold will not accept immeasurable current for
several hours after starting the charger.
The charging area should be well ventilated.
Do not charge the battery when the built in hydrometer is clear
or yellow. Replace the battery if there is a cranking problem.
Do not charge a battery that appears to be frozen.
Batteries showing a green dot in the hydrometer do not need to
be charged unless they have just been discharged (such as cranking the engine).