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).