Auto Service World
Feature   January 1, 2002   by CARS Magazine

Charging Batteries

Battery charging is easy with modern automatic equipment, but if you need to do it the "old fashioned" way, here are some guidelines for safe charging. According to Bosch, a good digital VOM is the pl...


Battery charging is easy with modern automatic equipment, but if you need to do it the “old fashioned” way, here are some guidelines for safe charging. According to Bosch, a good digital VOM is the place to start by checking static voltage. Check the following chart to determine what the battery’s charging state is:

Static VoltagePercent Discharged

12.8 V0%

12.5 V25%

12.3 V50%

12.1 V75%

11.9 V100%

Then use this formula to find the safe fastest charging time at the battery’s Amp Hour rating:

Amp-Hour Rating X % Discharged

10 % of Amp-Hour rating

= charge time in hours

Remember, however, that a charged battery isn’t necessarily a good battery. Always load test when troubleshooting charging system problems.