Step 1: Determine how long to recharge a battery by calculating how much capacity your battery has. For example, an Interstate battery with the part number MT-34 has 120 minutes reserve capacity (RC). In order to calculate the amount of amp-hours in a battery, multiply the reserve capacity by 0.6. In the case of a MT-34, 120 minutes RC x 0.6 = about 72 amp-hours (at a 20-hour rate).
Step 2: Before charging a battery, use a voltmeter to measure the remaining voltage in the battery. For example, if the voltmeter shows a voltage reading of 12.4 volts, then the battery is about only 50% charged. This means there are about half of the 72 amp hours in the battery. It's necessary to put about 36 amp hours, plus 15% more to compensate for the internal resistance in the battery, for a total of 36 amps + 36 x 0.15 = around 42 amp-hours back into the battery.
Step 3: Charge the battery at a 10-amp rate. Take the amount of amp-hours needed by the battery and divide it by 10 amps. Using the example above, it would take around 4.5 hours to recharge that battery. (The best charger to use to charge a car battery is a 3-stage automatic 12 volt/10 amp charger.) The charger doesn't really output the entire 10 amps during the charge cycle because it automatically limits the voltage and the amperage during the charge cycle. You may actually only see about half the output over the time period that you are actually charging the battery. For that reason, it can easily take nine hours or more to fully recharge the battery. Even after nine hours, because of the reduced voltage, the battery may require more charging to get it 100% charged.
Step 4: Use a voltmeter or check the battery cells with a hydrometer to make sure the battery is fully charged.
How do you know the best method to charge your battery? We've outlined the difference between a slow charge and a fast charge for you.
It is best to slow charge the battery. Slow charging rates vary depending on the battery's type and capacity. However, when charging an automotive battery, 10 amps or less is considered a slow charge, while 20 amps or above is generally considered a fast charge.
Repeated fast charges on a battery may overcharge a battery and reduce service life.
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