How long does it take to charge a battery?

The more discharged that a battery is, then the longer it will take to recharge it.  Usually it takes several hours to effectively recharge a battery.   

If a battery is very discharged then it can take up to 12 hours or more to recharge it. While recharging a battery, if the battery becomes hot when you touch it then stop charging it.

Slow charge: 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.

Fast charge:  Repeated fast charges on a battery may overcharge a battery and reduce service life.

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. In order to calculate the amount of amp-hours in a battery, the rule of thumb method is to multiply the reserve capacity by 0.6.  In the case of a MT-34, 120 minutes reserve capacity multiplied by 0.6 = approximately 72 amp-hours (at the 20-hour rate).  

Step 2: 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 approximately only 50% charged. Since the battery is 50% charged, then this means that there are approximately half of the 72 amp-hours in the battery. Therefore it is 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. 42 amp-hours needed by the battery divided by 10 amp charge  = it will take around 4-1/2 hours to recharge the battery. (The best charger to use in order to charge a car battery is a 3-stage automatic 12 volt /10amp charger.)  However, the charger really doesn't 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 1/2 the output over the time period that you are actually charging the battery. For that reason, it can easily take 9 hours or more to fully recharge the battery.  Even after 9 hours, because of the reduced voltage, the battery may require more charging to get it 100% charged.

Step 4:  Use a voltmeter or, better, check the cells with a hydrometer to make sure the battery is fully charged.

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