Determining Charge Time

The more discharged that your battery is, the longer it will take to recharge it. Batteries usually take several hours to recharge; if the battery is severely discharged (12.2 volts or less for an flooded battery) then it may take up to 12 hours or more to recharge. While recharging any battery, if the battery becomes hot when you touch it, stop charging it (a battery being charged should not get above 125 ferinheight).  It is preferred to slow charge all batteries, slow charging rates vary depending on the battery’s type and capacity. When charging an automotive battery, 10-amps or less is considered a slow charge. Fast charging is NOT recommended, 20-amps or above is generally considered a fast charge. Repeated fast charging an automotive battery may overcharge the battery and reduce service life.

Step 1:
To determine how long it will take to recharge your battery, you must determine how much reserve capacity your battery has (refer to manufactures specs). In order to calculate the amount of amp-hours in a battery, the rule of thumb is to multiply the reserve capacity by 0.6. For example, if you have an automotive/flooded battery with 100 minutes reserve capacity, multiplied 100 by 0.6 = your battery would have approximately 60 amp-hours (at the 20-hour rate).

Step 2:
Use a voltmeter to measure the open circuit voltage of the battery (see chart below for state of charge). Open circuit voltage is the voltage of the battery with no load or disconnected from any circuit. Do NOT charge the battery before you test the open circuit voltage. If the voltmeter shows a voltage reading of 12.2 volts then the battery is approximately 50% charged. Since the battery is 50% charged, this means that there is approximately half of the 60 amp-hours in the battery or 30 amp-hours. Therefore it is necessary to put about 30 amp hours plus 20% more to compensate for the internal resistance in the battery for a total of 30 amps x 0.2 = 36 amp-hours to fully charge your battery.

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Step 3:
If you charge your battery at a 10-amp rate and you need to replace 36 amp-hours in the battery. To determine how long (in hours) you will need to charge your battery, divided 36 by 10 = 3.6 or 3.6 hours to recharge your battery.

A 12 volt / 10 amp 3-stage automatic charger is the best charger for most automotive batteries. Unfortunately most chargers do not 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 12 hours or more to fully recharge the battery. Even after 12 hours, because of the reduced voltage, the battery may require more charging to get it 100% charged.

Step 4:
After the battery has cooled to room temperature use a voltmeter or check the cells with a hydrometer to determine the battery is fully charged.

The chart below is for quick reference, the method described above is the best way to determine how long it will take to charge your battery. The chart below shows how to determine the amount of time need to charge a battery based on the batteries CCA, open circuit voltage and charge rate. Batteries by Fisher does not recommend charging your battery above 10 amps.

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You should always wear personal protection when charging and/or handling all batteries. The data and information contained herein are intended for use by persons possessing proper technical skill and knowledge, as they may deem appropriate and at their own risk. Batteries by Fisher, Inc. assume no liability whatsoever for results obtained or loss or damage incurred as a result of application of any data or information presented herein. Batteries by Fisher, Inc. believes the information contained herein is accurate as of its publication date. Information contained herein is subject to change without notice.

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