When car batteries are charged beyond the manufacturer’s recommended voltage typically 14.5V for a 12V battery, overcharging is said to occur.
Overcharged car batteries get extremely hot – you may literally boil out the battery solution, have a battery explosion causing irreparable damage to the battery.
Signs of an Overcharging Car Battery
The following signs though not conclusive may be a sign that your battery is being overcharged:
- High battery temperatures during charging batteries
- Battery case covered with battery solution in the case of flooded lead acid batteries
- Bulging of battery sides
- Depending on external charger or car model, overcharge indicator light displaying on your external charger or car’s dashboard
- Battery voltages as high as 17V measured across the battery terminals with the car engine running
What Causes Overcharging of a Car Battery
Car alternators can overcharge and may kill the battery if they are:
- faulty with the inbuilt regulator exposing the battery to higher than recommended voltages for a long time
- replaced with non recommended manufacturer alternator that produces higher than recommended charging currents that are bad for
the battery
External chargers
External battery chargers too can overcharge batteries.
Always make sure the charger setting is set according to the manufacturer’s recommended settings to ensure optimum charging and prevent overcharging.
Typically this will require you to set the battery type to AGM, Lithium, or other depending on the chemistry of the battery you need to be charged.
Can a bad battery cause overcharging?
Note: A bad battery is unlikely to result in the battery being overcharged provided that the alternator is working well.
How to Prevent your Car Battery from Overcharging
To avoid damage to your car battery as a result of overcharging, use good quality chargers that have an automatic shut-off feature when the battery is fully charged or one recommended by the manufacturer.
This way, you can leave your battery connected indefinitely to the charger – you need not worry about forgetting to turn off your battery charger when the car battery is fully charged!
Check that the charger settings are set for the battery type you are charging – AGM or Flooded Lead Acid.
When replacing a car’s alternator, replace it with the car manufacturer’s recommended alternator to avoid the danger from overcharging.
How to Fix an Overcharged Battery
If your battery has not been subjected to excessive voltages (17V) for a long time, it is possibly fine.
If however, it has, then your battery will likely suffer permanent damage and needs to be replaced (check out the section on signs of a bad battery).
How to Stop a Battery from Overcharging
To stop your car battery from overcharging, if using an external charger, make sure:
- the charger setting is set according to the battery type. If it is an AGM battery, the charger should be set to charge AGM.
- The maximum current and voltage limits of the charger set do not exceed the battery manufacturer’s recommendations.
For example for a 100Ah AGM battery, the typical current and voltage settings are 10A and 14.5V respectively.
Excessive voltages and currents destroy your battery.
Charging with the car’s alternator
A common reason why a car’s alternator may overcharge the battery is damage to the alternator’s internal voltage regulator.
For signs of overcharging of a battery as a result of a faulty alternator, check here.
If the car’s alternator is damaged, replace it right away with the manufacturer’s recommendation.
Related questions
Undercharging a battery (Causes+fixes)?
Undercharging a battery limits the battery’s ability to attain maximum charge.
Common causes of undercharging of the battery are:
# 1. incorrectly set external charger settings. For example, when the external charger is set to GEL for an AGM battery, the battery doesn’t attain full charge.
Fix: Always confirm that the external charger is set to charge the correct battery time
# 2. Not providing enough charging time for a manual charger that does not automatically turn off at full charge.
Formula for estimating car battery charging time
For an estimate of charging time (for a fully discharged battery), multiply battery capacity x 1.2 divided by charging current.
For example, a 100Ah battery that has been deeply charged when charged with an external charger set at 10A will take approximately 12 hours from (100Ah x1.2)/10.
Fix: Use the formula above as a guide to estimate the charging time.
Alternatively, use an automatic charger that turns off when the battery is fully charged.
# 3. A damaged alternator may not be able to supply a high enough voltage for the car battery to charge fully.
Possible causes of damage may be faulty diodes in the alternator.
Fix: Replace the faulty alternator with the one recommended by the car manufacturer.
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