Why Car Battery may not be Charging Fully (How you May Fix this)

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Is your car engine cranking slowly in the morning?

Or, does the battery meter for your EV show less than 100% even after charging overnight?

Though either of those observations is not conclusive, they can point to a car battery not charging fully.

In fact, a car battery that does not appear to charge fully should not be ignored as it can point to an impending failure of the battery that could leave you stuck on the road.

There are other potential reasons why the battery may not be charging fully beyond the battery itself such as the charging source.

So, this post explains the possible reasons why your car battery may not be charging fully, and if and how they can be fixed.

5 Reasons Why a car battery may not be charging fully

Your car’s battery may not be charging fully because of the following reasons:

#1. Loose battery, alternator, or ground cable connectors. Confirm that the battery cable, ground, and alternator connections are clean and firmly connected.

#2. Car battery not sufficiently topped up during short trips. How often is the car being driven and how long are the trips?

The car battery will likely be undercharged if the car is parked for long periods and only driven for short durations.

This is because the current draw from the battery by the car electronics combined with the power draw when the car starts likely exceeds how much charge is replenished by the alternator as you drive for short durations.

#3. Aging battery. The car battery may be near its end of life. Unfortunately, some batteries are not able to attain full charge as they age.

If you suspect that the battery is damaged, try charging it with an external charger to see if it charges fully.

You can measure the final voltage when the charger stops charging. A fully charged car battery when resting should measure about 12.6 – 12.7V.

#4. Faulty alternator. If the car alternator is faulty, it will not be able to charge the car battery fully.

You may also be interested in this post: Is my car alternator undercharging the battery?

#6. Heavy battery drain by car accessories. What appliances are switched on in the car? If the heater or AC is on in an EV, the battery drains faster than usual. This can give the mistaken impression that the battery is not charged fully.

Your car battery may not charge fully when using an external charger if:

#7. The charger settings selected do not match the battery type. Check that the car battery charger is set to charge the battery type that matches your car battery.

For example, an external charger set to the GEL type charge to 13.6- 13.8 V only compared to about 14.5V for AGM battery type.

Mistakenly setting the charger to GEL instead of AGM will result in the battery not charging fully.

#2. Car battery requires more time to charge because of slow charge rate. A 5 amp charger charges about 5 times as fast as a 1 amp trickle charger.

If you have to manually switch off the charger (not an automatic charger) when the battery is full, confirm that the expected charge time for the battery is based on the charge rate selected otherwise you’ll undercharge or overcharge the car battery.

How to Check if your car battery is charging fully

Measuring the battery voltage can give you an idea (albeit it is not accurate) of the state of charge of the car battery.

For instance, a fully charged battery’s voltage should read about 12.6 – 12.7V. A much lower voltage, after recommended charging time, is a possible sign that the battery is not charging fully.

A good quality battery level tester can also give your a numeric and a visual display of a battery’s state of charge.

Keep in mind that battery level testers may need to be calibrated and many are not accurate.

Signs that the Car battery is not charging fully

Your car battery may not be charging fully if :

  • the battery charge level meter is no longer able to attain 100% charge level status
  • The car battery lasts for a shorter time with the same appliance when compared to previous run times.

For example, car headlights may only last for 45 min of headlights while they could last for 1 hour previously.

Note: These signs are not confirmatory as they may point to a faulty alternator or charger, weak or loose connections, aging or damaged battery as well in addition to the battery not charging fully.

Can you fix a battery that is not charging fully?

In some cases (if the battery is not damaged), you may be able to get the battery charging fully again.

For example, if the reason why the battery was not charging fully was because of:

  • loose, corroded alternator and battery cable connections.
  • faulty external charger or alternator

How to Fix a Car Battery that is not Charging Fully

Here is a list of checks you can do if the car battery does not appear to be charging fully:

#1. Confirm that battery cables make good clean contact with the battery posts

#2. Check the alternator voltage against the expected voltage. For example, the battery voltage should read between 13.5- 14.5V when connected to the alternator with the engine running.

If using an external charger, measure the charge output voltage.

Is it less than 12V for example? If so, then there might be a problem with the charger.

#3. In case you are using an external charger, confirm that the charger is set to charge the battery type.

If the car battery cable and charger connections are in good condition, the charger or alternator voltage is within expected limits and the charger is set to charge to the battery type then you might have a bad battery.

Get the battery tested at an Autozone to confirm it is in good condition.

How to Prevent a Car battery from not Charging fully

While it is not possible to completely prevent a battery from being undercharged, doing the following can help minimize the likelihood that the battery will be undercharged.

  • Regularly inspect the car battery, alternator cable connections, and ground and make sure that they are clean, free of corrosion, and firmly connected so that there is efficient charging of the battery.
  • If the car is going to be charged for an extended period, connect it to a good quality trickle charger to keep it topped up. Do not rely on regularly driving it as it may not charge the car battery fully.

Related questions

Should you replace a car battery that is not charging fully?

If the battery does not hold charge -drains overnight, shows signs of damage (leaking, swollen) then you should replace it.

In case of car battery draining quickly though, you should rule out parasitic battery drain and confirm battery condition by having it tested at an Autozone.

You may also be interested in this post: Why your car battery may be draining overnight

Final Word

If your car battery is not charging fully, then you should as the first step check the battery cable and charger connections are in good condition, the charger or alternator voltage is within expected limits and the charger is set to charge to the battery type.

Additionally, you should have the battery tested. A faulty battery may also not be able to charge fully.

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