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Charging A Car Battery While It Is Still Connected

Jumping in your car and hearing just a click when you turn the key is one of the worst sounds in the world! You want to get on with your day. But now the car won’t start because the battery is flat.

One of the first questions is: Can I charge my car battery while still connected to my car?

If you can, it should be quicker and easier. But before grabbing the charger, we need to make sure it’s safe to do.

Can I Charge The Car Battery While Still Connected?

The first thing to do is to check the car owner’s manual. Any smart battery charger on the market can safely charge your car battery while connected.

In some rare cases, the manufacturer does not recommend charging your battery while connected. This is to protect sensitive electronic equipment that could be damaged by using the wrong charger.

Older chargers will supply very high charging currents that could easily damage electronic components. Modern trickle chargers only provide low current charging, which is perfectly safe. Your car battery is designed to be charged by the alternator, which supplies a low current.

The other problem with charging too quickly is that you can boil the acid in the battery.

This could force battery acid out of the vent or, in severe cases, cause the battery case to split. Battery acid is very corrosive and could easily damage anything it touches.

Pros of Charging a Car Battery When Connected

Won’t lose your radio and clock settings: If you disconnect your battery, you must reset your clock and probably enter a security code into your radio.

You may also lose all the settings in your radio, although this depends on the particular radio.

It’s easier: It’s less hassle and quicker to charge your car battery while it’s connected. You probably won’t need any tools, connect the charger cables to the battery, plug it in and wait.

Cons of Charging a Car Battery When Connected

Trickle charge only: You can only trickle charge your battery while it is connected to your car. As explained earlier, this is because a high current could damage electronics on your vehicle.

If you are not in a hurry, then this isn’t a problem.

You can’t “revive” a totally flat battery: Batteries that have gone completely flat may need a high current charge to recover them. Here, you will need to remove it from the vehicle.

In Which Cases Do I Need To Disconnect The Battery From The Car Before Charging?

If you have an older type charger

Modern chargers are microprocessor controlled and limit the charging current. The older style chargers may provide too high a current and damage the car’s electronic systems.

When your battery is totally flat

In this case, a trickle charger may not be able to charge it. This is because the trickle charger’s low current flow is insufficient to overcome its internal resistance.

For this, you need a modern maintenance type charger that can recover batteries. However, these use higher currents and pulse currents that could cause damage. So you cannot use them with a battery connected to your car.

When you charge an old lead-acid battery

Most car batteries are now sealed for life, but older versions vented hydrogen gas, which is extremely explosive. Charging these types of batteries when inside the engine compartment could spark an explosion.

How To Charge The Car Battery Correctly?

The best way to charge your battery is with a modern microprocessor-controlled charger. These monitor your battery while charging and adjust the charging current accordingly.

It should supply around 10% of the batteries’ capacity as a rule of thumb with an older charger. So if your battery is 100 amp-hour, the charging current should not exceed 10 amps.

Older chargers can also overcharge your battery, possibly causing damage if left connected for too long.

There are some simple tips to follow when charging your car battery:

Step 1: Make sure you have the right polarity. Don’t trust the color of any cables or battery covers on the car, but physically check the battery’s markings.

The red connector must always go to the positive terminal. The black cable must always go to the negative terminal.

Modern chargers will detect incorrectly connected cables, but older chargers may not.

Although the charger may detect reverse polarity connections, as described above, it could still damage the diodes in your rectifier.

Step 2: Connect the charger terminals to the battery first, and double-check they are correct.

Step 3: Connect the charger to the mains and switch it on.

Step 4: Now check the charger current is flowing by checking the meter or indicators.

Step 5: Once the battery is charged, switch off at the mains, then disconnect the terminals. This prevents sparks that may ignite any escaped hydrogen gasses.

Can I Start My Car While The Battery Charger Is Attached?

This depends, but the bottom line is that it’s not a good idea.

Chargers are only designed to provide a relatively low current, say ten amps max, for a standard car battery charger. When you try to start your car, the battery could be supplying 100 amps or more.

Trying to draw 100 amps from your charger will definitely blow a fuse and may even damage it. The only exception is if the battery is charged enough to supply the required current.


Finding your car has a flat battery is a real pain. You’re going to miss that important meeting or day out you were hoping to have while the battery charges.

But at least you know you can charge the battery while it’s still connected to your car. No need to get tools from the garage or get your hands dirty. Just connect the charger and wait.

One small tip, though. Find out why your battery went flat; otherwise, it will probably happen again.

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