Can You Charge A Car Battery While Still Connected?

by Phil Borges // in Car

Charging a car battery while still attached to the vehicle is risky, as there is a significant danger associated with overvoltage or short circuits. However, you can still charge a car battery when connected by exercising preventive measures to lessen injury or car damage. This article will let you know the consequences of charging the battery, tips on the best time to disconnect your battery before charging, proper ways to charge the battery, and many more.

However, most car manufacturers will recommend that you turn the engine off before charging a battery, allowing the alternator to supply enough power to charge and run your car.

It is always best to check the manual for your car to see what is recommended. Some of the best battery chargers on the market can charge your car’s battery while it is still hooked. And mostly, quality batteries take at least four hours to charge.

When is the best time to disconnect the battery before charging?

    1. If the battery is fully drained, it is best to disconnect it before charging. This will prevent the battery from leaking or causing damage to other parts of your car.
    2. If you have an old version of a charger that is hard to find in the market nowadays. The older chargers are more likely to cause hazardous situations such as sparks or overcharging.
    3. If you are using a lead-acid battery for charging. Car batteries used to be unsealed, which was a problem because they would vent hydrogen gas. This is highly explosive and could cause an explosion if ignited in the engine compartment while charging them.

Ways to properly charge the car battery

If you have a modern microprocessor-controlled charger, it would be the best way to charge your car battery. This device will monitor the battery and charge it at the proper voltage.

Use a good quality charger for your vehicle, whether it is an older model or not. But, if your charger is old, it can be dangerous to leave it connected for too long. It cannot tell the difference between full and overcharged batteries, so they keep charging until you unplug them. In case of a reverse polarity connection, your charger could still damage diodes in your rectifier, so be very careful.

Modern chargers will charge your battery at around 10% of the capacity. It can tell when the cable is plugged in backward and stop charging your battery right away. For example, if you have 100 amp-hour batteries, the charging current should not exceed 10 amps to avoid damaging them.

Here are some ways to properly charge the car battery:

      • Ensure that the battery terminals are in the correct orientation. The red cable must always go with the positive, and the black one needs to be attached to the negative terminal of your battery.
      • It is always best to connect the terminals in order, no matter how many steps are involved, then plug in the charger and switch it on.
      • The indicator will tell you when the charge is working.
      • It is always essential to unplug the charger once you are done charging, or else sparks may ignite any gas that has escaped while it was being charged.


Would it be possible to start the car with the battery charger attached?

It is possible to start the car with the battery charger attached, but it is not advisable. Chargers are not designed to provide a high current, say 100 amps max. When you try to start your car, the battery may be supplying only ten amps of power. The charger will blow out and even get damaged if it tries to draw in more than that amount.

What can you get if you charge the battery while the engine is running?

If you leave the charger on while running, it will keep trying to charge. It may also supply power for other components in your vehicle if they are not drawing enough on their own, like lights or dashboards.

What can happen if I do not take my battery out of my car?

Some of the chemical reactions within your car battery will cause it to become damaged. Also, if you do not take the battery out, there is a chance of leakage and the battery shorting out.


Getting a flat battery while you’re driving can be indeed annoying. So, you are tempted to charge it while still connected to your car’s system. It is essential to understand that it would be better to take the battery out of your car when charging to avoid a car accident or injury. Lastly, it would be best to have a backup battery not to compromise your time and energy of waiting for it to charge fully.

About the author, Phil Borges

Phil Borges is a battery aficionado. He's written extensively about batteries, and he loves nothing more than discussing the latest innovations in the industry. He has a deep understanding of how batteries work, and he's always on the lookout for new ways to improve their performance.