Charging a Car Battery While Still Connected to your Vehicle – Safe?

Car batteries typically only last 3-5 years, so there will come a time in most of our lives when we will come across a dead flat 12v battery that won’t start our car.

When this happens we basically have 3 options.

• Jump start the car from another vehicle or a portable jump starter
• Replace the 12v battery with a new one.
• Try recharging it up to see if it will hold a charge.

Once 12v car batteries go dead flat say under 11.5 volts the damage is already done. This means the battery won’t hold or output the full cranking amps it was designed to. We can try to resurrect the battery and this process is called reconditioning or repair.

Fully charged 12-volt batteries are around 12.8–13 volts. There are 6 individual cells in a car battery which can each produces up to 2.2v per cell. So it is very important to keep each cell topped up with water and fully charged.

Only a few manufacturers that make quality chargers support this feature and I must say it works quite well. I have brought a few deep cycle batteries back to life with this process using a Ctek smart charger. The process will repair heavily sulfated and old 12V batteries that won’t hold a charge.

Charging a car battery while still connected

In the old days, it was always recommended to remove the battery completely from your car and charge it in a well-ventilated area. Now we have smart car battery chargers that monitor and limit the number of amps delivered into the battery.

Companies like NoCo and Ctek make permanent battery charging leads that you can bolt onto the terminals of your car or boat battery. This makes charging a simple plug-and-play experience.

I leave my battery connected to the vehicle’s electrical circuit while charging and there has never been any problem. I even leave my Sea-doo battery connected to the electrical system when I give it a 3-month top-up charge to its gel battery.

Because the battery is so far down into the hull it is hard to get the charge clamps onto it. This is where the permanently mounted charge leads are worth their weight in gold. They just plug straight into the CTEK charger.

A velcro strip makes sure the lead end is secure and some zip ties hold the leads in place.  The Sea-doo battery replacement procedure was a challenge but we got there in the end.

My battery charger of choice is the CTEK MUx 5 this is because it has a very sophisticated multi-step smart charging circuit to maximize battery life and performance. If you would like to read more about this charger you can see how it compares Ctek vs NoCo.

The CTEK smart charger comes with an extra set of bolt-on charging fly leads. I like that it has a rubber o-ring to stop water from getting into the terminals. Being a quick disconnect it makes charging so much easier with permanently connected charging leads. 

Safety steps to take charging your car battery in the vehicle.

• Always charge your battery in a well-ventilated area with the bonnet open and out of direct sunlight.
• Check the water level in each cell and top up with distilled water only.
• Charge the battery with the correct voltage and current for the size of the battery. Never charge a 6v battery with a 12v charger.
• Wipe down any spilled or bubbled-over battery acid with a damp cloth.
• If your car battery has expanded on the side stop charging your battery immediately and replace it.
• Check all electrical cables for tightness and any broken insulation.
• Always place the red positive charge lead onto the positive terminal on your battery. Never onto the negative terminal.
• Continuously monitor while you are charging a car battery while still connected to your vehicle.
• Always charge the battery with the car bonnet open.

Will my car electrical system be damaged from the charger?

The simple answer is No, as long as you are using a quality smart 12v battery charger and follow the safety steps mentioned above you can leave your battery in the car. Even though the charger uses a higher voltage up to 15V. 

Most 12v car alternators will output between 13.8V to 14.8 volts, to keep your battery fully charged. If your car electrical system is happy at 14.8V you should be able to charge a car battery at the same or less voltage.

I always like to remove the vehicle battery if I am doing a recondition or repair on the battery. This is because it may take a few days to complete the recondition cycle and I may need the car. It is also easier to check the battery water level. Be aware you will need a spare replacement battery if you are wanting to drive the car.

Can I leave a 12v charger connected to my car battery?

As long as it is a smart charger then there won’t be any problem. Be sure to check the battery water level every few days. Many boat owners do this as there is often a small power draw associated with alarm systems and boat electronics.

Smart chargers will float the battery voltage at the optimum level and prevent overcharge.

Conclusion

So the question still remains “Can you charge a car battery while still connected to your vehicle?” Yes, you can.

Car alarm systems and always-on video recording cameras will constantly drain your car battery. This is why if you don’t plan on using your car for a few weeks then you should either set up a trickle charger or top up your battery once a month. If you don’t your battery will deplete and it will fail completely.

Be sure to follow all the battery charging tips mentioned above and you will have a trouble-free battery that will last for many years. If you follow basic battery maintenance and keep the water level topped up you should get over 5 years out of your deep cycle battery.