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Charging a Car Battery While Still Connected

Conventional wisdom used to be that you need to remove a battery before you charge it. Thanks to today’s smart chargers and trickle chargers, however, this is no longer necessary. Some devices can even be attached to a car battery full-time to keep them at their maximum charge when still in use.

Still confused about the details? Let’s get into some tips and facts that will help you the next time you need to charge your battery. 

Is It Safe Charging A Car Battery While Still Connected?

It is completely safe charging a car battery while still connected — as long as you follow a few precautions. Your battery is designed to charge while connected to a vehicle (that’s how it’s recharged by the alternator while your car is running). While there’s no risk of damage to the battery, however, the likelihood of shocks and sparks does go up when the battery is still in the vehicle.

Charging a car battery while still connected
Charging a car battery while still connected can have unexpected consequences

There are a few simple steps you can take to make sure you’re charging your battery safely. First of all, it’s advised that you wear safety gear, including both gloves and eye protection when charging a battery still connected to a vehicle. 

You should also be careful about the charger’s voltage. Don’t use a power supply that exceeds 14.7 volts at any point during the charging process. The good news is, any trickle charger, jump starter, or battery maintainer designed for a car will fall below this benchmark. Smart battery chargers are the safest option for this purpose. 

Finally, make sure you’re following other safety guidelines associated with charging a battery. This includes charging in a well-ventilated area, as well as avoiding direct sunlight that could overheat the battery and accelerate its charge. Also, make sure you inspect the battery for splits or acid leaks before charging. If you see any, you should replace the battery rather than charging it.

How To Tell If Your Battery Needs To Be Recharged

It might seem like an obvious sign if your car won’t start, but keep in mind that’s not always the fault of a drained battery. A faulty alternator or corroded terminal can be the true cause of your troubles. 

So how do you know if the battery is the problem? Here are some tips:

  • Test the battery. If you have the right tool available, testing your battery is by far the easiest way to tell if it needs to be recharged. Many battery chargers, tenders, and maintainers also come with a diagnostic mode. If not, you can use a digital multimeter. A fully-charged battery should read at least 12.6 volts, and as much as 14.7 volts when the vehicle is running.
  • Check the lights. Don’t have a way to test the battery? Turn on your vehicle, then turn on the headlights. If they’re normal brightness, the battery and alternator are functioning correction. If they’re dim initially but get brighter when you rev the engine, you may have an issue with your alternator. 
  • Inspect the terminals and connectors. Before you try to charge or replace your battery, do a quick visual inspection of the terminals, connectors, and cables. Corrosion can prevent a charge from getting into and out of the battery. If you see any white build-up or powder on the terminals or connectors, clean it away with a hard brush, and see if your battery’s operation is restored to normal. 

Bonus: Check out our guide on power banks!

How To Charge A Car Battery While Still Connected: Step by step

  1. Ensure that the vehicle is turned off and that the keys are not in the ignition. This is especially important when charging a battery that’s still connected. If the car is on, attaching the clamps of the charger could cause serious issues, including shocks and other injuries.
  2. Open the hood and determine how you’ll attach the clamps. If you are using the O-ring connectors for a permanent tender installation, follow the manufacturer’s instructions for correctly attaching them. If you plan to use alligator clamps, evaluate whether there is enough terminal exposed to attach the clamp safely. In some situations, removing the connectors is the only option to access the battery terminals. Even if this is the case, you can leave the battery where it is in the engine while you charge it.
  3. Verify that the battery charger is set to the correct voltage, then turn off or unplug the charger while you’re connecting it.
  4. Attach the positive clamp to the positive terminal. This will usually be red and marked with a plus sign (+).
  5. Attach the negative clamp to a grounded point on the vehicle’s chassis. This will typically be black and marked with a minus sign (-).
  6. Turn on the battery charger and begin the charge. 
  7. When the charge is finished, turn off or unplug the charger and remove the clamps in reverse order (black first, then red). If you’re using a permanent trickle charger or battery maintainer, you can leave it attached full-time as long as you need to.  

What Should I Do If My Car Battery Won’t Charge?

Every battery has a maximum lifespan. It can be as much as 10 years for a car battery, but even so it will die eventually. That being said, all may not be lost simply because your battery won’t hold a charge one time.

Your first step should be to attempt to desulfate or recondition the battery. Many advanced modern battery chargers have a setting for this, especially smart chargers. If yours doesn’t, you can take your battery to an auto parts store for testing and reconditioning. 

The Bottom Line

It’s frustrating when your car won’t start, but you don’t have to fret. A bad battery isn’t always the problem. Many times your battery just needs some TLC in the form of charging, maintaining, or reconditioning. 

If your battery seems healthy but you find yourself charging it frequently, there’s likely another cause for your problem. Check if there are any electronic systems in your vehicle draining your battery when it’s turned off. Common culprits are lights in the passenger or trunk area, which could be continuously receiving power due to a faulty latch mechanism. 

If you don’t see any lights on, check that all the bulbs in these lights are functioning. A dead bulb won’t prevent a socket from receiving power, and it doesn’t take much to drain your battery when it’s sitting.

For all of these tests and activities, a smart battery charger is your best way to go. Being able to diagnose and repair batteries at home will save you a lot of money at the mechanic. A smart charger is an especially good investment for those having chronic battery issues—and they’re perfectly safe to use on your battery while it’s still connected.

Bonus: Check out our guide on how to charge a phone battery without chargers!

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