A car battery charger being used

All car batteries lose power over time. This isn’t a factor when the car is driven regularly, allowing the alternator to replenish the battery. But if the car is parked for an extended period, such as for winter storage or while working at home, it is wise to trickle energy into the battery so that it is ready to start the car when you need it. This can be done without removing the battery.

Many modern cars have significant draws on the battery, meaning that parking them even for just a week or two can drain the battery. (The owner’s manual may indicate how long the car can park without draining. In some cases, draining the battery may void warranty coverage.) A chief concern is that draining a battery completely can shorten its lifespan. Plus, if you jump-start a vehicle, there is a risk of damaging electronic components or requiring assistance from a technician. (These days, it is best to leave the jump-starting to professionals, rather than good Samaritans.) 

There are two main types of chargers: trickle and maintainer. Basic trickle chargers slowly replenish the battery, but they need to be monitored and manually disconnected when the battery is topped off. A maintainer may be all most people need. True to their name, these affordable chargers are meant to preserve the charge level, not resurrect a dead battery. Unlike trickle chargers, maintainers turn on and off as needed.

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A battery maintainer will charge as needed, automatically. Advanced maintainers, often called “smart” chargers, typically have modes to address the needs of absorbed glass mat (AGM) batteries, ensuring that they are charged fully and properly. And there is often a feature to de-sulfate the lead in traditional flooded batteries; that reverses some crystallization that occurs when batteries go unused, extending the battery’s life. These full-featured chargers typically have modes for maintaining a lawn tractor or motorcycle battery, adding to their value.

For those who park outdoors, away from a power source, there are solar-powered battery maintainers. These connect to the car battery, and some cars can be charged via the 12-volt power point inside. Because of their low amperage, a solar unit is used to offset any natural battery discharge, rather than recharge a weak battery. Most solar chargers come with a reverse charge blocking diode to prevent it from draining the car battery at night. Verify that the model you are considering has this feature.

“My preference is a smart battery maintainer,” says John Ibbotson, Consumer Reports’ chief mechanic. “These chargers address a wide range of needs, and they work automatically.”

Ibbotson suggests using quick connectors, often called pigtails. These go onto the battery terminals and allow the charger to quickly plug in and disconnect. Further, they can be safer than clips because they reduce the chance of causing an underhood spark. These eyelet-style connectors often come with smart chargers, and they can be purchased separately.

Before buying, be sure to read the specs for the charger to ensure that it has the features you want. There are many products to choose from, and it can be easy to select one meant for other purposes.  

Tips for Using the Charger

Connect the charger per the instructions. Then run the cable into the engine compartment, leaving the hood ajar. Make sure not to close the hood on the wires. You also don’t want to leave the hood completely open because that risks leaving a hood light on if your car has one.

Resist the urge to route the wire through the grille or down along the engine. This hides that the car is plugged in, creating a risk that a driver in the household might hop in the car and drive away without disconnecting the wires.

Be sure to check the car’s owner’s manual before charging because some cars have specific guidance. This is especially important for models with the battery mounted in the trunk or tucked under the windshield. With those, including some BMWs, there are jump-start points under the hood that could be used for charging.

Batteries can emit harmful and/or explosive gases. Be sure to charge in a well-ventilated area and wear safety glasses or goggles when handling a battery.

Final tip: Red is positive, and black is negative. It does matter. Always confirm with the instructions before connecting anything to the battery.