What Is a Trickle Charger?

The term "trickle charger" refers to a battery charger that charges at a low amperage.

How Trickle Chargers Work

Many battery chargers put out a variety of amperages, the idea being to charge a battery slowly or quickly depending on the need. Some are also designed to be left connected long-term without overcharging. So when you hear people talk about trickle chargers, that's typically what they're referring to.

For general use, any battery charger, or trickle charger, that puts out between about 1 and 3 amps will do, and you don't really need one with float mode monitoring unless you want to be able to leave it connected for some reason.

As to why you should charge your battery instead of driving it around, there are two issues. One is that the alternator can only put out a limited amount of amperage, so the battery is likely to still be low on a charge if you only drive to work or run some errands. The other issue is that alternators aren't designed to charge completely dead batteries.

Trickle Chargers vs. Normal Car Battery Chargers

There are two main ratings for car battery chargers: amperage output and voltage. To charge a typical car battery, you need a 12V charger, but many car battery chargers have 6, 12, and even 24V modes.

In terms of amperage, car battery chargers typically put out anywhere between 1 and 50 amps for the charging mode. Some also have a jump start mode, where they can put out upwards of 200 amps, which is what it takes to turn over most starter motors.

The main thing that defines any charger as a trickle charger is that it either has a low amperage option, or it only puts out a low charging amperage. Most trickle chargers put out somewhere between 1 and about 3 amps, but there is no hard and fast rule on that.

Smart Trickle Chargers

In addition to providing a low charging amperage, some units are referred to as "automatic" or "smart" trickle chargers, to contrast with manual chargers. These units include some type of mechanism to automatically switch off, and sometimes turn back on, according to the charge level of the battery.

This is a nice feature to have if you want to maintain the charge level of a battery that isn't going to be in use for some time, and trickle chargers with float mode monitoring are often used in applications like golf carts, or when storing a car, motorcycle, or truck.

How to Connect a Trickle Charger

Set the switch on the front of the trickle charger to the correct voltage for the battery and then connect the clips to the battery terminals. The black clip connects to the battery's negative (-) terminal and the red clip connects to the positive (+) terminal. Next, plug the charger into an outlet and turn it on.

Why Charging Faster Isn't Better

The reason why charging a battery slowly is better than charging it quickly has to do with the science behind lead-acid battery technology. Lead-acid batteries store electrical energy via a series of lead plates and an electrolyte solution of sulfuric acid, so when a battery discharges, the lead plates undergo a chemical transition into lead sulfate, while the electrolyte turns into a dilute solution of water and sulfuric acid.

When you apply an electrical current to the battery, which is what happens when you connect a battery charger, the chemical process reverses. The lead sulfate turns, mostly, back into the lead, which releases the sulfate into the electrolyte so that it becomes a stronger solution of sulfuric acid and water.

Although applying a higher charging amperage speeds up this reaction and causes the battery to charge faster, doing so has its costs. Applying excess charge amperage can generate heat, and may cause off-gassing. In extreme cases, it's possible for a battery to explode.

To prevent this, smart trickle chargers can detect the charge level and automatically adjust the amperage. When the battery is very dead, the charger provides more amperage, and it slows as the battery nears full charge so that the electrolyte doesn't off-gas.

Who Needs a Trickle Charger?

In most cases, a trickle charger is more of a luxury than a necessity. However, they aren't expensive, and it's a nice tool to have around. If you can afford to leave your car with your mechanic for a day and have them fully charge your battery—and check out both it and the charging system while they're at it—then that's great.

If you can't afford to be without your car, picking up an inexpensive trickle charger would be a smart move. Make sure you follow safe charging practices and avoid overcharging the battery, especially if you go with a cheap manual trickle charger.