Why do Battery Chargers get so Hot

If you’ve ever used a battery charger to keep your car battery juiced, you may be surprised at how hot it can get during operation. It’s natural to wonder how safe this condition is; after all, nobody wants to inadvertently burn down their garage.

Thankfully, a hot battery charger isn’t an issue. In fact, it’s entirely normal: battery chargers usually heat up during charging, sometimes alarmingly so. But the heat isn’t a reason to panic. It’s just the battery charger doing its job. Some significant differences may occur between gas, hybrid, and electric cars, of course, but the heat from the battery is generally not a cause for concern.1

Three main culprits create heat in battery chargers: resistance, inefficiencies, and battery charge rates. But to really understand why battery chargers heat up requires a bit of a primer on electronics.

Electrical Theory 101: Volts, Amps, and Ohms

We’ve covered some of this already in a previous article on battery chargers, but it warrants repeating that what we think of as electricity is actually a mishmash of three separate but related phenomena: voltage, amperage, and ohms.

It helps to think of these three terms as analogous to a water pipe and faucet. Voltage is like water pressure, driving the electricity along at a certain speed. Amperage is the volume of electricity being driven along. Ohms is the measure of resistance, or how much fight the electrical conduit – be it wire, water, or your body – puts up against the electrical energy trying to force its way through.

Ohms is often the culprit of a hot battery charger, so let’s take a deeper dive into what resistance really is.

Cable Resistance


Cables for a Battery charger Image courtesy of Pixabay

Think of the act of resisting: whether it is a tantrum-throwing toddler or a stuck zipper on your jacket, the idea of resistance boils down to two opposing forces fighting with each other for dominance. In the process of trying to overcome the other, friction develops. That friction manifests either as emotion in our toddler example or as a physical and mechanical strain in our zipper example. In electronics, it manifests as heat.

With electronics, all wires produce some level of resistance. As the power is refuted by the wire, energy is dissipated into heat. This is when it’s helpful to recall from your middle school science class that energy cannot be created or destroyed; the power that can’t get through due to resistance turns into heat.

This is partly why battery chargers heat up during use – they are attempting to force electricity through their cables at a steady rate during charging, but the resistance inevitably turns some of that potential battery juice into heat, or lost energy.

Battery Charger Inefficiencies

You may have noticed we said resistance is partly why battery chargers can get hot. There are other reasons as well, not the least of which is the inherent inefficiencies of the inverter that turns 120-volt household AC (alternating current) into 12-volt DC (direct current) required for automotive applications. Their inefficient efforts to convert that current suck up energy that, like resistance, is dissipated as heat. It’s a major reason why battery chargers get so hot.

We should note that efficiency refers to how successful a motor or device is at doing work. Something working at 100 percent efficiency is losing zero power to resistance, friction losses, or anything else that may interrupt or sap power. In a battery charger, the cables, wires, and circuitry all put up some level of resistance or cause some sort of unwanted power loss.

The EPA has done excellent research on the efficiency of electric vehicles.2 They may have initially begun the research on charging and driving your electric car, but the principles at play are the same whether you’re charging a Nissan Leaf or the battery in your 1969 Corvette.

Battery Charging Resistance

Another point to consider: batteries become more resistant to charging as they get closer to being fully charged. To understand why, consider your drained battery as an empty chamber. To start recharging it you hook the battery up to your charger, and which at once begins to dutifully siphon power from the electrical grid via your household outlet. It feeds that power via cables to your battery.

At first, the charging goes quickly – the electrons rush into the empty chamber. As the chamber fills, the rate of charging slows down – the electrons need to be a little more careful entering the chamber, just as you would be more careful entering a crowded elevator. When the battery is 80 or 90 percent charged, its chamber is nearly full and the charging rate slows significantly, as now the electrons have to forcibly stuff themselves into what space is left.

At 99 percent charged the last few electrons to get in the door must wiggle, squirm, and fight their way to the little remaining space. At that point, the charger is doing what’s known as trickle charging – feeding in very little power to the battery at a very slow rate of charge.

Once the battery is entirely charged it will refuse all additional juice. This can be dangerous if the charger is still hooked up to the battery, as it may continue to try and charge the fully charged battery. If that is the case, all that energy will be trying to dissipate as heat – and all that heat is a recipe for a fire. Luckily, they now sell chargers that cut off once a battery is charged, making for a safer charging experience.

Parting Thoughts

Charging a Car Battery

Image courtesy of Pixabay

Battery chargers will inevitably get hot. It’s an unavoidable byproduct of transferring energy from one source to another; somewhere along the lines, inherent inefficiencies and resistance will cause some power to be lost in the transfer. That lost energy is the heat we feel when we touch a surprisingly hot battery charger.

For the safest, coolest charging scenario, it’s recommended to charge at the slowest rate of speed you can. The slower the charge, the more efficient the charging, and less likely the charger will build up so much heat. It’s like sprinting 100 meters versus running a mile – one you maintain a constant pace, the other you blow out all your energy at once. Both will tire you out, but which is more efficient?

Article Sources

Ride Digest uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. Ni P, Wang X. Temperature field and temperature difference of a battery package for a hybrid car. Case Studies in Thermal Engineering, 2020; 20. doi: 10.1016/j.csite.2020.100646
  2. Where the Energy Goes: Electric Cars. US Dept of Energy. Accessed 23 Sept 2021.