If you’ve ever had a car battery that was hot to the touch, you’re likely to hit full panic mode if you don’t know why.
Why is my car battery getting hot?
Car batteries can become hot due to overcharging, an internal short, a rapid rate of discharge, or the battery was heated by the car’s engine without proper cooling. A hot battery should not be ignored as it can lead to fire or an explosion.
Why is my Car Battery Hot?
|Being Overcharged||Too many amps from an external charger, or the alternator/voltage regulator are faulty|
|Internal Short||Conductive debris from sulfation is bridging the gaps between the plates|
|Rapid Discharge||Heat generated from inefficient discharge when draw is too high|
|Hot from car's engine||Heat from under the car's hood was absorbed and being released by the battery's case|
|Terminals hot from loose connection||Loose connections generate heat. Tighten to fix.|
This is the quick and easy answer without getting into how to fix the problem and you were probably alerted to your car battery overheating due to steam or bulging.
You likely have permanent damage to your battery at this point, but the extent to which it is damaged might be mitigated by the steps you take.
I’m going to touch on each of the 4 main reasons a battery gets hot, a 5th reason why the terminals might be getting hot, and what you should do if any of this happens.
Let’s get started!
Overcharging will happen for two primary reasons:
- Wrong Charger or a Charger on the Wrong Settings
- Your Car’s Alternator or Voltage Regulator is not Functioning Properly
Wrong Charger / Incorrect Charger Settings
Choosing the incorrect charger for the job or having the incorrect settings on a charger can lead to costly mistakes by damaging or destroying your battery and by possibly causing it to explode, leak or cause a fire.
First, we want to steer clear of any charger above 10 amps to be safe when charging a car battery. I prefer to stay at 6 or under, as it leads to a slower but more complete charge anyway and maximizes the amps the battery is able to absorb.
A car battery will generally have an amp-hour (AH) equivalency between 50 and 80. The rule of thumb is to keep your charging amps at 10% or less of the total AH capacity of a battery. The other rule of thumb is that it if you have a 10 amp charger, it will replace about 10AH in the battery per hour.
If you use a battery charger with a 50 amp rating, you’re essentially trying to fill the entire battery in one hour’s time! While a speed charge sounds nice for getting you on the road sooner, it places an enormous amount of strain on your battery.
When being charged, the battery is converting electrical energy into chemical energy but it can only do so at a certain rate, and that rate changes based on the increasing charge of the battery. The more charged it becomes, the slower it charges as it cannot convert as quickly.
Think of it like you went without water for nearly an entire day and were working outside. Then someone placed a half-gallon of water in front of you but said that if you wanted some you had to drink the entire half-gallon.
The first quart would go down smooth as you quenched your thirst but that second quart would be a struggle and you’d be taking sips as your stomach tried to make more room and as your body tried to rehydrate each cell of your body. That’s what it’s like for your battery when using a charger with too many amps.
If you couldn’t get the water down fast enough you’d throw up. Your car battery, on the other hand, has to do something with the excess electrical energy that it is receiving but that it cannot convert.
The excess current is passed through the electrolyte solution and begins to essentially boil the fluid in your battery which then releases the energy in the form of heat. This heat is contained within the battery and will start with the battery bubbling, then hissing as the steam escapes the vents, then feeling hot to the touch on the battery casing, then bulging, and finally cracking/exploding/causing a fire.
The rate at which this happens depends on the number of amps you’re pumping into the battery.
If you have a flooded battery (one with removable caps so that you can check the water levels) you’ll have a little more wiggle room with the occasional overcharge since you can always replace the water that evaporates from overcharging with distilled water.
Sealed batteries like AGM of GEL (they typically say, “Maintenance Free”) will vent off the overcharging electrolyte solution and it cannot be replaced. Do this enough times and you’ll ruin the battery.
You’ll also want to make sure your battery charger is safe for a sealed battery before you use it. Some chargers are specifically designed for flooded batteries and tend to be a little higher on the voltage during the charge. Sealed batteries require a charger with a lower voltage to avoid the overheating of the electrolyte.
If you’re looking for a charger that is perfect for both flooded and sealed batteries alike, you’ll want to check this one out at Amazon. I’ve used it for 6 years without fail and it never overcharges my batteries. Better yet, you can set it and forget it if you’re not planning on using your car battery over the winter or whatnot.
Regardless of what charger you go with, it is best to go with a modern smart charger (like the one I recommended above). These sense the internal resistance of your car battery and provide the correct voltage and amps to maximize the batteries ability to absorb the current.
An old-school charger simply had a setting or two from which you could choose and you had to be mindful to set a timer and not leave it hooked up too long. These chargers force current into a battery regardless of the batteries ability to absorb it.
Alternator or Voltage Regulator
This runs in the same vein as having an incorrect charger or one on the wrong settings.
After you turn your car’s key, the battery releases a short but high amperage to start up the engine and electronics of your vehicle. Immediately thereafter, the alternator is engaged and converts mechanical energy into electricity.
The voltage regulator is responsible for taking the electricity from your alternator and making it suitable for the battery to absorb.
If either of these malfunctions, your car battery could easily overheat from being overcharged or by failing to charge which will lead to the battery discharging at a high rate which will also lead to overheating.
2. Internal short
This occurs when there is a complete circuit occurring within the battery itself. Physical damage of some sort has happened and the battery is no longer able to store its energy but depletes it on itself by simply sitting there.
The self-discharge rate when an internal short occurs can take place rapidly or slowly and will produce heat proportionate to how quickly the short drains the battery. The worse the short, the quicker the heat will build.
A common reason for a “soft short” is when the lead on the plates within the battery naturally flake off over time by simply discharging and recharging.
These flakes can conduct electricity and will start to accumulate at the bottom of the battery. Because car batteries are starter batteries, the lead plates have lots of surface area which increases the amps they can put out and these plates extend down nearly to the bottom of the battery (unlike deep-cycle batteries which have less surface area and a “well” for these flakes to accumulate without hurting anything).
Eventually, these flakes will number enough that they will bridge the distances between the plates and the battery will conduct electricity within itself from one terminal to the other.
An internal short can be a very dangerous situation and gets even worse if you try to charge a battery with this condition. Newer chargers should sense the issue and cease the process. Older chargers, however, can simply keep pumping energy into the battery and this can lead to a fire or explosion.
3. A Large Draw Being Placed on the Car Battery
Just like a rapid charge can overhead a battery, a rapid discharge can do so as well.
A battery can only convert electrical energy into chemical energy at a certain rate efficiently, and the opposite is true as well.
If you demand too much of a draw from the car battery, it will try to release its chemical energy into electrical energy at a rate faster than it can efficiently do so. The end result is that heat is generated.
This could happen if your car’s alternator gives out and your car is forced to run off of the battery until the battery dies. Once that happens, the car will die until a new battery is introduced. The rapid drain though can overhead the battery.
Using your car battery as a “deep cycle” battery can also cause such a thing to happen if the load placed on it is high enough. Trying to run your refrigerator’s compressor with a car battery and inverter would be a great example of this.
It’s a bad idea, by the way.
4. Engine Heat
Engine heat, when combined with radiant heat from the ground below and high ambient temperatures can also cause a battery to feel hot to the touch.
If you’re in such an environment, it’s best to not let your car simply run stationary at idle. The car needs to be moving to keep air flowing through the engine compartment to cool things down.
5. Terminal Connections are Hot
A fifth and related reason to experience heat with your battery is that you might have hot connections at your terminals. This is likely due to a loose connection (or loose connections). Remember not to overtighten your connections, but they do have to be tight.
If you have a loose connection, every time the connection separates sightly from the terminal, it can arc with electricity as it tries to bridge the short gap and will heat up the area.
Also related to this is using wires with too small of a gauge for a particular task can also result in the wires overheating. If you’re trying to use your car battery to power something in a deep cycle fashion and the amp requirements are high but your wires are tiny, you’ll push the wires passed their capacity to transfer electricity and they can overheat/melt.
How To Know if My Battery is Damaged?
The first thing you’ll want to do is put on a pair of safety goggles if dealing with a battery that’s overheating.
Then, you’ll want to disconnect anything charging the battery or any draws being placed on the battery.
Then, check the voltage with a voltmeter and let it sit for a while to see if it cools.
Check the voltage again periodically to see what happens. If you start high (12.5 ro 13+ of just connected to a charger), and gradually work down to 11.7 or less, your battery probably has an internal short since it is now considered dead after doing nothing but sitting there.
If you have a sealed battery (AGM, GEL) you will need a new battery as nothing can be done by you to affect the internal conditions of the battery.
If you have a flooded battery (where you can check the water levels) go ahead and pop off the caps to check the water levels.
If any lead plates are exposed, you have a problem. Fill the cell with distilled water just enough to cover any exposed plates by 1/8 of an inch.
You can then proceed to try to recharge your battery (I would recommend this only if the battery is weak but wasn’t completely dead because of a likely internal short). After the battery is charged, fill the cells to just below the bottom of the fill tube which will have a v-notch in it.
I then put it on the charger again to top it off.
Once done, you’ll want to test the specific gravity of each of the cells using a hydrometer. Each cell on a 12-volt battery should be around 12.09 or 12.1 when 90%-100% charged.
If all of them are reading that but one is reading 1.75, then you know that cell is bad and you will need a new battery.
If you’re not dealing with a bad cell, an internal short, a batter that’s showing signs of bulging, or one that doesn’t overheat again after you take corrective measures, you’re just going to have to play it day by day.
You might have caught the problem early and may still get years of service out if the battery. Only time will tell.
Why is my Car Battery Hot and Smelly?
This is due to overcharging. It can be either accidental in that you’re using a charger with too many amps or have left an old-school (non-smart charger) hooked up for too long, or it can be purposeful through a controlled overcharge to “equalize” or “desulfate” the battery.
The smell is hydrogen sulfide which is released during overcharging. It is an irritant to the throat and sinuses and is poisonous at high enough concentrations.