An ATV battery can get somewhat warm from normal charging, but should never get hot to touch or overheat. This applies both when riding or charging it with an external battery charger. A battery that heats up more than normal is a sure sign of an issue that needs immediate attention. 

The most common cause of an ATV battery that is getting hot is from overcharging or due to a defective battery. Overcharging may occur if the voltage regulator on the ATV or from chagrin the battery at too high of an amp setting with an external charger.

In this post we will have a closer look at all of these scenarios.

A bad regulator/rectifier can cause overcharging that will overheat and potentially damage the battery

The voltage regulator rectifier is a crucial part of the ATVs battery charging system that serves two key purposes.

Firstly, it converts the alternating current (AC) coming from the stator to direct current (DC) that can be utilized by the electrical components of the ATV. 

Next, it regulates the voltage to a stable output of about 14.5V. The input voltage from the stator fluctuates with the engine’s RPMs and can reach levels as high as 20V or more. 

Since the electrical components of an ATV, including the battery, are not designed to handle voltage levels higher than about 14.5V, we must regulate the input voltage to a stable operating voltage. 

When the regulator fails, it will distribute excessive voltage that can damage components and overcharge the battery. Overheating of the battery is a sign that it’s being charged with a too high voltage. 

You can use a multimeter to test if the regulator if it’s working correctly. Just follow these steps:

  • Set your multimeter to 20VDC.
  • The red lead goes to the positive battery terminal, while the black goes to the negative terminal.
  • With the engine off, you should get a reading of about 12.4 to 12.6V. This indicates a healthy battery
  • Start the ATV and read the voltage while running at an idle, then rev up to about 3000RPM
  • The voltage should increase to about 13-14.5 volts. Some regulators max out as high as 14.8V
  • If the voltage fluctuates or you get a reading outside this range, it indicates that your regulator is defective.

Another way of telling that the voltage regulator is bad is when the lights fluctuate in brightness when you rev up the ATV. At levels above 14,5V, the bulbs will glow much brighter than normal, until they eventually burn out completely. 

To further test your voltage regulator, you can use the multimeter to see if the diodes inside the regulator are working correctly. Partzilla.com has made an excellent video demonstrating this procedure:

Older bikes or some budget ATVs use a stand-alone regulator/rectifier while most modern ATVs now has a combined ECU module.

ECU voltage regulator ATV
This is how the ECU module on most Polaris ATVs looks like.

This module controls the charging of the battery, but it also controls fan operation, controls the starter, has the main circuit breakers and battery protection, controls the hot engine light amongst a range of other things. 

The downside with combining many features in one unit is that it’s the regulator portion that most commonly breaks. Instead of having to replace just the regulator, you now have to replace the whole ECU unit at about three times the cost of a regular rectifier regulator unit. 

Click here to learn more about how the charging system on an ATV works.

A bad cell will cause the battery to overheat when charging

If your regulator/rectifier checks out fine, the problem is likely with the battery itself.

A battery is divided into cells, where each cell holds a specific portion of the battery’s overall voltage. When one cell goes bad, it will negatively affect the combined voltage of the battery.

A 12V battery is typically 2.2V x 6 cells, which equals 12.6V nominal voltage. They are designed to be charged with a charge voltage of about 14.5V. If one cell shorts, often from internal sulfation, you are left with only five working cells. 2.1V x 5 cells equal only 10,5V nominal voltage. 

The ATVs charging system or external charger will see it as low, forcing the five working cells to accept the total charge. This will cause the battery to overcharge and heat up.

If you suspect that your battery is bad, you test it yourself by performing a so-called load-test (you will find instructions on how to do it in this post) or you can take it to your dealer, and they will test it for you.

Charging a battery with too high current or too long causes overcharging

If your battery is not getting hot from regular riding, but when charging it with an external battery charger, you are likely either:

  • Charging it using too high of charge current
  • Charging it too long using a manual charger
  • Charging it in the wrong charge mode

Make sure you use the correct amp-setting

ATV batteries are relatively small and subsequently has to be charged using a relatively small charge current (amps, A). 

A good rule of thumb to know the right amperage to charge your battery is to divide its Ah- rating by ten. A 15Ah battery should use a charger with a power output of no more than 1,5A. Most major ATV battery brands also provide charging data printed on the battery casing. 

This illustrated step-by-step guide will tell you how to charge any ATV or UTV battery.

You should never charge an ATV battery with more than 3A. If you do, it will overcharge and therefore get hot. 

Do not connect the battery to a manual charger and leave it unattended

A manual charger will continue sending amps into the battery, even after it is fully charged and can no longer absorb any more current. This will cause the electrolyte fluid inside the battery to “boil” until it evaporates completely. 

When this happens, you may notice a funny odor from the evaporated electrolyte. Some say it smells like rotten eggs. This is a sign that you are overcharging the battery.

Caution! The gasses created from overcharging a battery is highly explosive. If this happens, you should unplug the charger and leave it connected to the battery so that you don’t risk creating any sparks. Let it vent for at least 30 minutes before removing the positive and negative charge leads. 

Automatic chargers, on the other hand, know when the battery is getting near a full charge. They will gradually decrease the amperage until it stops completely when the battery is full. 

After the battery is fully charged, the charger automatically switches over to a maintenance mode. It will only add a small charge when necessary to keep the battery topped off. This is great when the ATV is sitting for an extended period without being ridden. 

Also, note that a typical automotive trickle charger can be too much to handle for an ATV battery. 

Using the wrong charge mode can also cause overcharging

Different types of batteries (lead-acid, AGM, GEL, and so on) require a slightly different charging current. GEL batteries, for example, use a higher current than you would use on a typical flooded lead-acid battery. 

So if you charge a conventional battery using the GEL mode on your charger, it will charge it at a higher voltage than the wet lead acid battery is designed to handle. This may cause it to overcharge and heat up.

A lead-acid battery may be running low on electrolyte

A lead-acid battery needs to have a minimum electrolyte level, or it may get warm or even hot from charging.

On conventional wet lead-acid batteries, you need to make sure the electrolyte level is not below the minimum mark or expose the electrolyte plates inside the battery.

Some ATVs tend to be harder on the battery than others. 

If you search the forums, you will learn that Honda ATV is notorious for boiling batteries, leaving them low on electrolytes. Some find it takes only 2-3 years before their battery is dry. 

It’s a good precaution to have a bottle of distilled water at hand and inspect the electrolyte level regularly. Top off any cells that are getting low.

Is a hot battery dangerous?

Do not ignore signs of a battery that is getting too hot. Continuing running the battery hot will more than likely cause permanent damage to the battery or other components of the ATV.

The gasses created from overcharging a battery is highly explosive. A small spark can lead to a potentially fatal explosion. So if your battery is getting hot, bkeep this in mind and be careful not to create any sparks.