Can a Solar Charge Controller Overcharge the Battery?(How to Check Battery is Fully Charged)

Most times Solar Charge controllers are ‘connect and forget”. Once installed, you probably only need to access the battery status information.

For the most time, they work reliably protecting your battery from failure by protecting it from being overcharged and over-discharged.

So, what if you find your solar battery overheating?

Or, you measure the battery voltage and you find it unusually high?

Can it be that the solar charge controller is overcharging the battery?

Well, read on to learn if a solar charge controller can overcharge a battery, the signs that show that your battery is being overcharged and what you can do to stop and prevent your battery from being overcharged.

Is it Possible for a Solar Charge Controller to Overcharge a Battery?

Yes, it is possible for a charge controller to overcharge a battery.

This can happen for example, if it is not able to read the battery system voltage correctly.

There are reports of some charge controller settings changing during use and charging a 12V battery system with voltages meant for a 24V battery system – overcharging the battery in the process.

Can an MPPT controller Overcharge a Battery?

Yes, it is possible for an MPPT solar charge controller to overcharge a battery if the controller is defective or if fails to read the battery voltage correctly.

Some versions of the Victron 75/15 MPPT controller need to undergo a manual reset process to detect the correct battery voltage.

If you do not follow the recommended reset process, the controller does not update to read the current battery bank voltage resulting in the battery being overcharged.

Common Causes Why a Solar Charge Controller Can Overcharge Battery?

The Common reasons why a solar charge controller may overcharge your battery are:

#1. If the solar charge controller is not set up correctly. Solar charge controllers have configurable charge settings depending on the type of battery – AGM, GEL, or Lithium being used in the system.

For example, AGM batteries should be charged to about 14.5V compared to 13.6 ~ 13.8V for GEL batteries.

Using the AGM battery voltage settings on a GEL battery can result in overheating of the batteries.

#2. The Solar charge controller fails to read the correct battery bank voltage because the manufacturer recommended sequence for connected batteries to a charge controller is not followed.

If the charge controller is not able to read the battery system voltage correctly, as an example the charge voltage settings for a 24V battery system are used on a 12V battery leading to overcharging.

#3. The solar charge controller is defective and fails to regulate the charging process and overcharges the batteries.

Signs Charge Controller is Overcharging Battery?

Here are some common signs that can point to your battery being overcharged by a solar charge controller:

#1. Charge controller status is continuously in “Bulk Charge” mode yet batteries are fully charged.

#2. High battery temperatures. Batteries should be cool or warm to touch during the charging process.

If on the other hand, the batteries are extremely hot or boiling then there are being overcharged.

Panels Drain At Night
Panels Drain At Night

You need to stop the charging process immediately – you can disconnect solar power going into the charge controller.

#3. You measure the battery voltage and it is higher than the recommended voltage for that battery type.

For example, a 12V battery is charged to voltages in excess of 15V.

How to Tell that your Solar Battery is Fully Charged

You can usually tell that a solar battery is fully charged by monitoring the charge controller status.

Solar Charge controller status showing float mode signals that the battery is fully charged.

If the Solar charge controller is in bulk or absorption mode then charging is still going on.

Measuring the battery voltage can also provide an indication of the charge status of the battery though is not completely accurate.

This is because there are often elaborate algorithms used in charge controllers to ensure that a battery is fully charged which may make taking a one-off battery voltage reading as an indication of the status of charge misleading.

But as a guide, AGM batteries at 14.5V are close to or fully charged while GEL batteries at 13.6 ~ 13.8V are close to or fully charged.

Should you Leave the Solar Charge Controller On all the Time?

Yes, if it is a good quality charge controller. It needs to be installed and set up correctly to read the correct battery voltage and charge the correct battery type with temperature compensation (if required) then you should be able to leave it connected without danger of overcharging the battery.

How to Stop a Solar Charge Controller from Overcharging the Battery

If you suspect that the charge controller is overcharging the battery, then as a first step, you need to isolate the solar panels immediately to stop the charging process.

You can switch off the solar panel power using the double pole PV isolator (if installed) then allow the batteries to cool down.

  • Check that the charge controller is set to charge the battery type in your solar battery bank and adjust accordingly if not.
  • Confirm that the charge controller is set to correctly read the battery bank voltage – whether 12V or 24V.

Check the solar charge controller manual on how to do this as it can vary from one controller to another.

  • Is the temperature probe properly installed? Confirm that the correct type of temperature probe is installed and properly installed as per the manufacturer recommendations so that it is able make the necessary adjustments to the battery charging voltage.
  • You may need to consult the solar charge controller manufacturer for additional support.

If the charge controller is confirmed as defective then replace it accordingly.

Final Word

A charge controller can overcharge your battery especially if the wrong battery type is selected or it does not read the battery voltage correctly or if it is faulty.

Should your battery overheat during the charging process a sign of overcharging, then check that the charge controller is set to charge the correct battery type, that it is reading the correct battery bank voltage, and that the temperature probe is properly installed.

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