You’d think that once a charge controller is installed and wired correctly you’re set.
Your solar system battery will then be maintained in the best condition possible without any danger of overcharging.
After all, that is what the charge controller is touted for that is counted upon to protect the battery is a connect-and-forget appliance.
Once installed, you’re set and your battery is safe against being overcharged.
Well for the most part with a few settings applied, the above expectation is realistic.
Unfortunately, your batteries may be overcharged damaging your batteries even with the charge controller in place.
What causes this and how can you best guard against your batteries being overcharged even with a charge controller connected?
Well, read on to find out why your batteries may still be overcharged and what you can do to stop and prevent it.
Can a Solar Charge Controller Overcharge a Battery?
Yes, unfortunately, it is possible for off-grid solar batteries to overcharge even with a charge controller (that is meant to protect them against it) connected.
If not discovered in time, your batteries can be damaged irreparably. There are several reasons why this can happen.
Signs a Charge Controller is Overcharging
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 the touch during the charging process.
If on the other hand, the batteries are extremely hot or boiling then there are being overcharged.
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.
Why a Solar Charge Controller May Overcharge the Battery (+What to Do)
If a charge controller is installed in our off-grid solar system and the battery is being overcharged, check the following:
1. Incorrect battery settings may have been configured into the charge controller. You need to select the correct battery type (AGM/GEL/LiFEPO4) on the charge controller for it to set the most suitable voltage for say float charging.
If not set right then you risk damaging the battery.
Consult your battery’s owner manual and confirm that the correct battery type has been selected on the charge controller.
2. The charge controller may have failed to determine the battery bank voltage correctly. Several charge controllers are able to automatically determine that it is a 12V or 24 battery bank that they are connected to.
If a charge controller has auto voltage recognition, then you need to follow the recommended manufacturer connection procedure for the charge controller to correctly detect the correct voltage.
Consult your charge controller owner manual on how to correctly set the battery bank voltage.
3. Battery may be faulty. If a battery is damaged, for example, has an internal short, the charge controller may fail to switch from bulk charging to float charging and continuously pump the high bulk charging current into the battery, boiling it and causing bulking of the sides and damage to the battery!
A battery with a voltage under 11V or so may have an internal short. Conduct a load test to confirm that it is in good condition.
Does the battery show any signs of damage? Perhaps cracked sides, bulging sides. It is advisable to continue use as you probably have a battery on its way out.
Do the batteries get extremely hot during the charging process? If so then you might have a faulty battery too.
Arrange to have the battery tested at a reputable AutoZone.
4. The Charge controller may be faulty. The charge controller can fail too allowing the high solar panel current into the batteries regardless of the battery voltage settings selected.
It helps to have independent voltage and current metering (besides the charge controller) allowing you to compare the expected voltage and current at that state of the charging process to catch this one.
For example, a 100-watt solar panel charging a partially charged 50Ah AGM battery through a charge controller should be able to fully charge the battery in about 5 hours of full sunshine.
At the end of the charging process, you should expect the charging current to reduce to a trickle.
Is your current meter still reading 4-5 amps instead? This is a red flag- that there’s an issue.
This though requires that you have separate meters in addition to the charge controller display which I’d recommend.
Some charge controller voltage and current settings are inaccurate and misleading!
5. The charge controller temperature probe may not be installed (correctly). The suitable battery voltage needs to be adjusted depending on the battery temperature to avoid overcharging or undercharging the battery.
To achieve this, the temperature probe should be installed and properly installed as per the manufacturer’s recommendations.
So, check that the charge controller temperature probe is correctly set and installed.
When Is Solar Battery Fully Charged?
A fully charged AGM battery that’s been disconnected from the charge controller and has rested for about 30 min or so for the surface charge to dissipate will measure a battery voltage of about 12.7-12.8V.
When still connected to the charge controller and fully charged, the battery voltage may read as about 13.2V.
You can use this as a rough guide to tell if a battery is charged.
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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