Maintaining your deep cycle batteries is essential to ensure a maximum life span and to prevent damage. As such, it is important to detect battery problems at an early stage. Troubleshooting deep cycle battery issues is fairly easy to do yourself with the aid of a multimeter, volt meter, or watt meter.
The outside of deep cycle batteries can show early signs of failure. As such, troubleshooting battery problems can begin with a simple inspection. Ensure that the top of your battery is clean and dry. When a battery is covered in dust and dirt, it can discharge across the grime. Also, inspect the battery for broken or loose terminals; they are dangerous as they may result in short circuits. Flooded deep cycle batteries will need to be checked for leaking and damaged battery cases that may have been caused through overcharging or overheating. This problem won’t occur with AGM deep cycle batteries as they are designed with glass mat technology preventing leakage, even when damaged. Generally, cracks and holes will not prevent deep cycle batteries from operating, but they can be unsafe. As such, it is recommended to discard any batteries that have reasonably damaged battery cases.
It is suggested to test your battery’s life when it is fully charged. If you find yourself in a situation where you cannot charge the battery, let it sit for approximately one hour before testing. As a result of charging or discharging, an uneven mixture of acid and water can arise on the surface of the plates. This phenomenon is referred to as a ‘surface charge’, and will need to be removed before the testing begins as it may influence your data. Surface charge can make a bad battery look good and vice versa. To remove the surface charge you can simply leave your fully charged deep cycle battery to sit for at least four hours. Also, ensure your battery is not connected to any appliances or a battery charger as this will influence the data.
Note: if you have multiple batteries connected together, each battery should be disconnected and charged/tested separately.
You can test your deep cycle battery’s charge level in several ways. The most common methods use a multimeter, voltmeter, or watt meter. When you decide to test with a voltmeter, we recommend using a digital meter rather than an analogue meter as it will be more accurate in measuring millivolt differences. For a detailed guide on how to test the voltage of your deep cycle battery, you can have a look at our video ‘How to Check Your Battery Charge Level and Troubleshoot Issues’.
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Once you tested your deep cycle battery’s voltage, you can analyse its state of charge. Simply compare the measured voltage with a state of charge table to discover your battery’s estimated charge level. For example, if your AGM deep cycle battery rates at 12.30V, it's at a 70% state of charge as shown on our State of Charge graphic. This charge graphic relates to 12V AGM deep cycle batteries, but can also be used as a general guide for other battery types though keep in mind that there may be slight differences in the voltage rating.
Typically, a fully charged deep cycle battery will have a voltage of over 12.8V - 13V. Below are a few common battery problems you can identify by the voltage measurements.
If a fully charged AGM battery tests more than 20 percent lower than the fully charged voltage level, it’s probably due for replacement. This is typically a symptom of battery age, damage from over/under charging, or sulfation. You can sometimes improve a battery in this condition by using a desulfation device, otherwise you’ll have to live with the low capacity or replace the battery.
If your battery shows a good voltage when it’s fully charged, but quickly drops voltage to 11V or less when using power, this usually means the battery has a faulty cell and needs replacement. This can be caused by excessive vibration such as driving over corrugations without adequate shock absorption for the battery, or a manufacturing fault. Faulty cells are difficult and usually impractical to repair, so you’ll need to replace the battery.
When a deep cycle battery is fully discharged (dead flat), it should reveal a reading of approximately 10.5V. If your test shows that this voltage is below 10V, this typically means that the battery has been left in storage for too long without a charge or left with a load running on it that doesn’t have an automatic cut-off. Once a battery is below 10V, it is difficult to bring it back up as most battery chargers won’t recognise it as a battery due to the very low voltage. You can sometimes bring a battery back up by using an old bulk battery charger with no smarts (one that you just switch on, and it starts powering away with no stages or battery detection), but this usually depends on how long the battery has left at this low voltage.
Although you can easily troubleshoot any different battery issue yourself, accurately testing a deep cycle battery’s capacity requires a ‘deep discharge’ test. This kind of test can only be done with specialist discharge testing machines that can be found at reputable battery stores.
When troubleshooting a battery bank set up with a series or parallel circuit and one of the batteries is faulty, it will pull all the other batteries down. For example, if one of your batteries has dropped down to 8V, you may notice that the other batteries within this bank will have low voltage ratings as well. To troubleshoot this, you’ll need to disconnect and perform a full test on each battery individually. The battery with the lowest rating will be the faulty battery that needs to be replaced.
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