How to Troubleshoot Your Deep Cycle Battery

Power - by Bryan Veldboom - updated on 7/7/2023

A man in a red coat sitting out side a camper van

Deep cycle batteries are used to power a variety of applications including wheelchairs, golf carts and floor scrubbers, and serving as "house" batteries on RVs and boats. Today, we'll be taking an in-depth look at how to troubleshoot and maintain a deep cycle battery so that you can get the longest life possible.

What is a Deep Cycle Battery?

While deep cycle batteries are used in a number of vehicles, they function much differently than SLI batteries like those found in cars or trucks.

SLI Batteries

Work by providing a short burst of power necessary to start an engine (usually less than a minute or two).
After the engine has started, an SLI battery's job is essentially done.
SLI batteries should only be discharged up to 50% of their total overall capacity.
SLI batteries are recharged by a vehicle's alternator.

Deep Cycle Batteries

Are designed to provide a constant flow of power for a sustained period of time (usually hours).
Deep cycle batteries are designed to withstand deep discharges (up to 80% for some batteries) of their overall capacity.
Deep cycle batteries must be recharged using a battery charger.

Why Do Deep Cycle Batteries Go Bad?

Deep cycle batteries have an average lifespan of around 150 to 250 cycles. When a deep cycle battery reaches the end of its life, its performance will begin to drop. The performance of a deep cycle battery can also be negatively impacted if:

The battery has become sulfated due to undercharging.
The battery has been damaged due to overcharging.
The battery's plates have become corroded because the electrolyte has not been adequately replenished with distilled water (this is for flooded batteries only).

How Can You Tell If a Deep Cycle Battery is Bad?

Some signs that your deep cycle battery is dying include:

The battery suffers from shorter run times.
The battery has difficulty holding a charge.
The battery has been used for more than 250 cycles.
If you notice cracks, bulges or swelling on the battery's casing, the battery is no longer safe to use and must be discarded.
Broken or damaged terminals are also a sign that the battery must be replaced.

Testing Deep Cycle Battery Voltage

If your battery looks to be in good shape, but is suffering from a drop in performance, the next step is to test the battery. You can do this by using a multimeter to test the battery's voltage, or by performing a load test (details further below). Here are the steps for using a voltmeter.

Always wear protective eye, face and hand protection when charging or testing a battery.
Fully charge the battery and then let it sit for roughly four hours disconnected from the charger before performing a test.
Disconnect the battery from all applications and battery chargers.
Turn on the multimeter.
Set the multimeter on the voltage setting.
Connect the multimeter leads to the battery's terminals, with the red lead going to the positive terminal and the black lead going to the negative terminal.
Take the voltage.

What Should a Deep Cycle Battery Read On a Multimeter?

A fully charged 12-volt deep cycle battery should have a voltage of 12.7 volts or higher. If your battery tests at around 12 volts or lower, your battery will probably need to be replaced.

How Do You Test a Battery with a Hydrometer?

Using a multimeter is a fast and easy way to test a battery's voltage, but there are other, more accurate testing methods. If you have a flooded deep cycle battery, you can use a hydrometer. A hydrometer is a tool resembling a syringe with a glass float inside, which is used to test the specific gravity (its weight compared to water) of your battery's electrolyte solution. The experts at your neighborhood Batteries Plus store can perform a hydrometer test for you, or you can pick one up yourself and follow these steps:

Perform the test in a well-ventilated area.
Always wear protective eye, face and hand protection when testing a battery.
A hydrometer should only be used on a flooded (wet) battery. Do not attempt to use a hydrometer on an AGM or other fully sealed battery.
Make sure that the battery is fully charged before testing it.
Do not take a hydrometer reading right after you have added water to the battery. The water must have enough time to thoroughly mix with the electrolyte in order to provide a true reading.
Remove the caps to reveal the battery's cells.
Place the hydrometer's nozzle into the battery's electrolyte and draw some of the liquid into the hydrometer's bulb. Draw the minimum quantity of electrolyte into the hydrometer to permit the float to float freely without contacting the top or bottom of the cylinder.
When reading the hydrometer, be sure you are holding it vertically so that the float is not rubbing against the side of it.
Test each cell and note the readings (corrected to 80°F or 27°C). A variation of fifty points between any two cell readings (example 1.250 and 1.200) indicates a problem with the low reading cell(s).
As a battery ages the specific gravity of the electrolyte will decrease at full charge. A fully charged battery will normally test at 1.275 - 1.280 while a discharged battery will read in the 1.140 range. This is not a reason to replace the battery providing all cells are within fifty points of each other.

How Do You Load Test a Deep Cycle AGM Battery?

In an AGM battery, the electrolyte is absorbed and held in a special glass mat which makes it impossible to perform a hydrometer test. If you're looking for a more accurate method of testing an AGM deep cycle battery, you'll want to perform a load capacity test. Performing a load test requires special equipment which most people aren't likely to have.

Fortunately, there's an easier way. Simply take the battery or batteries to Batteries Plus and ask them to perform a capacity test for you. This will let you know for sure if you need to replace them. Be aware that performing a load test may take around 20 hours to complete, so be prepared to leave your battery or batteries overnight. If they cannot perform a capacity test, they will perform a crank load test. This will also provide you with an accurate assessment of your battery's condition.

How Do I Maintain a Deep Cycle Battery?

Here are some basic tips you can follow to help get the longest life possible out of your deep cycle battery.

New deep cycle batteries do not perform at full capacity at first. Be sure to discharge a new battery with a short runtime for the first few cycles and then gradually increase its depth of discharge.
Even though they are built to withstand deep discharges, you should not fully discharge a deep cycle battery if it can be avoided.
A deep cycle battery should be fully charged every day that it's used, even if it is only discharged by 25% of its overall capacity
Do not leave a deep cycle battery partially charged; always recharge up to 100%.
Do not overcharge a deep cycle battery as this will affect the battery's capacity and shorten its lifespan. An automatic battery charger or battery maintainer will help prevent both under and overcharging your battery.
If you have a flooded deep cycle battery, check the electrolyte level regularly and add distilled water any time the plates are visible.
Also, be sure not to add too much water to the electrolyte. The liquid level should sit just above the battery plates.

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