How to Troubleshoot Your Deep Cycle Battery

Power - by Bryan Veldboom - updated on 6/29/2023

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.

What is Load Testing?

Load testing is the most accurate test you can perform and is intended to gauge how well a deep cycle battery holds its charge. Performing a load test requires special equipment, which most people are unlikely to have.

Fortunately, Batteries Plus can help. Take your deep cycle battery to your nearest store and our specialists will perform a true discharge test. Keep in mind that such testing takes roughly 20 hours to complete, so you'll have to leave your battery overnight.

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 short runtimes 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.