Also known as a maintenance-free battery, a sealed battery works in the same basic manner as a traditional wet cell, or unsealed, battery. Like an unsealed battery, a sealed car battery still contains electrolytes. However, you cannot refill a sealed battery with fluids to replenish the electrolyte level. Because it's sealed, you are also not able to test this type of battery with a hydrometer. Rather, a digital voltmeter, which can be purchased for as little as £13, will be needed.
Remove any type of terminal covers on the sealed battery. You must be able to access both the negative and positive terminals. Also check your voltmeter to ensure it is set to the DC power scale and 12 volts.
Attach the digital voltmeter's leads to the sealed battery. The positive lead must be securely connected to the positive terminal and the negative lead securely affixed to the negative terminal.
Observe the voltage readout displayed on the digital voltmeter. If your sealed battery is fully charged, the readout should be approximately 12.8 or 12.9 volts. A reading that is extremely low, such as 10.0 to 11.0, may indicate a shorted cell. In this case, a new battery is likely needed. A reading in the 11.0 to 12.0 range means the battery is low and needs recharging.
Recharge the sealed battery if necessary. Use a 12-volt battery charger, set on a low amp-rate, to slowly recharge the battery. Depending on the severity of the battery's discharge, the time to fully charge may be anywhere from a few hours to a full day. Check the battery charger often to ensure the battery is charging correctly.
Turn on the vehicle's headlights for about five minutes after the battery has been fully charged. Then turn the lights off and wait about 10 minutes. This will remove the surface charge, which could interfere with the accuracy of the test. If the sealed battery has been removed from the vehicle, allow the battery to rest for anywhere between two and eight hours to remove the surface charge.
Retest the sealed battery by connecting your digital voltmeter in the same manner as before. If the reading is low once again, the sealed battery is not capable of holding a charge and needs replacing.
The vehicle cannot be running during the test. Also, all electrical accessories or features must be switched off. If a switch is left on, it will drain power from the battery and skew the reading on the voltmeter. If you notice the terminals are dirty or corroded before you begin the test, scrub them with a mixture of water and baking soda. Then rinse them with clean water. If the sealed battery is more than five years old and is repeatedly losing its charge, a replacement battery is likely necessary.
Due to the release of hydrogen gas through any possible crack in the battery case, explosions can occur. Keep the battery away from any flame or intense heat. Wear protective eyewear as a safeguard.