A gel-cell battery is different than most traditional batteries in that it is sealed and does not utilise liquid electrolytes. Rather, silica is added to the electrolytes, causing it thicken into a gel that is suspended within the battery. Because of this, gel batteries are spill-proof and more resistant to corrosion. Gel batteries, however, are generally more expensive than traditional batteries and are not able to be refilled. This also means gel batteries cannot be tested with a hydrometer. The simplest and quickest way to test your gel battery is with a digital voltmeter.
Access the gel-cell battery by removing the terminal covers. Then disconnect the battery from the vehicle. Detach the gel battery's negative terminal first, followed by its positive terminal. For most vehicles, you will need assorted box wrenches in order to completely disconnect the battery.
Attach the voltmeter's tester leads/clamps to the gel battery. The negative tester lead must connect to the battery's negative post and the positive tester lead to the positive post.
Turn on the voltmeter and look at the reading shown. If your gel battery displays a charge within the 12.85 to 12.95 range, it is 100 per cent charged. A voltage of 12.65 means it is only 75 per cent charged, while a 12.35 reading correlates to a 50 per cent charge.
Recharge the gel battery if you receive a low voltage reading. Be sure that you use a voltage-limited battery charger, also known as a float charger, for this task. A traditional charger should not be used on gel batteries, as it may overcharge and damage the battery. A voltage-limited charger will charge at a set rate and turn off when a full charge is achieved.
Permit the gel battery to sit for 24 hours after charging. This will allow the surface charge to dissipate and provide a more accurate reading. Retest the gel battery with a voltmeter after the 24-hour period has passed. If your gel-cell battery shows a low charge once again, chances are it will need replaced.