As part of its HOT ROD torture test, this Optima AGM battery was deliberately dealt a long, slow, 2.2-amp discharge until it showed less than 4 volts across the terminals. At reduced voltage levels, many conventional battery chargers will refuse to deliver an effective charge—producing the frequent complaint that an AGM battery “won’t take a charge.” In reality, the battery simply has insufficient potential for the charger to “see” it in the circuit.
What Voltage Do I Need To Recharge an AGM Battery?
Here’s the magic fix: When an AGM battery won’t charge by ordinary means, simply connect a second, well-charged battery (12.4 volts or greater) in parallel with the dead unit—positive to positive, negative to negative. Then connect the charger to the pair. This will in effect trick the charger into delivering the necessary current to the discharged battery. Using this method, we brought several deep-discharged batteries back to life. By the way, this trick also works with conventional batteries, though it’s not quite as effective. For maximum battery life, Optima advises that normal charging should be limited to 10 amperes. Other manufacturers advise similar limits. So-called sealed batteries can be especially sensitive, but in no case should any automotive battery be allowed to heat significantly while charging. Excessive charging current and temperatures will age a battery before its time and can even be dangerous; when you overcharge a battery, essentially you are manufacturing highly explosive hydrogen.
Some AGM batteries, like the Optimas shown here, use spiral-wound plates in cylindrical cells—hence the six-pack appearance. Among other things, spiral-wound cells allow purer lead to be used in the plates (it is claimed) since they need not support their own mass. Other AGM makers say conventional construction is superior, claiming greater plate area for a given case volume. In the Optima line, the RedTop is the performance piece, while the YellowTop is a dual-purpose battery for both performance and deep-cycle use. (A race car with no charging system would be an ideal application.) A dark-gray case denotes a conventional Optima battery, while light gray indicates a deep-cycle model. Deep-cycle batteries deliver lower peak current but can withstand deeper discharges. A BlueTop Optima is for marine applications. It will start your car but is not optimized for automotive use.
Picking the Right Battery Terminals To Use
For maximum model coverage, many aftermarket batteries are equipped with top and side terminals, and there is no reason you can’t use them both. In fact, battery manufacturers recommend that you connect the vehicle’s main cables to the side terminals in high-load applications, then use the top posts for winches, inverters, and other high-current add-ons. Don’t throw away the plastic terminal caps—save them in case you want to store the battery later. When bench-testing and charging a GM-style battery, don’t just run a bolt into the delicate side terminals. There are only a few threads inside and they can easily be stripped or burned due to arcing, or the terminal lug can be loosened in the case when the bolt bottoms out. Instead, always use a washer and jam nut on the bolt for a tight, secure connection. Also make sure to keep the terminals clean—corroded battery terminals produce more unnecessary battery replacements (and a zillion other electrical problems) than any other single cause.