How to charge a dead agm battery

At HOT ROD, we hear stuff. Lately we've been hearing about hot rodders' love/hate relationship with their sealed, leakproof batteries. You adore their ability to take punishment and their paint-friendly nature, but they have developed a reputation for refusing to take a charge once they have gone dead. Here's how to bring them back to life.

First, some background. While the spill-proof batteries currently on the market are often referred to as gel batteries, most are AGM batteries-short for absorbed glass mat. Optima and Odyssey batteries are common examples. Unlike a gel battery, in which a silica agent is added to the electrolyte to form a semisolid, an AGM battery uses an ordinary sulfuric acid solution like any standard automotive battery (about 60/40 water and acid at full charge). However, here the electrolyte is absorbed and retained by layers of boron-silicate glass matting between the lead plates. While both battery types can rightly be considered leakproof and spill-proof, AGM is currently considered the superior technology, especially for automotive use. Some AGM batteries employ spiral-wound, cylindrical cells (Optima), while others use flat plates and box cells (Odyssey) like conventional batteries. Also, sealed is something of a misnomer-lead-acid batteries generally are capable of venting when necessary.

Hot rodders are tough on batteries, but the extreme heat and vibration we deal them aren't the half of it. Between these rounds of serial abuse, often we let our cars sit unused for extended periods-like between race events or cruise nights, or even for the entire winter. Also, the electronic gadgets in newer cars (engine and body computers, entertainment and security systems, and so on) often employ keep-alive memory functions that place a small but constant current draw on the battery.

That's an awful thing for automotive batteries, because they all will self-discharge over time-even the more expensive deep-cycle models (just more slowly). At room temperature, a standard battery loses nearly 5 percent of its capacity per month just sitting on the shelf. And once the battery's voltage falls below around 1.75 volts per cell, permanent damage begins to take place in a process known as hard sulfation. Essentially, the plates become coated with gunk (dense lead sulfate crystals), reducing the battery's capacity. If a battery is allowed to discharge long enough or not properly recharged between discharges, it's a dead player. Its plates are completely sulfated and nothing will bring it back.

To replicate these diabolical conditions, we obtained two new Optima AGM batteries, a standard RedTop and a deep-cycle YellowTop model, and submitted each to a constant 2.2-ampere load. At regular intervals over many days, we removed the load and attempted to recharge the batteries using a standard, repair industry-style, 2/10/40-ampere, roll-around charger, known in the biz as a bulk charger. At no point did we have any trouble getting either battery to accept a charge-that is, until the around-the-clock discharge drove the batteries below around 4.2 volts. Then the batteries refused to charge, replicating the common complaint-"won't take a charge."

However, the trouble isn't really with the battery or even, necessarily, the charger. Here's the real problem: Once the battery's voltage falls below a certain level, the charger can no longer sense the battery's presence and will refuse to deliver the current. In some cases this can occur at any point below 10.5 volts. Older and more basic chargers are more prone to this hang-up than newer, fancier models, which are equipped with control circuitry to deal with these issues. So there are two solutions available: Buy a new, expensive charger equipped with the extra brainpower, or employ a simple trick the Optima engineers laid on us.

Here's all you need to do: Grab another car battery with a decent charge on it (12.4 volts or better) and connect it in parallel to the problem battery using a set of jumper cables. Then activate the charger and charge the battery normally, being careful to follow the manufacturer's instructions for current and voltage limits. It's as simple as that. The second battery supplies the voltage that tricks the charger into supplying the necessary current. After an hour or so, you can remove the second battery and continue charging. We tried it and it works. No matter how deep the discharge we applied, we were able to bring the battery back to a full state of charge using our cheap but trusty bulk charger.

You can now find chargers that are specifically marketed for AGM batteries. They're nice but by no means necessary. However, some recent chargers have a switch position on their control panels marked AGM/gel. Since AGM and gel batteries have rather different charging requirements, that position is somewhat bogus, according to several AGM battery manufacturers. They recommend selecting the conventional switch positions with these chargers.

Of course, you can avoid all these headaches with deep discharge and parasitic loads in the first place simply by using a float charger. Also known as battery tenders, these units deliver a small (500 to 800 milliamperes) but constant charge to the battery, keeping it fresh and fully charged. Just connect the float charger to a wall socket and to your battery when your car is not in use and you're good to go. We've seen basic versions of these chargers on sale for as little as 10 bucks-which is the deal of the century, especially when compared with the hundreds of bucks you'll pay to replace a dead AGM battery. As an added bonus, you'll know that whenever you want to drive your rod, the battery will be ready and waiting.