Battery charging, like tire busting, is one of those automotive service tasks that are so simple it’s often relegated to the apprentice or junior tech in the shop. Charging automotive lead-acid batteries can be ‘plug-in-and-go” simple, but maintaining a healthy state of charge and solid cold-cranking ability requires a more sophisticated approach.
They’re called “batteries” for a reason. Each cell, when fully charged, carries a potential of about 2.2 volts, which is why six cells are connected in series to make a “12-volt” automotive battery.
Like cheap Christmas lights or any other series circuit for that matter, one bad element or cell will cripple the whole assembly. “Five out of six” isn’t bad; it’s terrible at engine cranking time. Ideally, each cell ages like the others, allowing the battery to degrade gradually for maximum service life. Unlike large industrial batteries, however, we can’t replace cells individually, so driving current through the terminals is the primary tool technicians can use to maintain optimum battery health in today’s sealed units.
Charging a battery, whether by shop charger or the vehicle’s alternator, drives a chemical reaction inside the battery which is the opposite of the discharge reaction:
While this is a simplified version of the discharge chemistry, the key is what the products of the reaction represent. The water dilutes the sulfuric acid, which is why a battery hydrometer is a great way to measure state of charge in batteries with removable cell caps. A weaker acid concentration (lower numerically) represents a lower charge, as well as a freezing risk in sub-zero weather as the dilute acid concentration behaves like a weak antifreeze solution. The other product of the discharge reaction, however, has larger consequences for battery performance; lead sulfate.
Lead sulfate is the compound responsible for the well-known killer of lead-acid batteries: “sulfation”. While the lead sulfate is a natural component of the discharge process, it has a harmful effect on battery chemistry. The negative electrode in each cell, in the form of a lead grid to maximize its surface area, is a natural site for stray sulfite ions in the acid solution to attach. Lead sulfate forms on this grid surface, eventually coating it. The coating hurts battery performance in several ways. One is by blocking the lead surface, which prevents the acid (with its charge-carrying ions) from completing the charging reaction. The sulfite also clogs the pores of the grid, reducing its effective surface area. And a heavily sulfated battery, if subjected to a heavy, high current charge, generates excessive heat, which can warp the grids, boil away the electrolyte, or in extreme cases split the case or separators. The white powder seen around the terminals of an old battery is lead sulfate.
At that point, the battery is generally scrap. Charging prevents sulfation from taking the upper hand by reversing the discharge reaction:
This is just the discharge reaction operating in reverse, except that neither reaction goes to completion in either direction, so that water, sulfuric acid, lead sulfate, lead and lead peroxide are all present in each cell. Storing the battery on the shelf, or in the car in a discharged state for an extended time causes sulfation, a major cause of poor cranking performance, especially in the cold weather season when a strong reserve capacity is essential for rapid cranking. But can sulfation be reversed?
Reversing sulfation has been widely studied by engineers, and some charger manufacturers claim limited desulfating capability. One solution, commonly engineered into commercial lead acid chargers for fork lift and standby power batteries is the “equalization charge”. Equalization is just charging, only more so. Driving cells at a higher potential, basically a controlled overcharging, drives the charging reaction equation above, which converts lead sulfate to sulfuric acid. The result is clean plates. Unfortunately, overcharging also generates potentially damaging heat. Industrial lead-acid battery chargers are connected daily and many use microprocessors that equalize batteries after a preset number of conventional charges to keep batteries desulfated. Vehicles use alternators, so any rejuvenating charge is strictly a service procedure.
There are two other options. One is to use an additive, typically EDTA, a metal ion deactivator and preservative found in everything from pharmaceuticals to shampoo. EDTA dissolves the lead sulfate, cleaning the plates chemically. Disadvantages are several. The resulting sludge falls to the bottom of the battery, where it may short the plates and the complexed sulfate ions aren’t available to the solution, weakening the acid solution. The final problem is simplest: getting at the cells in modern sealed ‘maintenance free’ batteries.
Another desulfating approach is more interesting: high-frequency waves. Most technicians understand how ultrasonic waves can clean deposits from fuel injectors, but operating at much higher frequencies, radio waves at approximatley 2 to 6 megahertz, break up the sulfate crystals and promote their dissolution back into the acid. Commercial and several interesting homebrew devices are available, but claims vary from total rejuvenation to a marginal salvage of a scrap unit.
Whether desulfating a weak battery makes sense depends on the service circumstance as much as the technical obstacles. If the alternator or starter has been replaced, installing a new replacement battery is almost always justified. Chargers capable of equalization are expensive compared to conventional units, but “rejuvenation”, might be a marketable add-on service to customers with mid-life batteries, especially in vehicles with lots of accessory current draw under cold, stop and go driving conditions.
BATTERY CHARGING TIPS
Safety First: Eye protection, gloves and an apron or shop coat are a good idea. Think about the hydrogen gas issue as well as possible ignition sources.
Disconnect and clean terminals before charging. It’s surprising how many amps are blocked by terminal resistance.
Avoid quick charging. Slower is better, at currents preferably less than 20 percent of the amp-hour capacity of the battery.
When bench charging, keep the unit upright, and think about the vents. Where will the hydrogen gas or bubbled acid go?
Clean the case. Dirt absorbs moisture and can provide a discharge path, as well as concealing replacement signs like cracks, post pullout or bulges.