Batteries are simple. Put electricity in, take it out, put it back in again and there you have it, right? It may seem that automotive starting batteries are simple electricity tanks, but inside they’re a miniature chemical plant that does some very interesting things to get the engine going in the morning. Understanding batteries isn’t difficult, and it’s useful knowledge when choosing or helping a customer to choose from features and benefits among multiple brands.
If you’re a qualified automotive technician, then you have all the knowledge you need to understand the inner working of lead-acid batteries. Ohm’s Law works here, and so does series and parallel circuit theory. Even the word “battery” is a clue to what’s inside the box, namely six smaller cells that are connected in series. And like any series circuit, this causes the voltages of each individual cell to add to the desired 12 nominal volts. Unlike most electrical and electronic devices in cars and light trucks however, batteries are the electron source, not the load, so from the popular “water in a pipe” analogy that describes so much of circuit theory, the battery is the overhead water tank, fed by its pump, the alternator. Voltage is simply the “pressure” available from the “tank” to do useful work.
Inside the Box
Unfortunately, we can’t store electrons as easily as water in a tank. This is where the “electrochemistry” happens, and keeps happening whenever the engine is cranking or the alternator is charging. Inside the box (usually made from polypropylene plastic) there are just three main components, the electron pathways in and out of the battery and the acid solution that connects the path, called the electrolyte.
What we call the “negative terminal” is in the arcane language of chemistry the “anode”, which reacts chemically with the sulfuric acid electrolyte to release electrons that are free to flow through the vehicle’s electrical system to do work. The anode is also the lead in a lead-acid battery, and is one of the major industrial uses of the heavy metal. Once through the electrical system they return to the battery through the “cathode” or positive terminal, where another chemical reaction, this time between cathode and electrolyte, absorbs the electrons, and a circuit is complete. Chemists write these reactions in a form of shorthand with the “ingredients” of the reaction on the left side and the products of the reaction to the right of the arrow:
At the anode (negative terminal):
Pb + SO4 PbSO4 + 2 electrons
The two electrons are free to flow into the car’s circuits.
When they return to the cathode, which is made of lead dioxide paste, each electron must be consumed, or else they will “pile up” and the electrical current will stop flowing. At the cathode (positive terminal), the reaction is:
PbO2 + 4H + SO4 + 2 electrons PbSO4 + 2H2O
The two reactions neatly produce and consume electrons, but not until they’ve passed outside the battery through the vehicle’s electrical system. At both electrodes, lead sulfate is a product. What happens when all the lead and lead dioxide is used up at each terminal? The reaction stops, and the current stops flowing, too. The solution, of course, is to recharge and inside the battery, recharging is simply the reverse of the two reactions, combining electrons (from the alternator or charger) with lead sulfate at the anode to restore the lead plate, and regenerating lead dioxide at the cathode. Lead-acid batteries are literally about corroding away the internal components and then electroplating them back into their original form, at least as close to the original form as possible.
It sounds simple, and it is, but it’s not a perfectly reversible process. Like any electrical device, there is an internal resistance and when any current flows through an internal resistance, it generates heat. Heat kills batteries in several ways, but the two most common are by boiling away the electrolyte, which exposes internals to the corrosive outside environment and by mechanically warping the internal grids and plates, which can short out internally to create a dead cell. Too much current, too fast is a common cause of excessive heat, either through high-current quick chargers, or from a defective, unregulated alternator. Rugged plate separators are one answer to the problem, as is patient charging and topping up of electrolyte levels where access to the cells is available.
The electrolyte itself can contribute to premature failure if unwanted reactions or foreign material causes sludge or sediment to settle at the bottom of the cell. Look for lots of room below the grids to accumulate gunk without shorting the plates and always use distilled water to “top up” lead-acid batteries.
Inside the Equations
The equations reveal a lot about the process of energy conversion inside a battery, including a couple of important service issues that aren’t widely known. One is the use of hydrometers to check battery condition. It’s not as easy as it used to be to get access to the cells with today’s “maintenance-free” designs, but if you can, the two reactions show why hydrometers are an excellent way to peek inside a battery. At the cathode (positive terminal) water is one of the products of the discharge reaction. The more the battery is discharged, the more dilute the acid solution that is the electrolyte becomes. Hydrometers measure “specific gravity”, which is the “weight” of the solution relative to pure water. That’s why higher numbers are better, since less water is present to dilute the solution, meaning less discharge has occurred. Better than a load test? It can be, since it allows the technician to test each cell individually. One marginal cell can be enough to trigger a host of electrical problems, and for the customer who knows just enough to test his or her battery with a DIY Radio Shack voltmeter, it can demonstrate that the unit really does need replacing, despite a no-load reading over the magic 12 volts.
A quick look at the reverse (charging) reaction at the positive terminal (cathode) and another issue becomes clear: hydrogen. It’s a flammable gas, and since it’s a product of the charging reaction, it will be present, unfortunately right beside an excellent ignition source in the battery terminals themselves. Flash arresting labyrinth vent designs and absorbent traps help reduce the risk, but the chemistry is clear: it’s a danger that warrants protective clothing and eyewear when servicing batteries.
Another aspect of lead-acid operation revealed by the equations is that the acid doesn’t disappear; it’s converted back and forth between lead sulfate and sulfuric acid, so keeping the fluid level over the plates is about maintaining the largest possible effective surface area between the grids or plates that form the anode and cathode. That’s why adding more battery acid instead of water won’t revive a dead battery. In fact it can inhibit recharging by adding excessive sulfur into the equation, upsetting the back and forth switch between sulfate and solution.
Basic lead-acid cell technology is 150 years old, but there’s more to the internal workings of a automotive cranking battery than can be covered in any single article. Watch SSGM for more on lead-acid as well as new-technology cranking battery updates.
Sulfation How and Why
“Sulfation” traditionally means death to lead acid batteries, especially in automotive cranking applications, but little is known at the bay level about the process. Dennis Kissack, manager, ACDelco product service, shared a few tips on preventing sulfation and its effects with SSGM.
What is sulfation?
“Sulfation occurs when you have a battery that is discharging. You have the lead dioxide of the positive plates combining with the sulfuric acid to form lead sulfate. It appears on the paste grids as a white material. You have a similar reaction at the negative plate, where the lead or “sponge lead” of the negative plates combines with the sulfuric acid to also form lead sulfate. You start off with two plates of dissimilar paste materials and in discharging they attempt to become the same. When that happens, the electrolyte really becomes like water the more the battery is discharged.”
Is it reversible?
“Yes, as long as it doesn’t go too far. On some batteries, if the electrolyte level gets low enough to expose the plates to the air, the sulfation may reach the point where it isn’t reversible. It would show as a stratified layer across the cells.”
Should dealers charge display batteries?
“Shelf life is affected by temperature. It’s shortened by excessive heat. If dealers don’t turn batteries in a timely fashion, they should check to see if the batteries need charging. Any kind of discharge forms some amount of sulfation. It depends on how quickly you reverse it. If you deeply discharge a battery and leave it sitting, the grids are sitting in more of a water solution and grid corrosion becomes a factor. It can get to the point where even with recharging, the battery won’t recover. If a dealer is selling batteries over the counter, checking open circuit voltage with a voltmeter will give an indication of the state of charge. Charge levels are correlated to both voltage and specific gravity. A 100-percent charged battery would show approximately 12.6 volts. At 50-percent charge, you’ll see approximately 12.24 volts. 11.98 would indicate a very discharged battery. It won’t tell you if a battery is good or bad, but it will give you a sense of the state of charge.”