How a car battery works, EFB and AGM technologies, and what kills a battery

How a battery is built

A storage battery is a rechargeable chemical current source made of cells combined so as to give a higher voltage or a higher current, depending on how they are connected — in series or in parallel.

Cars normally use lead-acid starter batteries. Compared with other types they offer the highest energy density together with the ability to deliver a very high current for a short time.

How does it work?

The battery converts electrical energy into chemical energy while charging, and chemical energy back into electrical energy while discharging.

The positive electrode is dark brown lead dioxide.

The negative electrode is grey spongy lead.

The electrolyte takes part in the reaction as well — an aqueous solution of sulphuric acid with a density of 1.28 g/cm³. During discharge the active mass of both the positive and the negative electrode turns into lead sulphate, a process known as double sulphation. By the end of the discharge the electrolyte density drops to 1.08–1.10 g/cm³.

What EFB and AGM batteries are

Enhanced Flooded Battery

EFB batteries are improved lead-acid batteries with liquid electrolyte. The plates are also made of lead, but they are thicker than in a conventional battery. Each plate is wrapped in a special microfibre envelope that stays saturated with electrolyte and sits tightly against the active surface. It also protects that surface from damage such as plate sulphation, which occurs during deep discharges.

Because the plates are thicker, the battery lasts far longer and tolerates deep discharges much better. A conventional starter battery loses about 5 per cent of its capacity with every deep discharge, whereas an EFB recovers to almost 100 per cent.

EFB batteries hold roughly three times less electrolyte. The smaller volume of liquid and the use of purified lead make the battery accept charge 45 per cent faster. This helps in high ambient temperatures and cuts electrolyte corrosion activity by 40 per cent. The battery works from −50 °C to +60 °C — good figures for a price segment below that of AGM.

Another advantage of EFB is the advanced Power Frame grid. Optimised geometry and precise design give the battery fast charging.

AGM technology means batteries whose compartments are filled with a porous glass-fibre mat soaked in liquid electrolyte. The micropores are not completely filled, because the free volume is used for gas recombination. AGM batteries differ from conventional flooded lead-acid batteries purely in their design and manufacturing technology.

AGM batteries are produced either with a spiral cell design, mainly in North America, or with a flat electrode configuration in Europe.

Spiral cells have a larger contact surface, which allows them to deliver very high currents briefly and to charge faster. The downside is lower specific capacity — the ratio of electrical capacity to size — compared with the flat configuration.

Today AGM car batteries with flat cell blocks are the most common. The higher charge and discharge current of AGM batteries therefore depends on the production technology. Batteries with flat cells usually have current ratings similar to those of the same batteries of classic design.

Why batteries fail

Car batteries need careful treatment. Without proper inspection and service their life shortens with every cycle. Both severe cold and extreme heat are harmful. But most often a battery is killed by incorrect use.

Common causes of failure:

1. Overcharging by the alternator. Excessive charging current leads to rapid evaporation of the electrolyte, which can even cause the battery to explode, a falling fluid level, rising acidity, destruction of the separators, rapid shedding of the active mass and so on.
2. Undercharging by the alternator. The consequences are no better than overcharging: loss of capacity, lower cranking current, plate sulphation and eventually a shorted cell.
3. Contaminated electrolyte. Dirt getting into the electrolyte can short the battery. Topping up with non-distilled water is just as bad: the electrodes lose their working ability very quickly and the battery fails.
4. Double load when jump-starting. If the car will not start, jump-start it from a battery that is disconnected from the other vehicle electrical system. Otherwise the load from the donor alternator and from the alternator of the car that has just started can short both batteries.
5. Neglect and delayed diagnostics of the electrical system. This is how a parasitic drain appears. In the end the battery is flat after a single night. Repeating that leads to a loss of capacity and to a battery that can no longer start the engine. Replacing the battery will not fix the problem — you need an auto electrician, because a permanent drain will destroy a new battery as well.