Vehicle batteries
Essential tips for fitting and maintaining your vehicle’s battery and alternator.
Despite the huge evolution in motor car electronics and powertrains, the humble leadacid starter battery is one of the few components that has not been usurped. Thanks to being comparatively inexpensive, having a moderate lifespan and being able to deliver relatively consistent performance in a variety of ambient temperatures, they remain a critical component in modern vehicles.
Today, even the most technicallyadvanced production car is equipped with at least one lead-acid battery. Should you investigate the technical specifications of a pure electric vehicle, for example, you will find that one is used to power the ancillary 12-volt electrical circuits. Understandably, battery technology has evolved over the years but considerable re-engineering was required to bring the humble leadacid battery into the 21st century.
How it works
A battery is a device that stores electrical energy in chemical form. Every car battery can be cycled – charged and discharged – a set number of times, which is established as a natural part of its design life and varies between batteries.
Conventional lead-acid car batteries, which are the sole focus of this feature, contain plates that consist of two electrodes, positive and negative. The electrodes consist of a skeleton grid frame made from a lead alloy, with an active material paste applied. After charging for the first time, the electrodes consist of lead dioxide (PBO2), positive, and porous lead (Pb), negative. The plates are bathed in a diluted sulphuric acid solution and this type of car battery is defined as ‘flooded’.
The grids are made from a lead alloy, through which electrons flow. A separator, made of an electrically inert material, separates the positive and negative plates to prevent short circuits and battery failure. These plates are arranged alternately in a pack, with all positive plates connected together, as are the negatives. One pack, capable of delivering 2.12 volts, is inserted into
each of the cell cavities within the battery case. The packs are connected together, in series to form what we know as a 12-volt battery. However, the total voltage, when the battery is charged fully and in good condition, should be around 12.72 volts.
Electrons flow through the negative to the positive plates via the electrical 'consumer' that has been switched on. This causes the plates' chemical composition to change, with lead sulphate building on both electrodes. Consequently, the strength of the sulphuric acid weakens, with its ph value increasing and becoming closer to that of water.
The chemical changes, caused by the discharging process, can be reversed by charging. This involves feeding a direct current into the battery from an external source, such as from the car’s alternator (producing an AC alternating current initially, but converting it subsequently to DC) or a mains-powered charger. The lead sulphate dissolves back into the electrolyte and the lead-based materials on the electrodes revert to their original materials: lead dioxide (+) and porous lead (–). The water in the electrolyte solution breaks down, with the hydrogen recombining with the displaced sulphate to create sulphuric acid and the oxygen with the lead on the negative plate to form lead dioxide These gases can also be seen as bubbles escaping from the electrolyte solution. This is why a battery ‘gases’ as it receives current and is why charging should be carried out in a well-ventilated area, even if your battery claims that it is ‘sealed’.
Keeping cool
Although batteries can be charged and recharged, they are a consumable item and their performance will deteriorate over time through normal use.
“One reason for this is that the battery is a cannibal – it literally eats itself as it is used,” says Ian Newham, training manager at GS Yuasa Battery Sales UK, our technical partner for this article. Yuasa is an OE supplier to a number of major vehicle manufacturers and also has a manufacturing facility for industrial batteries in the UK.
The natural process of the sulphuric acid electrolyte reacting with the active lead-based material on the plates is the main cause of natural ageing. This chemical reaction is hastened as the battery’s temperature rises. With the cold-start ability of a healthy battery
dropping by approximately 30% at 0°C, not helped by cold engines becoming harder to turn over as ambient temperatures drop, it is unsurprising that any damage that has been wrought on the battery by wear-and-tear, as well as corrosion over a long, hot summer, is not revealed to the car owner until the first cold morning of autumn.
As sulphuric acid is corrosive by its very nature, the destructive reaction between the electrolyte and lead alloy material cannot be avoided completely, although it can be slowed by ensuring that a battery is not overcharged and kept in a cool environment. While the ageing process can be hastened by drawing continual high currents, flooded lead-acid batteries that are exposed to brief heavy discharges, encountered in conditions such as a cold engine start, tend not to be affected that badly, unless there is a fault present that necessitates prolonged cold-start cranking.
The cycling process causes small amounts of the active lead-based material to fall from the plates. This is electrically conductive and, should it become trapped between a positive and negative plate, a short circuit and loss of performance from the cell concerned can result. Therefore, this discarded material is collected in a sediment trap, moulded in the bottom of the battery case.
Sections of lead alloy breaking from the plates, or the plates themselves breaking loose, result typically from poor build quality, or physical mistreatment.
“While the better design and build of more expensive batteries will slow the ageing process and increase the number of cycles,” says Ian Newham, “they all have a finite lifespan and should be viewed as a consumable component.”
Keep it charged
Flooded lead-acid car batteries prefer being kept at 90-100% charge to achieve the longest lives. The ideal scenario would be an engine cold-start that depletes only a small amount of power, followed by a drive cycle that is sufficiently lengthy for the alternator to restore the battery’s optimum charge level. However, even in an ideal world, every discharge robs the battery of a small amount of capacity, just as using the brakes removes a small amount of friction material from the pads. The key to maximum battery life is to keep this loss as small as possible and to replenish the charge quickly afterwards, without causing overheating.
Should the battery be unused, even when disconnected from the vehicle, it discharges naturally at around 0.1 volts per month at 10°C. This rate of self-discharge increases as the ambient temperature rises, so you should avoid storing a battery in hot conditions. Instead, find a cool, dry and wellventilated place, keeping the battery at peak charge, using a smart charger. When the battery is connected to the car, maintaining a high charge level is not that easy. Short journeys might provide inadequate opportunity for the battery to recover its lost charge, especially during winter, when powersapping accessories, such as heated seats and windows, are used continually.
Consider also that ECUS, alarm systems and clocks all discharge the battery gradually. Even if it takes weeks for a battery to become ‘flat’, the internal chemical reactions that take place during that period will damage the battery permanently. For the same reason, flooded lead-acid batteries are not designed to be discharged and charged heavily. This ‘deep cycling ’ increases the risk of lead material dislodging from the plates, which reduces the battery’s capacity. Leisure batteries are more suited to this pattern of use, but they cannot tolerate short and intense bursts of discharge.
Maintaining a 90-100% optimum charge level and the voltage above 12.5V is critical also because of sulphation. When a battery is discharged, lead sulphate builds on the active lead-based material on the electrode surfaces. Provided that the battery is not kept below its peak charge for long and is recharged promptly if it is not, the lead sulphate changes back into the lead dioxide/porous lead material. This process is time-critical. Should the battery be left below its optimum charge rate, the lead sulphate will crystallise and become difficult, if not impossible, to eradicate on charging, even for domestic smart chargers with special desulphation modes. The result is that the battery can never be restored back to its full capacity.
Water loss poses a further threat. For older cars, routine maintenance procedures dictate that the vent cap on the top of each cell be removed and the electrolyte level verified as being higher than that of the plates. Deionised water is recommended for topping-up, because tap, or rain water, for example, introduce other chemicals that can damage the battery. While all batteries ‘gas’ and release the explosive mix of hydrogen and oxygen when charging, older car batteries
used to contain around 10% antimony. Unfortunately, a proportion of antimony would dissolve in the electrolyte, increasing the tendency of the battery to lose water. Superior manufacturing techniques saw the antimony level reduced to less than 2%, heralding batteries being advertised as ‘low maintenance’. Replacing the antimony with calcium has reduced water loss even further, negating the need for occasional manual top-ups, enabling battery manufacturers to remove any plugs, or cell caps, from the casing, effectively sealing the battery for life.
Incorporating clever lid designs that capture the hydrogen and oxygen gas that builds during charging and release it back into the electrolyte as water, valveregulated lead-acid battery (VRLA) tend to be advertised as being ‘maintenance free’. This indicates that the battery will never need to be topped up with water, or charged, when in regular service on a vehicle. Periodic recharging will be required if it’s not in regular use.
Although maintenance-free, these batteries can still be damaged if subjected to undercharging and overcharging. Notably, if they are overcharged, gases will be vented into the atmosphere should the internal pressure become excessive. This causes the electrolyte levels to drop and, because there is no access to top up the cells with distilled water, both the capacity and lifespan of the battery are curtailed.