What you need to know:
Maintain your car’s battery power by driving it frequently and for longer periods of time. If you do not use your car often, consider investing in a battery charger to help maintain the correct voltage.
Most cars still use old-fashioned lead-acid batteries, which are nowhere near the best design by any measure (weight, size, capacity, charging speed, reliability, longevity)…except one:
Even with just 12 volts, they can “discharge” a tsunami of amps like no other, and that is exactly what cars need to get started. A car’s starter motor has to turn all the moving parts of an engine and a heavy flywheel (which weighs plenty), against friction and often gloopy oil, from stationary to a high speed rotation in a second or two.
That demands a huge “draw” of electricity, say, 300 amps – enough to melt metal and drain a 50 ampere-hour battery from full to flat in six minutes.
Other “affordable” types of battery which are superior in every other respect cannot do that.
The moment an engine fires up, the battery is out of business as the alternator is now being turned by the engine and is generating all the electricity the car needs and recharging the battery, too.
The lead acid battery can go through this discharge and recharge cycle many thousands of times, indeed almost indefinitely because it works using what is known as a “reversible chemical reaction”.
Fully charged, it contains plates (such as thin slices of bread with gaps between them) made of pure lead, completely submerged in electrolyte (a mixture of water and acid).
When electrical power is demanded, the acid reacts with the pure lead and starts to form lead sulphate, and the electrolyte becomes more dilute. This reaction is what generates electricity, and the battery is “flat” when the plates are almost completely lead sulphate and the electrolyte is almost pure water. The potential to generate more has gone.
But, by reversing the flow of electricity from demand to supply (charging the battery), the chemical process reverses. The acid leaves the plates and re-joins the water, returning to pure lead plates and the right mix of acid and water for the electrolyte when the battery is fully recharged. Ready to start the process all over again.
Three things can change that from an “everlastingly reversible process” into progressive battery ageing and failure; heat and/or chemical change and/or structural damage. Knowing what causes those, and avoiding them, is what makes batteries last longer.
Car batteries can cope with some heat, after all, any chemical reaction creates heat and batteries have to live in hot engine compartments.
Even the extreme chemical reaction caused by the starting download is usually brief and the heat is quickly dissipated. But prolonged cranking of the engine without pause during hard starting is another matter. That can generate enough heat to buckle the lead plates and cause them to touch each other, creating a short, which generates even more heat. That will quickly damage and destroy a battery.
Excessive heat is also caused by excessive load, for instance if the starter motor is jamming or by recharging too rapidly or too much (say if the car’s or the charger’s voltage regulator is faulty).
Loose or fouled connections at the battery terminals also generate heat and/or allow the battery to “leak” discharge even when not in use. This is made worse if the outside of the battery casing is dirty. Therefore, one should always wipe it clean regularly, especially near the terminals.
There is a precise balance between the amount of lead and the quantity and strength of electrolyte for the “reversible chemical reaction” to continue indefinitely.
Things that can upset that balance are evaporation of the water in the electrolyte (that should be regularly replaced by topping up with distilled water), allowing the electrolyte level to drop too low (leaving the top of the lead plates to oxidise in the air instead of being completely submerged) and any impurities in the water used for topping up (distilled water only should be used, even the cleanest drinking water will damage the balance).
Others include failing to fully recharge the battery for an extended period (some of the residual acid in the lead will become “fixed” and the plates will be permanently sulphated to some extent), and too often and for too long running the battery at a low level of charge (for instance a faulty alternator), which also causes some permanent sulphation.
If a battery will remain unused for more than a week, it should be fully charged and disconnected from the terminals to avoid electrical leakage that will discharge the battery and cause permanent sulphation.
This can be external, for instance damage to the casing from rough treatment when fitting terminals to the battery posts (never use a hammer), or overtightening the battery clamp, causing distortion (which can misalign the lead plates inside) or cause cracks, which allow electrolyte to seep out.
From the constant bumping and shaking on rough roads in general use (even worse if the battery clamp is too loose and allows the box to bounce) which can cause the plates to come loose and either spill fragments of lead into the bottom of the box (eventually causing a short circuit between plates). The plate can also be displaced so it touches another or no longer contributes to the circuitry and capacity (dead cell) of the battery.
Fit a battery of the correct capacity for your engine. A 50AH battery might be able to start an engine designed for a 70AH battery, but it will work too hard and fail sooner.
Fitting twin batteries has the opposite effect; both have an easier life. They can last more than twice as long and you have back-up if one fails. Lead acid batteries will last longer if kept as fully charged as possible, and are never allowed to fall below half-charge (do not keep trying to start until a battery is completely flat). Find and fix the starting fault or ask for a push.
Check your battery’s voltage once a month
A lead-acid battery’s life will shorten dramatically the longer it is left partially or fully discharged, so checking the voltage with a voltmetre once a month is a great way to keep an eye on your battery’s health. A healthy, fully charged lead-acid battery should have a voltage of around 12.7 volts or above.
Should the voltage drop below 12.5 volts, recharge the battery as soon as possible. It is also important to remember that a lead-acid car battery is considered to be half charged at 12.4 volts, and completely flat/dead at 12.0 volts, so do not get complacent.