Why Car Batteries Do Not Last Here

Almost everybody associates flat batteries with cold mornings, and there is a reason: cold thickens the oil, so the engine is harder to turn, and cold reduces the chemical activity inside the battery. That is when a marginal battery finally cannot do it.

But that is the moment of failure, not the cause. What wore the battery out was heat, and that is the part that matters here.

What heat actually does

High temperatures speed up the chemical reactions inside a lead-acid battery, including the ones you do not want. Grid corrosion inside the battery accelerates. Water evaporates from the electrolyte. Both are cumulative and neither reverses.

Under-bonnet temperatures in this climate are high and they stay high, so a battery that might reasonably do six years in a cool climate will commonly do three to four here.

That is normal. It is worth knowing so that a battery failing at three and a half years does not look like a defect, and so that nobody talks you out of testing one on the grounds that it is practically new.

Short trips make it worse

Starting an engine takes a lot out of a battery, and the alternator puts it back over the following period of driving. A car doing four or five kilometres at a time never fully replaces what starting took.

Run that pattern for months and the battery lives permanently in a partially discharged state, which causes sulphation and permanently reduces capacity. School runs, the shops, and short work trips all produce it.

Cars that sit unused for weeks have a related problem from the other direction: modern vehicles draw a small current continuously even switched off, so three weeks standing can leave one flat, and repeated deep discharges cause lasting damage.

A flat battery is often somebody else’s fault

This is where money gets wasted, so it is worth checking before replacing.

If a car keeps going flat, the battery may be fine and reporting a different problem. An alternator that is undercharging leaves it flat; one that is overcharging cooks it, which is why a replacement battery sometimes fails surprisingly fast. Something drawing current with the car switched off, a parasitic drain, produces the same symptom and is measurable. And corroded terminals or a poor earth strap, which are common in this climate and cheap, produce slow cranking and strange electrical behaviour that look exactly like a dying battery.

Testing the charging system and checking for a drain at the same time as testing the battery is what stops you buying two batteries for one fault.

Getting the right one

Vehicles with stop-start need a battery built for it, either AGM or EFB as the manufacturer specified. Put an ordinary battery in a stop-start car and it will fail early, because the duty cycle is nothing like the one it was built around.

Many modern vehicles also require the new battery to be registered to the vehicle’s electrical system, so the charging strategy treats it as new. Skip that and the car keeps charging as though the battery is four years old, shortening the life of the one you have just paid for.

The batteries and charging page covers testing and what gets checked alongside it.