Modern car batteries are rated for hundreds of charge cycles. The cycle is straightforward: the engine starts (a heavy momentary draw), the alternator then charges the battery back to full over the rest of the drive. The chemistry is happiest when each discharge is followed by a meaningful charge.
In central London, that pattern often breaks down. The classic case is a two-mile commute, twice a day, in stop-start traffic, with the heated seat, infotainment, headlights and stop/start system all drawing power. The alternator never quite catches up. The battery slowly drifts to a chronically low state of charge, which sulphates the plates and shortens its working life — often by half.
What actually wears the battery out
Three things, in order of frequency in our diagnostic reports:
- Chronic undercharging — the most common cause. Easy to fix.
- Parasitic drain — a glovebox light, dashcam or aftermarket alarm pulling current overnight. Easy to find with an ammeter.
- Heat — engine bay temperatures over 60°C in summer accelerate plate corrosion.
Three small habits that recover years of life
- Once a fortnight, drive for at least 30 minutes with the engine warm. Even a single 20-mile motorway run on a Sunday is enough to fully recharge a deeply-cycled battery and reset the chemistry.
- If the car sits unused for more than five days, plug in a CTEK or NOCO smart charger overnight. They cost less than a single replacement battery and look after the car automatically.
- Get a free five-minute battery test once a year. A printed slip showing cold-cranking amps and state of charge tells you whether the battery is healthy long before you get the click-of-doom on a January morning.
When to give up and replace
If the cold-cranking amps drop below 70% of the rated value, no amount of overnight charging will bring it back. Sulphation is a one-way process. At that point, replacing it is straightforward — and the new battery will benefit from the same three habits. We've seen plenty of London batteries reach their rated six-year life when an owner adopts a once-a-fortnight long drive.