Car clinic: Will my car's automatic stop-start system drain the battery of power?

Our experts address readers' technical and consumer issues

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Q. I’m concerned about the impact on the battery of the stop-start technology on my new Volkswagen Golf. Surely sitting in traffic with the engine off but, say, headlights, wipers and radio on is going to take a lot out of the battery, especially if it’s frequently being asked to restart the engine.
GT, Farnborough, Hampshire


A. Stop-start technology is increasingly common, generally works well and helps to keep down fuel consumption and emissions. As most traffic lights (and jams) are in urban areas, it also cuts the pollutants that cause smog and breathing disorders in our towns and cities.

You are correct in one respect: if the battery has to power much of the car’s electronics for most of the time, there will not always be enough charge to restart the engine. That is why your car’s computer oversees the stop-start technology. If it detects that the battery may have insufficient charge to restart the engine, it will keep the engine running.

Other conditions that would prevent the engine from stopping are having the air-conditioning running with a high outside temperature, the engine not being fully warmed up and a diesel car’s particulate filter being in its cleaning cycle. In addition, in many stop-start cars the battery has a higher capacity (so it takes longer to drain) and the starter motor is made more robust to cope with the extra demand. Changes are also made to the engine management system so that the engine can restart more quickly.
Sunday Times Driving car clinic expert: Tim Shallcross

Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motoring


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