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. You’ll find stop-start technology on the vast majority of new cars, and it generally works very well. Not only does it save you from using fuel unnecessarily, but it also keeps pollution and smog down in urban areas (where you’ll find most of the traffic), and that’s good news for everyone.
It’s a key feature in mild-hybrid vehicles, which can harvest energy normally lost while decelerating, then use it to help power important features when the engine is switched off. That means the car can switch off the engine while still moving and then coast to a halt, but the driver still has power steering, and the climate control and radio systems will still work. When the driver is ready to move off, the engine will start again quickly.
While it’s true to say ancillary features, such as air conditioning systems and radios, will slowly drain the battery, car makers have thought of that. According to Tim Shallcross, a mechanic and adviser to the Institute of Advanced Motoring, the on-board computers know how much charge is in the battery and can modulate the stop-start technology accordingly.
“Your car’s computer oversees the stop-start technology,” he said. “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.”
It’s worth noting that although stop-start technology might stop the engine when you stop the car, it may also restart the engine if the conditions become unfavourable. That’s why the engine will sometimes come back to life if you’ve been stopped for a while – particularly if you have the radio and heating on.
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Every starting process is associated with a large amount of energy from a car battery. The battery must be in a very good condition to reliably supply a high starting current. Especially with modern vehicles with automatic start-stop systems, the engine is switched off several times during the journey. During these phases the battery continues to supply all of the electrical consumers with energy. A conventional starter battery (SLI) is not designed for this charge output (i.e. continuous discharging and charging).
Only a battery with start-stop technology can deal with these demands and withstand the strain.
In a car which is not equipped with an automatic start-stop system, the starting process is usually a one-off event. While driving, the generator supplies the electrical consumers such as the entertainment system and navigation device, so that if the generator is working, there is no discharge of the starter battery during the journey.
The case is different for cars with start-stop technology. During a journey through the town, the battery is subjected to a series of start and stop periods. If the car is stationary at traffic lights with the engine stopped, electrical consumers such as lights, windshield wipers, radio or displays still need power. The load is increased by continuous discharge and charging.
In particular, short journeys present a challenge for the battery. Especially with old batteries and in winter, the charge acceptance of the battery reduces, so that adequate recharging during short journeys is not always ensured. This can result in gradual discharge of the battery, so that at some point, the battery no longer has enough energy to start the engine. Due to the special technology, the internal resistance of AGM batteries remains considerably lower than that of conventional batteries over their entire life, so that sufficient charge acceptance is also ensured over a long period and short journeys with many start-stop phases can be handled better by AGM batteries.
AGM batteries have a large cold start current. They powerfully start the starter motor and reduce its running time. Thanks to the cycle stability of AGM batteries the engine can be switched off and started again several times at short intervals, without the risk of difficulties when restarting.
With modern vehicles, the AGM battery is not only charged by the generator, but is also fed by the braking energy recovery system. Normal starter batteries must be kept permanently fully charged and can therefore not absorb the energy which is generated by recuperation. AGM batteries can be operated in the partial charge range and therefore provide sufficient “extra” capacity to store the energy which is generated on braking.
The high reserve capacity ensures the power supply for all types of electrical components. The cycle stability of AGM technology ensures the power supply to components while also providing a high starting power. Even if the battery is not fully charged, because, for example, electronic assistants and consumers which ensure comfort are supplied during a stop, or the generator has been temporarily decoupled to reduce the load on the engine.
Start-stop systems require a battery with modern technology, because normal starter batteries are not designed to meet the higher demands of these vehicles.
The architecture of the electrical system in modern vehicles requires a compatible battery technology in order to function properly and reliably. For this reason, in many vehicles, new batteries must be “registered” – the Battery Management System (BMS) must know what type of battery is installed in the vehicle in order to exploit its full potential. If an incorrect battery is installed in the vehicle or is not correctly registered, this can result in premature battery deterioration and another breakdown.
For this reason, only EFB or AGM batteries should be installed in vehicles with automatic stop-start systems. If an AGM battery is already installed in the vehicle, it must always be replaced with another AGM battery.