How to look after your car battery during lockdown

In the current climate, we’re helping lots of our members with questions about car batteries and how best to look after them while we’re not driving as often as usual.

Our Technical Adviser shares the most frequently asked questions and answers below…

Q1. Why do car batteries go flat if unused?

A typical 12-volt battery discharges naturally at around 0.1 volts per month at 10 degrees Celsius. In warm weather, the discharge rate increases. Additionally, the car’s electrical system draws power even when the car is unused. The rate varies between makes and models but we have heard about cars discharging a healthy battery entirely in under a fortnight.

Q2. Why do you recommend that a 12-volt battery is kept fully-charged?

Lead Acid batteries (i.e. flooded, Enhanced Flooded Batteries and Advanced Glass Mat types) prefer to be kept at 90-100% charge (above 12.5 volts), otherwise they sulphate internally. This reduces their capacity and shortens their lives.

Q3. Can I let the 12-volt battery run flat and simply jump-start the car afterwards?

We do not recommend this strategy. If the battery drains completely, there is a risk that it will never recover. The possibility increases the longer the battery has been left discharged and for how much its state-of-charge has dipped below 12.5 volts.

Q4. Should I start the car and let the engine charge the battery?

No. 12-volt batteries prefer to be charged slowly. The modern car’s charging system is very unlikely to charge the battery fully. This is also true in many day-to-day driving conditions and is a major reason why batteries deteriorate prematurely.

Q5. Is it OK to give my car a run once or twice a week to ensure the battery stays charged and to prevent potentially expensive recovery/repair bills? 

By starting the car, you will have to replenish the charge lost by doing so. Much also depends on for how long you leave the car running. Letting a car idle on the drive alone can also cause further issues with the engine, including moisture build in the exhaust/engine oil, particulate filter blocking and spark plug fouling. In any case, it is likely that relying solely on the car’s charging system alone will not charge the battery to its optimum 90-100% charge. Using a mains-powered Smart charger is preferable. 

Furthermore, this isn’t classed as an essential journey in the Government guidance, and if you are stopped by police, you risk being warned or fined for contravening COVID-19 regulations.

Q6. Should I use a battery charger?

To our knowledge, a battery charger is the only way to charge a car battery to its optimum 90-100% charge. Modern lead-acid batteries are vulnerable to overcharging, which shortens their lives. This is one reason why we recommend smart chargers, rather than old-fashioned trickle chargers. Smart chargers also have different charge setting options for Advanced Glass Mat batteries, used on the latest models.

Q7. Can I connect a smart charger directly to the battery terminals?

Not in every case. Many modern cars (especially those fitted with Start-Stop technology) have a battery monitoring chip on the negative terminal, which can be damaged if connected to an external power source. Consult your handbook for advice for jump-start connection points. Typically, a separate negative terminal post may be mounted to the bodywork.

Q8. Should I disconnect my car battery, when using a smart charger?

Check the advice in your car handbook and the smart charger manual. In our experience, disconnecting the battery is usually unnecessary but check both publications first.

Q9. My car is parked on the road and I cannot get an electricity cable to it. What can I do?

You can disconnect and remove the battery to charge it in your home but be wary that the battery releases hydrogen and oxygen gases during the recharging process, which are explosive. Therefore, ensure that the chosen room is very well ventilated.

Q10. I have a Hybrid car, equipped with a high voltage battery. I presume I do not need to worry about keeping it charged?

Hybrids (or, more specifically, Self-Charging Hybrids) have their high voltage systems activated by a separate 12-volt battery. Should this smaller battery be discharged, the car will fail to ‘start’. Therefore, check the car handbook and use a smart charger to maintain the 12-volt battery’s condition.

Q11. I have an electric car. Should I keep it permanently plugged-in to the mains during lock-down?

The high-voltage batteries used in Electric Vehicles (and Plug-In Hybrids) use different chemistries. Unlike 12 volt lead-acid batteries, high voltage packs prefer not to be kept fully-charged for long periods. Consult your handbook for more information but, generally, maintain the high voltage battery between 50% and 80% charge level.

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GEM Motoring Assist is a members’ motoring and road safety organisation in the UK. Our aim is to keep our members on the move… whether that’s through our breakdown recovery service or our motoring and road safety expertise.

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