why does my car’s battery keep going flat?

If your car has developed a fault, or for consumer advice, turn to Honest John by emailing [email protected] It’s draining again Our 55,000-mile 2002 Honda Jazz Sport 1.4i DSI had a new battery in November 2017. This has run flat on two occasions over the past year, when the radio […]

If your car has developed a fault, or for consumer advice, turn to Honest John by emailing [email protected]

It’s draining again

Our 55,000-mile 2002 Honda Jazz Sport 1.4i DSI had a new battery in November 2017. This has run flat on two occasions over the past year, when the radio has been used for about 20 minutes while parked. On the first occasion the RAC couldn’t find a problem with the battery or the alternator. The second time we restarted it ourselves. It has gone flat during lockdown and started easily with jump leads but, despite taking it for a three-mile run, was then flat again within days. It’s just been back to the garage: again no fault with alternator or battery. Can you help? GA

The first thing to tell you is that current-generation batteries and charging systems are not designed to allow you to sit in the car using the battery with the engine switched off. The reason is to lessen the load on the alternator and thereby improve the fuel economy of the car. There should be a warning in the handbook. A common villain that discharges batteries of Japanese cars is the interior light delay switch (which keeps the interior light on for 15 seconds or so after you shut down the engine). Even after the light goes out, the switch can remain live and draw current. The answer is to switch off the interior light completely. And three miles will not be enough to recharge the battery – it needs 20-30.

Sodden impact

My wife bought a 10,000-mile 2016 Nissan Leaf Tekna in March, shortly before the lockdown. As we were then very soon into self-isolation, it has only been used a few times – one of those in rain. Underneath the dry surface of the boot mat, the car is now sopping wet the spongy insulation literally oozes water when touched. There is no sign of wet anywhere else in the boot area. The obvious thing to do is remove the mat, both to let it dry and also try to see the cause, but a Bose speaker box is bolted through the mat and prevents this. We have informed the supplying main dealer of the problem, but in the meantime we are concerned about ongoing damage such as corrosion. When folded flat, the rear seats show no signs at all of any dampness at their base. Is there anything we can do? BC

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