Battery Drain

[davidanthony]

I recently saw a battery drain problem was due to a faulty heater LSR Last Stage (Hedgehog) Resistor on a BMW 5 Series even when the ignition was switched off.
Is this little critter in heater circuits known to be the cause of battery drain problems on other makes of vehicle ?

[davidanthony]

It was definitley the LSR draining a 60 amp hour battery in 12 hours. Thats more than an interior light. BMW confirm this is a problem. When changed all was OK

[Buenchico]

There are often several circuits in a car which remain powered independently of the ignition switch. The obvious one is the clock but many people are unaware that, if they don't dive the car for extended periods, they should disconnect the circuit to the interior light. The timer circuit remains active and draws power even when the light is off.

[Hymie]

A colleague had the same problem on a 3 Series BMW about a year ago. He searched the web to find it was a common problem on BMWs. He said it was a bit of a pig getting it out and replacing.

In relation to other vehicles suffering similar problems – it is now common practice for high power electrical car components to be connected (via fusing) directly to the battery positive terminal, with the control achieved through switching the 0V (using a transistor or relay via the ignition switch).

So with the vehicle ignition switch off, 12V is being supplied to many of the electrical components – a short circuit in the control circuit is all it takes to permanently energise the component, draining the battery.