There are people who feel they have to move their cars every day, almost like others who take the dog out for a walk. Why?, because they are afraid that if they don't, the next day the car won't start. Where older vehicles are concerned, there is the alternative (and cheaper) method of disconnecting the battery after every trip. All this leads us to the question: why do some (motor vehicle) batteries discharge themselves?
Assuming that the battery is OK. Perhaps it has already been replaced and the problem is still not solved. In this case one speaks of a hidden consumer, which has to be determined. The classic (although not undisputable) method is to disconnect the negative pole with the ignition switched off and the serial connection of a multimeter between pole and pole- terminal.
Before you disconnect a battery, find out, whether or not the radio is protected by a code, and also whether there are other control devices which could possibly cause problems. Perhaps the battery may not be disconnected at all, without the assistance of a slave-battery ...
The multimeter is set to 'mA', which of course, does not require a large amount of stand-by current which would immediately make the instrument useless, at least for such measurements. Definitely better, in this case, would be the use of a current-measuring caliper, which should however, encompass, all the minus leads. With this method, one can save onesself the job of disconnecting the battery.
Let's consider the indicated current once again. First of all, we assume that (almost) all of the control devices are dormant. This can take some time in modern vehicles, unless the manufacturer states otherwise, approx. 30 minutes. In some cases it may be necessary, among other things, to also shut all the doors (motor car).
How high may the stand-by current now be? We would like to assume 50mA or 0,05A. This would be 1,2Ah per day. This would roughly half discharge a 50Ah battery in approx. 3 weeks. This would be just about acceptable, maybe however, only in a vehicle with a stronger battery. Under normal conditions, a battery with only half its capacity, can still start the engine. Of course the manufacturer's data should always have priority.
If the value is higher, an invisible user is sitting in the car and stealing the current. The removal of single fuses gives us a certain idea of where it's hiding place is. If the amperage drops clearly in a circuit switched off in this manner, we are already getting a little nearer to the source of the error. However, this does not mean that it's now going to get any easier.
Because we are not acting on on the assumption of stored mistakes, the on-board computers can't really help us either. The good old method of reading the wiring diagram now becomes reality. That which would interest me the most, would be easily accessible plug connectors in the recently identified circuit with which one can disconnect certain sub-areas.
Sometimes it also helps to check for light or heat radiation with your own senses, or with one of those practical and relatively inexpensive remote measuring instruments. Ascertain the difference between the acceptable, and the measured value, and then calculate the additional consumption. Which components in the respective circuit, with 'ignition off', or after having left the vehicle, are still consuming? Find them, or at least make it possible to switch them on or off. 03/09