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Standby Current Measuring








Stubborn errors: bad starting behaviour

The beginnings are only too well known. For some time now, the vehicle has been unwilling to start. One has already replaced a few things, in the worst case, even the battery, starter and generator. The search for transitional resistance then goes to the wiring to and from the starter, all the connections are freed from rust and protected. The problem is, nothing helps, the error remains. At least, one has discovered, that it only occurs after the vehicle has been standing for a longer period. Just about then, the idea pops up, it could be caused by a permanently switched on consumer (parasitic drain).

Measuring the current to find any standby-consumers ...

There are any number of consumers, which are still active despite the ignition being switched off. In the past, it was only the clock. If one disconnected that, all was well. However, since the appearance of remote locking systems and anti-theft alarms, the whole thing becomes more difficult.

When measuring, consider also the 'sleep-mode' .

In particular, because not always the same amount of current is used. Unless the manufacturer states otherwise, one should wait, a good half of an hour after the last switching on. In this half an hour, nothing in the car should be altered. Should, for the purpose of testing, one of the doors has to be left open, it helps to de-activate the door-switch, perhaps with the help of a screwdriver.

Resistance measurement is not quite decisive.

How doe one measure anyway? In theory, one could disconnect the negative pole of the battery, and carry out a resistance measurment in the remaining electrical system. This however, has disadvantages. Not only because nowadays, one should not simply disconnect the battery. In addition, e.g., the cold- and warm resistance of an incessant burning light bulb, would differ by about the factor 10.

mode). Long term measuring is also normal. In that case, it doesn't matter if the built-in telephone rings during measuring. One can then observe, how perhaps several consumers are first woken-up, and later, one after the other, go back to sleep. By the way, we have, in the case of the telephone with perhaps unsuitable software, already one possible error source.

Measuring the resistance is not quite decisive.

How, in the first place, is the measuring done? In theory, one could disconnect the negative pole of the battery, and measure the resistance found in the remaining electrical system. This however, does have disadvantages. Not only because nowadays, one should not simply disconnect the battery. In addition, e.g., the cold- and warm resistance of an incessant burning light bulb, would differ by about the factor 10.

Best to measure with a calliper-ammeter

Therefore, we've decided on a calliper-ammeter, which nowadays, already has an accuracy right down to 20 milliamps or (better) even less. Now one only has to know the permissible threshold value. First of all, we once again, the manufacturers data. If we don't find what we're looking for, we assume a value of 50 milliamps. More should not be assumed, even though a 50 Ah-battery should be able to survive this load for a calculated 1000 hours. After all, if the vehicle, having only half the battery capacity, starts badly in winter, all that's left are 500 hours, and that, only in theory.

Most accurate method: Current measuring using a multimeter

Admittedly, this still leaves plenty of room for inaccuracy. Plan B would mean, connecting the multimeter with the selector on current measurement in such a way, that when removing the minus pole and also during measuring, the battery is not disconnected from the electrical system, a somewhat more demanding task. In addition, the measuring range must be set at '20 A' and also the measuring cables must be accordingly connected, since up to 10 A can be reached even when the ignition is switched off.

Trouble-shooting through selective switching off

The value during the measurement is exceeded. Now the search really begins. In older vehicles, at least one could remove the fuses, one after the other and check when the value drops. Modern often have communal areas, like e.g., the lighting system with integrated current check and/or sometimes very large communally fused areas. If the error memory then throws no light on the subject, the search within such a system is only possible by actually checking or disconnecting the individual consumers. 06/07




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Translator: Don Leslie - Email: lesdon@t-online.de

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