I have a Toyota Avensis. The battery ran flat and I needed roadside assistance to start the car. The AA man tested the battery and said it is leaking to earth (to the body). He said it could be a bulb on in the glove box or whatever. I have checked and found nothing obvious. I did change some bulbs in the clock displays recently.
The man had specialist amp clamps to see if the electrical system was leaking from the battery. I only have a multi-meter. Anyone know how I can test using this? I can remove fuses and isolate the circuit and test which circuit is the problem.
A clamp meter will only give an approximation of the current being drawn - especially at the very low level that should be the case with everything switched off.
You need to remove one battery terminal and wire the meter between the battery and lead - ie in series. The meter should be capable of reading up to 10 amps which should cover any switch on surge. A smaller one will likely blow its fuse. The 10 amp one might too.
Once things have settled the quiescent reading should be less than 0.05A
- 50mA. More usually about 20mA.
Much higher than this needs investigating. But unfortunately pulling fuses won't isolate everything - the alternator for example is permanently connected to the battery and a fault in that can result in a highish constant drain.
Yup, I use that method too. My meter is fused at 250mA and never blown the fuse yet with my special lead that has an old 21 Watt brake bulb soldered in series. As for clamp meters. I've never seen one that does low DC in the low mA region and even the very expensive one's that can are not as accurate as a standard £4.99 DMM. All I have ever used them for is high AC current, but even then it needs to be in a single core as measuring live and neutral at same time just gives a inaccurate reading.
A scope isn't a particularly convenient way to measure a voltage, though. And that's what the device converts current into.
Can you give details of its range(s) - and accuracy?
IMHO the problem is the field it is measuring is very distance dependant - so the thickness of the wire insulation would come into play. In other words you could make one which was reasonably accurate for a given wire - but most would want one (for a car) which will fit all likely sizes.
That's not how they work though, you're measuring the flux in the coil (or more accurately measuring the current required to null the flux in the coil). If it's a DC current then the output will be DC, so you can plug it into a scope or a multimeter. Fluke bought Lem/Heme so the current usefull small one is
My existing one can tell if there is a major problem with quiescent current ok - ie if it's a few amps rather than mA. But you'd need to have a lot of use out of this 1mA device to justify the price - the 10mA one wouldn't be accurate enough.
But for DIY a normal DVM is fine and accurate enough - provided you realise what can happen and take precautions.
I haven't used it for several weeks, and I'm not at work for a few days, so it's all down to a fuzzy memory and I've no idea who makes it or the price, I just use it when there's a problem a scope can't sort out by itself, but it doesn't look cheap as such stuff goes, a few K is my guess but it's several years old so perhaps they're cheaper now. It's about half the size of a tower PC case, mains powered, definitely works from DC to high MHz (I've seen good shape squarewaves at several MHz so my guess is about
100MHz). Its sensitivity is 20mV/A, so if the 20mV range on a scope is used then 1 division equals 1 amp. Our scopes have 2mV ranges, so 1 division is
100mA, so reading 10mA approx would be about the limit on a scope. It's ranges are basically just the one (or is it 2, not sure), so basically it's down to the scope's range switch to be used for all range adjustment. A DVM used with it should be usable for DC work and give more accurate low DC current readings, I haven't used it for this though as I'm usually more interested in spikey pulses that shouldn't be doing what they're doing. The clamp is small, you could almost get a pencil through the hole I reckon.
A Fluke type clamp meter would be best for most car usage but a quick look on their site only showed AC ones.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.