Call me wanker - again

[Dave Plowman (News)]

You don't need a traditional ammeter these days. You can measure the voltage drop across say the earth lead to the battery and translate that into current flow - either positive or negative.

[Dave Plowman (News)]

If only it was that simple. A charge rate only slightly higher than needed will result in a battery 'boiling' dry. Certainly only half a volt or so higher than it should be.

Most cars already have an 'idiot' light to tell you there is no charge. An alternator failing in the 'hard on' condition is pretty rare - I've only ever come across it once.

[Clive George]

They should do that anyway - I had a big arrow drawn on me for pinning my hip.

[dennis]

The wife's car has a sensor on the battery earth lead so the computer can see if its charging or not. Uses a hall effect sensor AFAICT, there is no actual electrical connection to the cable.

[Johny B Good]

I wonder how it copes with calibration (and the hundred or so amps from the starter)?

I'd expect it still monitors battery voltage using the hall effect sensor to get a more accurate indication of battery condition.

[ARW]

The same light bulbs that I normally pay less that a quid for

[Tim Watts]

On Tuesday 11 February 2014 19:41 Johny B Good wrote in uk.d-i-y:

Hall sensor for current, surely? And the worst that would do under starter load is max out the sensor - would not damage it.

[SteveW]

Applies to humans as well AFAIK.

SteveW

[Huge]

The surgeon will likely do that anyway. When I had my left large toe straightened (Dupuytrens) the surgeon saw me beforehand and drew a large arrow on the toe with a felt tip!

[Johny B Good]

Sorry for the confusion. I meant the "car's computer", not the hall effect sensor.

I appreciate that but my concern was over the 'zero' calibration point. If you've ever used a DC clamp meter, you'll be aware of the need to reset the zero calibration point every time you use it (and in between measurements every few minutes). I can't imagine that the hundred or so amps of starter current helps this in any way.

If a modern DC clamp meter avoids this, I'd like to know how they manage this neat trick.

[Capitol]

You can always arrange this by having an auto zero circuit.

[Tim Watts]

On Tuesday 11 February 2014 22:57 Johny B Good wrote in uk.d-i-y:

I guess it could be zeroed when the car is switched off (allowing for a very small standby supply).

[Johny B Good]

DC clamp meters have this auto zero function but you need to remove the clamp from the circuit you're testing before pressing the zero calibrate button or else you land up with a DC offset from the circuit being tested if you can't shut off the current.

In this case, you can't "Swich the Battery Off" so the issue of compensating for drift in the DC offset inherent to hall effect sensors when used to measure DC currents still persists. It's not an issue with AC since an HPF can be used to filter out the DC component.

[Adrian]

You can take the man out of Yorkshire, but...

[Adrian]

Do you often cook human lungs?

[Capitol]

All instruments can be designed with continuous auto zero.

[Johny B Good]

I suppose it could be as simple as that. Any alarm/wireless door key functions should be able to manage with less than 100mA drain (well less one would hope since a 36AH battery would be totally drained after only 15 days - not good news when you return from a fortnight's holiday abroad).

If we're looking at a drain of 50mA or less, I suppose setting the zero on that would be close enough (closer still when the average 'standby drain' is known and used as the 'calibrate' point).

You know, I do believe I may have answered my own question. :-)

Thanks for the hint, BTW.

[Johny B Good]

Assuming that was true (I just don't know) one has to wonder why the DC clamp meter manufacturers didn't incorporate this feature into their designs simply to reduce manufacturing costs (eliminating the auto zero re-calibrate button being the obvious cost saving in this case).

[The Natural Philosopher]

Well it is not true of course - if you continuously auto-zero a meter it will always read zero!

The meter manufactures are simply deciding that the operator has a better chance of detecting when there is zero current than an automatic system.

So there's the button guys!

[ARW]

My ancestors are Scottish:-)