Car Ammeter

[barry wilkins]

I did this test on my previous car to check the alternator was charging the battery properly.
The earth cable from the negative terminal to earthing point on the vehicle has a very small resistance. So I built a simple millivolt meter and connected from the said battery terminal and earth point and measured the voltage drop ,then I connected a DC ammeter in series with the earth cable and measured the actual current drawn.
Now I had the measurements I calculated a factor ,multiplying the mV by 0.921 which gave me the current .
Examples
Side lights - 3.8mV x 0.921 = 3.5A discharge.
Head lights -12.6mV x 0.921 = 11.6A discharge
With the engine running after starting gave me +7 to 4.5 mv ,so I could tell the alternator was charging the battery.

Barry Wilkins

[tom66]

Re: Car Ammeter

Useful info. But isn't it enough to see the battery voltage increase? A charging lead acid will show around 14.4V, whereas a battery that has been idle will drop down to around 13.5V reasonably quickly.

[stj]

Re: Car Ammeter

you can get a digital ammeter with a 200a shunt on ebay for under $10

[redwire]

Re: Car Ammeter

It's an interesting experiment.
Copper is not so great for a (shunt) resistor. The tempco of copper is about +2% per 5°C and you have to make a Kelvin connection with your multimeter.
If your probes are not matched- touching different metals like lead, zinc, or copper then thermocouple effects add mV too.

You have a 1.09 milliohm resistor, all for free. I looked at #4AWG wire which is about 0.25 milliohms per foot, so your crimp connections are looking dirty....