Car Battery Tester Measure Amps and Alternator Draw - Measure DC Amps

**Boyntonstu** · Sep 17, 2015, 09:43 AM

In order to measure 100 Amps DC I am using a multimeter and a 18.75" length of 0.128" Diameter solid copper wire (#8).

The voltage drop measured across the wire in mllli-Volts directly equals the current in Amps.

**Jon** · Sep 18, 2015, 01:05 AM

Thanks Boyntonstu! I've added your Car Battery Tester to our Diagnostic and Testers categories, as well as to your builder page: Boyntonstu's Homemade Tools. Your receipt:

Car Battery Tester by Boyntonstu

tags: battery

**mklotz** · Sep 18, 2015, 10:40 AM

Can you explain how you calibrate your device? Are you using a low-resistance ohm-meter to measure the resistance of the copper wire or do you have a calibrated current source?

**DIYer** · Sep 18, 2015, 09:41 PM

Great tip, Boyntonstu! Thanks for posting this.

**jere** · Sep 19, 2015, 11:55 AM

very neat tip! what amperage is the alternator in the vehicle rated at? rather than adding a battery you might consider changing the alternator for one with higher output.