# Cheap/easy way to measure DC Amps

#1

Car Battery Tester Measure Amps and Alternator Draw

#10 Copper Wire has a resistance of 1 Ohm/ft and 1 mA = 1 Amp.

In my Video there is 18" of #8 copper wire which as a resistance of .64 ohm/ft and therefore 18" (1.5 ft) also yields 1 Ohm.

#2

Don’t you mean ‘1 volt will be measured across this length (1 foot, etc) of wire per amp flowing through it’

#3

0.001 Volts per Amp

E = IR I=E/R

Since R is 0.001 Ohm per short length of wire.

1 Amp = .001 / .001

Copper wire resistance

#4

Your original post should have read 1 milliohm (0.001 ohm) per foot, or 1 ohm per 1000 ft, with a 1mV drop measured across 1 foot of wire indicating 1A flowing through the wire

#5

Use an Allegro ACS770 or similar. Using drop across wire is hugely thermally unstable, and results are nonsense.

#6

What are the temperatures that would make a copper shunt off by more than 5% if you are in in the temperature zone?

BTW I soldered the Voltage measurement points using a calibrated meter.

#7

20C to 30C would be around 4%. I suspect that the rise might be (much) more than that. If you have thermal compensation in place it can be accurate, but, if not, it will be pretty inaccurate for continuous current flow.

#9

I cannot imagine flying with my setup. It is for a 5 second measurement.

Gud’nuff for most without having to spend a dime.

#10

https://www.digikey.com/product-detail/en/allegro-microsystems-llc/ACS770ECB-200B-PFF-T/620-1538-5-ND/4473977 Not sure where you are, but this is just \$8.38 in the states. Limit is 200A, bidirectional. With a bit of heat shrink, you can fly with this also. Needs a 5V supply to the smaller pins, but then the output is 2.5V for 0A, 5V for 200A, 0V for -200A. I’ve used this device for FOC brushless DC motor drives, it is high bandwidth (120kHz), so will even be able to look at commutation/switching noise (to see how rough the ESCs are with the battery).

#11

How to measure 333 Amps?

#12

Personally, I would measure the current to one machine only.

However, if you want all 4 at the same time, then:

https://www.digikey.com/product-detail/en/tamura/L03S400D15/MT7185-ND/529417