How to Test Car Battery Resistance

by Anthony Smith

Overview

There are few things more frustrating than to turn the key to start your car and hear that clicking sound that means your battery is dead. This always seems to happen on the coldest day or night and at a time when you really need to be somewhere in a hurry. The best way to avoid this situation is to regularly test your car battery as part of your regular vehicle maintenance.

Step 1

Disconnect the car battery from its two battery cables.
Step 2

Set the multimeter to read voltage and then attach the positive and negative leads of the multimeter to the corresponding positive and negative terminals on the battery. Record this voltage reading. We will call it V1.
Step 3

Determine the current draw of the starter in the vehicle in amps, either by looking on the starter itself, or in the manufacturer's specifications. A typical number would be 60 amps. We will call this value A.
Step 4

Reconnect the battery cables to the battery terminals, leaving room to attach the multimeter leads to the battery terminals themselves. You will not get an accurate reading for this test if you connect the leads directly to the battery cable connectors.
Step 5

Reconnect the multimeter to the battery terminals and have a friend start the vehicle. Get a voltage reading while the starter is operating. We will call this value V2. Disconnect the multimeter and reattach the battery cables properly.
Step 6

Use Ohm's Law to calculate the car battery's resistance. Take the difference of the two voltage readings (V1-V2) and divide this by the current draw (A). For a car battery that measured 12 volts initially and then 11 volts when placed under a 60 amp load, the equation would read: (12-11)/60 = .02 ohms.

2
Digital multimeter
Digital multimeter
Most multimeters will not accurately read resistance in a car battery by simply placing the leads directly on the terminals because they cannot detect such small resistance.
Most multimeters will not accurately read resistance in a car battery by simply placing the leads directly on the terminals because they cannot detect such small resistance.