Voltage drop: the test that finds bad grounds

A bad connection can show 12.6V with nothing running and fall flat the second you turn the load on. Test it while it's working.

The short version

• Turn the load ON, then measure across a section of wire or a connection.
• Good connection: almost nothing, under about 0.2V.
• Check the ground side too. Bad grounds cause as many problems as bad positives.
• Move your probes in steps from the battery toward the load until you find the jump.

On the job

1. Load on (lights, pump, blower). Red lead on the battery positive post, black on the positive at the far load. That's the positive-side drop.
2. Then black on the battery negative post and red on the load's negative. That's the ground-side drop.
3. Over 0.5 V on either side is too much. Now measure across each connection in that run: the one over 0.1 V is your fault.
4. Fix it, retest under the same load.

Numbers

• One connection: under ~0.2V under load
• Whole 12V circuit: under ~0.5V battery to device
• Any single connection: under 0.1 V
• Battery to panel: under 0.3 V under load

Shop tip

Dim lights at the back of the rig but bright up front? Voltage drop. Usually a ground.

Try it on a service call

Put the lesson to work on a hands-on troubleshooting case.

• Back lights are dim

Keep going on 12V and batteries

Learn it

• Volts, amps and ohms, the simple way
• Practice: find the bad connection
• Batteries: lead-acid vs lithium
• Fuses, breakers and grounds
• Converter / charger

Prove it

• Back lights are dim
• Battery dies even on shore power
• Lithium won't charge on cold mornings
• Coach is dead, battery is fine
• Batteries dead after storage

On the job

• 12V system
• Converter / charger
• Lithium battery
• Coach dead, battery good
• Solar not charging