Voltage difference between Two negative cables

[Ericthemantis]

Hey all my ESS hasn't kicked on in awhile, so I was looking around at threads about isolating and charging the aux battery and main battery separately. I took my voltmeter to the different negative leads (big nut, little nut/L bracket, main negative terminal) and now I'm confused. I measure a 12V difference between the two negative leads! Here are my measurements:

• Positive to Negative Terminals = 12.5V
• Positive to Negative L bracket = 12.5V
• Positive to Negative big nut cable = 0-1V

The two negative leads spark when I touch them back together, but then I hear the car systems do their little boot up sounds. What does this mean?? Is it expected?? How the heck do I check or charge the aux battery??

EDIT: This is a 2023 2.0L JLU (Willys id that is important)

Sponsored

 

[garykk]

That’s because you have two batteries.

 

[Ericthemantis]

That’s because you have two batteries.

Ok, but why did the negative lead show a 12V difference between the other lead? I'm seeing that to charge the aux battery you connect Positive to Positive, Negative to L bracket when disconnected, but I'm not understanding how it could charge if the difference in voltage between the two is less than 1V.

 

[Country Roads]

Did you ever find an answer? I noticed the same thing with mine.

 

[Ericthemantis]

Did you ever find an answer? I noticed the same thing with mine.

Nope. But I did need a new aux battery.

 

[Mguy]

Ok, but why did the negative lead show a 12V difference between the other lead? I'm seeing that to charge the aux battery you connect Positive to Positive, Negative to L bracket when disconnected, but I'm not understanding how it could charge if the difference in voltage between the two is less than 1V.

To charge the JL's dissimilar AGM batteries correctly, isolate any and all cables on the IBS from the IBS and themselves, then charge batteries separately. Might not be a regular practice among JL owners using external chargers (and not done by the stock JL system).

When two dissimilar batteries are placed in parallel, technically speaking, there is potential difference between the two. That difference is measured when the voltmeter is put in the parallel sub-circuit (that's happening when test leads are each connected to negatives).

 

[THAW]

To charge the JL's dissimilar AGM batteries correctly, isolate any and all cables on the IBS from the IBS and themselves, ...

Charging of the IBS-connected battery should be done through the IBS to keep the IBS properly calibrated.

Sponsored