Unexplained Battery Discharge

Say When · June 12, 2020

On two occasions since I took delivery on 02/14/20, the batteries in my 2020 GMC Sierra 2500HD SLE Crew Cab, Long Box, 4WD, L8T Gas Engine truck have inexplicable gone dead. My truck is equipped with optional Dual Alternators and 220-amp Primary Battery & 170-amp Auxiliary Battery. I have not identified the source of the battery drain. Has this phenomenon happened to any other GM-Trucks.com members? Can anyone offer any advice on how to diagnose the problem?

Much obliged & kind regards.

Tomato · June 12, 2020

You might just have 1 bad battery. It will draw down the good one, and these trucks do not like low voltage. They do all kinds of strange things when you try to start them. Make sure the dealer replaces both, of that is the case...

Sask Farmboy · June 17, 2020

Very good points Tomato.

This can also occur when the batteries are still good on their own but one battery is slightly weaker than the other battery.

Always replace batteries in pairs, never only one.

Method to find a battery drain:

Shut off ignition and let sit 5 minutes for all the computers to power down to sleep mode
Use an amp clamp to check the current flows from both batteries to ensure the current flow is not too large for checking with the voltmeter.
When checking current draw with the voltmeter, be careful that you maintain the circuit continuity by ensuring the voltmeter is connected into the circuit before removing a battery cable. Otherwise, the computers will wake up again and draw a higher current until charged up again and this higher current might blow your voltmeter fuse.
Clip one voltmeter lead to one battery post or clamp and connect the other lead to the other battery post (might need help with this step). Then remove one battery clamp, without disconnecting the voltmeter from the battery post.
Read the current draw
Do the same for the other battery and add the 2 current draws together to get the total current draw -- I think GM's max allowable is still 50 milliamps.
If it is over 50 mA, start pulling fuses until the current draw drops to identify which circuit is drawing too much current.
Be aware that pulling a fuse might cause a computer to shut down and then start up again when the fuse is installed again, causing higher current draws temporarily until the computer completes its wake up.

gmckenzie · June 24, 2020

Here is my story. It was a 2001 with a Duramax, so 2 batteries.

Came to find the truck dead. Asked my wife what she did as she was the last to drive it. I was joking, but of course should have kept my mouth shut. Explain I'm joking and there is nothing she could have done to cause it.

Charged both batteries and it was good for a couple of days, and then dead again. Part of the problem is I have a short commute, so batteries never get a good charge, but they should last more than 2 days.

Weekend comes. I've stuck the truck on the charger so I have a good charge. Disconnect on batter entirely so I'm only fussing with one. Disconnect the negative and put an ammeter in line to see what's happening.

I see a draw of about 2 amps for maybe 30 seconds, then a slight draw for a minute or so. Repeat. Repeat.

I start pulling fuses one at a time. Of course I have to wait a minute or so as it's not a constant draw. One at a time. Cursing all the way. Well, I finally pull one and don't see the draw. Forget which one it is now, but it controlled stuff in the cab. Look online to see what was controlled by the fuse and somehow glean onto one item being the circuit breaker for the electric motors for the seat. Sure enough, I check the control and the seat cover has the control to push the seat forward jammed full back. Move the seat cover, replace the fuse and the mysterious draw is gone.

Put everything back together and go inside.

Remember above where I told my wife there was nothing she could do to cause it? Well, yeah dumb me (still kinda in a pissy mood) tells her I was wrong and it was something she did. God, some people never learn.

Anyway, if you do what the above post suggests, then know that circuit breakers can draw and then not draw. So be sure to check for more than a minute.

Sask Farmboy · June 26, 2020

Well done!

And as the French would say "Vive le difference"!

redwngr · June 30, 2020

Never would have thought about the self-resetting circuit breakers doing that --- but I'll try and keep that one stored in the troubleshooting memory bank.