BCM?

Good afternoon,

I would start by checking the battery voltage. Use a voltmeter and check for 12.4 to 12.6 volts. If it is less, charge the battery and then retry.

https://www.2carpros.com/articles/car-battery-load-test

Roy

Battery is dead after SUV sits overnight.
If I hook up the battery charger, with the battery in the SUV, it will not charge. But
If I disconnect one of the battery cables
it will.
Now, the battery stays charged, and I have only head and brake lights, but lost all cabin and dash lights, windows, door locks, blower, stereo, and can't use the fob.

Did you check the voltage? I need the reading.

As far as draining overnight, I will attach a guide for you to follow to determine the drain.

https://www.2carpros.com/articles/car-battery-dead-overnight

Roy

It's not draining now.
The battery stays charged, but I have only head and brake lights.
I've lost all cabin and dash lights, windows, door locks, blower, stereo, and can't use the fob.

Please, I need the voltage. You may have a bad battery and it needs to be tested.

Can you give me the voltage?

Roy

12.6 volts.

Okay, now with the meter attached, have someone turn the key to start and tell me the voltage.

Roy

Wait before we get too far ahead of ourselves answer me this, the battery will charge if the neg cable is removed from the battery. But if both cables are connected to the battery the charger won't do anything.
But yet I connected the cables after the battery is charged and the vehicle starts and ran good, no problem, so I kept disconnecting the cable at night so it wouldn't draw the battery down, again no problem. Until two days ago hooked up the battery and no interior electric including tail lights and dash lights.

It would not change because of the connections at the battery cable.

You still need to follow the guide for the battery drain for that issue.

Roy

Okay, here's a puzzler, and I hope someone out there understands what I'm trying to say, and can give me a straightforward answer.

The battery is in my SUV, a 2002 GMC ENVOY 4WD, is dead every morning, but it's a good battery, had it in another car, worked great!

Now if I connect my battery charger to the dead battery, with the SUV's positive and negative cables connected to the battery, my charger goes completely dead, no juice whatsoever.

But, if I remove the SUV's negative battery cable from the battery then hook up my charger it will work. All lights, horns, whistles, etc., and it'll charge the battery.

Now my query is, why? Why do I have to disconnect the negative battery cable in order for my charger to work AND charge the battery?

The one thing that comes to mind is the cable itself. Check both ends for corrosion.

Roy

The battery is good, I need to know why the battery holds a charge now that I have no tail lights, no interior lights, windows, locks etc.

Hello,

If I can jump in here is sounds like the BCM is bad but to be sure lets do a CAN scan which you can get a scanner from Amazon for about $50.00. here is a video to show you how:

https://youtu.be/InIlnsjOVFA

You don't need a subscription for the ones on Amazon and this will be handy in the future as well. Here is how you can change the BCM out in the diagrams below. Please let us know what you find. We are interested to see what it is.

Good information from Ken, but before you go to a BCM, I would follow the guide for the drain to be sure it is the BCM that is causing the issue.

Roy