2000 gmc jimmy battery drain.

My 00 jimmy's battery drops substantially when its in reverse and the brakes are applied or when the rear defroster is on and I hit the brakes. My dad tested the alternaters output and he said it was fine. So assuming that he is right what else could it be.

What exactly do you mean when you say; "battery drops substantially"? Are you talking about the amp gauge in the instrument panel? If so, how far does it drop?

What voltage is the regulator maintaining at 1500RPM, (all accessories off)? What is the maximum amperage output of the alternator? Did you have the battery load tested?

Sorry about the late reply. The gage on the insterment panel drops midway so its between the 9 and the 14 on the gague readout. Everything is off, vents stereo, drls are on though. Not load tested I don't think. It only drops in drive when the rear defroster is on and the brakes are depressed. The second I release the brakes the Hague goes back up to just under or on the 14.

Sounds like the brake lights are drawing too much current. Check to see if the brake light bulbs are OEM or some type of high output bulbs.

Do you have a trailer wiring harness/connector at the rear bumper? If so, is anything hooked into it like a third brake light in the receiver hitch? Also check the wiring harness, (if you have one) at the rear bumper to see if the connector is rusted out and see how the wire harness looks.

Sorry about the late reply. The gage on the insterment panel drops midway so its between the 9 and the 14 on the gague readout. Everything is off, vents stereo, drls are on though. Not load tested I don't think. It only drops in drive when the rear defroster is on and the brakes are depressed. The second I release the brakes the Hague goes back up to just under or on the 14.

Dash volt gauges are not very true volt reading.

Get a volt meter and test at the battery.
And then post back volt readings.

no wireing harness. I replaced two of the brake bulbs when I bought it about 9 months ago. The drain has only been happening in the last few months. If it were the bulbs drawing too much power then why is it only when the rear defroster is on and I hit the brakes that the drain occurrs.
When the drain occurs all the lights dim(dash headlamps)

no wireing harness. I replaced two of the brake bulbs when I bought it about 9 months ago. The drain has only been happening in the last few months. If it were the bulbs drawing too much power then why is it only when the rear defroster is on and I hit the brakes that the drain occurrs.
When the drain occurs all the lights dim(dash headlamps)

Get a volt /ohm/amp metor and give us the reading at battery on voltage and alt output.
The check amp draw when rear defroster and brake lights is on and then with brake lights off.

. . . If it were the bulbs drawing too much power then why is it only when the rear defroster is on and I hit the brakes that the drain occurrs. . . .
The brake lights are not enough drain by their self to cause the problem, but when applied when something else is on the electrical load gets heavier.

You also could have a bad ground with the brake light bulbs. Take the brake light bulbs out and see if the problem is still there. If it is then you have eliminated the brake light bulb sockets.