Charging Problem After Warm Up

'91 XJ, '93 4.0, '91 PCM.

In the morning or when the Jeep has been off for a while and is cool, the voltage is fine. After 20 mins of straight driving, voltage drops down to say 11 or 12. 40 minutes it's down to 10.5 or 11 and 60+ minutes it's around 10 to 10.5 volts and stays. I went to get the battery tested and the alternator. The alternator is new, about 3 months. Both battery and alternator tested perfect.

Any idea's on what may be draining the battery? I'm really out of idea's.. Ignition related maybe?? I sold the Jeep but I don't want to give it to him without trying to fix it. Thanks!

My 89 does the same. It's normal. There are some fixes though. The best I've heard it explained is it sinses the voltage in the dash after your A/C heater radio and other stuff has put a draw on the voltage. The fix would use a relay and the volt meter hooked in close to the battery. I've seen some diagrams with a fix on here somewhere. Maybe someone will chime in.

It's weird 'cause I can run with the radio off and the heater off and it still draws power. I thought it was just something with the gauge until I noticed my headlights dimming, turn signal slowly down extremely, dash lights getting dim, etc.. Ugh.

Mine used to do the same thing. I did the upgraded body ground and chassis ground modification and also replaced the large charging cable from the alternator to the battery with a #2 guage cable.

Replace all your ground wires/straps with larger heavy duty items making sure their contacts are clean and dry. Run an extra ground from the negative battery post to the fender or body somewhere. Run a heavier wire ground from body of alternator to the chassis.

I used a third negative battery cable to run a new ground from the firewall to the engine block as well.

Still, you may have an alternator that is failing when it gets hot. That is something that must be checked at the time you are having the charging problem as the alternator may check out fine when its cooler.

It is not normal for the alternator out put to drop to less than 12.8V, and it's not normal for your dash voltmeter to read less than what the alternator is outputting. So, when your dash gauge is readin10-12v, use a digital voltmeter on the battery to confirm the alternator is, in fact, deficient.
Bad grounds, like Tally-XJ stated, can create havoc with voltage levels, especially when we're talking about low voltage to start with (12v). What I would do, as a matter of "preventive maintenance", would be to disconnect all ground circuits connected between the battery and engine/body and between the engine and body. Clean and inspect each connector for corrosion/tightness and clean away any oil, paint, or rust from the area where the connector attaches.