Toyota Tacoma 2006 battery drains when started but returns to 11.8V when not running

Question

2006 Tacoma 6-cyl 4x4, and the battery is 3 years old. Truck would not start this morning, so I checked the voltage - 11.8V. When I cracked the ignition, it just clicked. To me that usually means a dead battery. I was able to start the truck with jumper cables. When I disconnected the cables and checked the voltage of the battery, it showed it slowly draining until it was below 10V and dropping...slowly, but dropping still. I turned the truck off, checked the battery voltage and it returned to 11.8ish range again. Dead battery or alternator?

Answer 1

What you are describing is your alternator being bad. You can try charging the battery separate from the vehicle until full (over 12.5-13 vdc), then try what you're doing over again. If you're seeing the battery continually lose voltage as the engine is running, this is indicative of a bad alternator.

Also, you talk about the battery returned to the 11.8 vdc range after stopping the engine. I don't know what the scientific name for this is, but I call it "rebound". You see it quite often in batteries of any kind which are under load, then once the load is relieved, the voltage goes up some.