Bad battery, alternator, or something else?

I fired up the DeLorean today for the first time in weeks after I completed repairing other things. It usually cranks for a while when it has sat for a week or more and it's super cold outside. I have a battery cutoff switch, so there's never a drain on the battery when it's sitting.

After only a few cranks the battery was losing power quickly. I was able to get it started, but I noticed that the dash lights, like the door ajar light, were really dim. I let the car idle for about 5 minutes and when I looked again, the dash lights were so dim that they almost weren't visible. I also noticed that the voltage on the dash was showing only around 8 volts, which is really odd. I decided to turn the car off.

The battery is all but dead now. I tried starting it again and it doesn't even have enough juice to crank. Me thinks that if the alternator were working properly, the car would register full voltage while running, regardless of the state of the battery, but I'm not sure. The battery is also suspect because of its inability to hold a charge for a few weeks, even when the battery disconnect has been used. The battery is from the previous owner and I have no idea how old it is.

I was just wondering what these symptoms point to. As a side note, I have never once seen the battery light on the dash illuminate and the car has always worked fine until today. Thanks for your help.

Batteries in (very) cold weather loose part of their capacity.
I don't recall the actual value but it could be up to 20%.

Besides that batteries deteriorate over the years.
You may be able to charge them and the charger tells it full, but the capacity under load may be insufficient.

So the age of the battery and the outside temperature (below freezing point) both influence the battery, especially when cranking.
The CCA (Cold Cranking Amps) of a battery tell it cranking force (only available very short moments).
Load testing of a battery tell you if your battery still is up to its job.
A bad cell in the battery is enough to have it refuse to do the job it's hired for.

BTW: You mention never having seen the battery light? Hmmm.... even not after turning the iginiton key ;)

very simple answer:

Alternator not working !

I've seen alternators that needed to be revved once to start working.
That was neede on old alternators if the battery bulb was shot.

Alternators don't live forever, the coal brushes wear over time and need
to be replaced. Or replace the alternator...

Test your battery bulb too. I believe one of the stock alternator types required that bulb be functional for the regulator to activate, and the other type didn't -always- require it. Another one of those quirks. If you have an aftermarket alternator, all bets are kind of off, but Hervey told me flat out that his alt will NOT charge the battery if that bulb is bad. More than likely it is your alt, but this is a 5 minute test and a 50 cent fix if it ends up being the culprit.

Either way, also check the small wire(s) coming from the alt - 2 on a stock, 1 on many aftermarkets. Make sure they are connected well and not broken or burned. Those control the regulator.

Good luck.

Been going through all the threads I've started to give a resolution to all the problems I've fixed. This issue ended up being a burned out battery light in the dash. The car charges fine with no issues since that light has been replaced. :)