If this is with the engine off -your battery is bad.
If this is with the engine running, your alternator is not charging and your battery has probably been ruined as well.

How many times must we tell you? You need a battery and PROBABLY an alternator.


Testing an alternator is very easy:

1) Engine off, measure battery voltage. You should see 12.2 to 12.6 or so.
2) Start engine - battery voltage should rise due to charging from the alternator: over 13 volts is good.
3) Turn on heater, all lights, wipers, seat heaters, etc.; rev engine to 2,000 rpm - if battery voltage is still above 13 volts, the alternator is charging fine. Without all this stuff on, you better see 13.8 to 14.2

Once you get close to 15 volts, you are getting near an overcharge condition.

Regulation of the alternator output is internal to the alternator - there is no separate regulator.

In case you wonder:
The path of charging is very simple (this is for LHD cars in the (USA):
Alternator stud - heavy cable to battery positive post - thru battery - thru ground cable - thru chassis - back thru chassis to engine ground strap(s) - thru engine block and back thru the frame of the alternator.

While it is true that high resistance in this path will cause the battery to not be charged, it would also cause the starter to not be able to sink enough current to turn the engine over.

(I've NEVER found the chassis to engine ground strap(s) but I know there are there, somewhere.)