Battery related faults:
To see if your battery is bad either because you have an outgoing battery or a bad battery check the following

Dead battery:

- A dc voltage reading on the multi meter lower than 10.6 Volt (meaning 1 cell has broken).
- Many warning lights and a faulty start with a loud click at the end.
- Slow crank or no crank and esp / no fuel warning light (can also point to ground issues).

Battery on its way out: (replace immidiately)
- Erratic locking of doors:
Doors locking/unlocking without synchronization.
Door locks randomly opening/closing while driving - may also point to an ISM or DDM issue
- Unexplainable warning lights (many at the same time).
Fuel meter suddenly dropping while driving and getting an ESP error (may also point to ground issues).

For the above issues your solution is the following: 

- Replace your battery every 4 years.

In case you have replaced your battery recently but still have voltage issues see to the following steps 1 by 1:

Grounding issues for the battery:

- In case of a negative cable feeling hot do the following: 
Follow the cable to where its attached to the chassis, clean the contact point. After that measure with a multi meter in ohm setting to see if any ohm resistance is present. If so do the following:
- Check the clamp of the negative battery lead for play or if its removable. Unfortunately the factory forgot to remove the plastic on the top of these cables from time to time.

Dynamo & regulator related issues: 
In case of a broken dynamo, or voltage regulator measure with your multimeter the voltage going to the battery while the car is running. This must be 14+ V if you dont have any grounding issues. Below 14V you have a grounding issue or your battery is very hot. (it then also reduces the charge it takes by about 1V.)
In case of a broken voltage regulator you will see fluctating voltage readings while measuring going from 12.4V up to 17V in those cases replace your regulator for an original voltage regulator. Do not use aftermarket, this will damage the ECU after a few weeks to years of driving causing a P2500.