Today I drove a few miles to the Post Office, did my business there, came back to my car, put the key in, pulled the starter knob and then "clickclickclickclick".
I got a jump and the car started just fine. Lights and accessories worked but seemed as though they weren't getting enough voltage. Revving the engine helped temporarily.
Got home and now the same problem.
What might be the cause of this? Alternator? Voltage regulator?
The belt for alternator is kinda loose but it still turns it just fine, might that be it?

__________________
1973 220D ~135k Mi
1979 Ford F100 300 I6 - 225k Mi

Fix the belt and install a new regulator, that'll probably get you there.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

x2, if the lights get a bit brighter when revving the engine, you have a worn out voltage regulator. Always fix what you know to be wrong, first. Belt too loose = replace it and adjust it before doing anything else.

__________________
1998 E300 turbodiesel

America's Rights and Freedoms Are Not The Enemy!

Recharge your Battery before doing further testing. Havine a Voltmeter helpful as it give you more information. Also clean the contact areas of the Cables and Ground. Look under the insulation of the Cable and see if there is that typical green crrosion has eaten up the cables.

Things are not always the typical Votage Regulator or bushes issue.
I just had to replace a battery on an 1986 US Car. It charged but it would not keep up the charge when the light, heater on and you were stepping on the Brake Pedal. That being said the fist thing I did was clean the terminals and connections.

Next I charged the Battery. After which it seemed to charge OK but it gradually deteroiated to back to the poor charging. A new Battery fixed it.

Your year and model shows a 35 amp AL64X or an AL74X Bosch Alternator. The parts place shows an external reguilator the Beru one is $23. The ones in the pic appear to be solid state regulators. Don't know if the original was a solid state one.

The external regulator means if your brushes are worn you are going to have to take the Alternator apart to replace them.

__________________
84 300D, 82 Volvo 244Gl Diesel