battery: bad or not charged??

[robwill2 at wxs.nl]

Hi Audifans,

For about 6 months I got a new battery for my Audi 100 
(1982)..
It's not of a wellknown brand, so this may be the cause 
for the following:

After last loading it (at home) about 2 months ago, today 
it appeared to be quite empty (only slowly churning the 
starter, but unable to let the engine start)..
Help from another battery started the engine, but after 
some hours standing it refuses again..
On making contact with my key (starting-lock) the load-
control light is on (so no bulb-failure) and on the run she 
isn't burning, so I assumed the generator is loading..

Would you first suspection go to a bad battery?
When yes, how can I test (got only a multimeter) or 
should I have the battery-cells tested in a workshop??

Is another cause for this emptying possible??

(I drive distances of about 20 km's, without electrical 
loads in the car (no extra lights and so on)..
Lowest temp's are about 10 degr. C. at night over here 
now, so this shouldn't be the problem...

Thanks for any suggestions,
Robert
robwill2 at wxs.nl