Disconnect or not is the question?
 

My '01 E320 4Matic is about to sit in the garage for 3 weeks. In past events a couple of years go, (sitting for 2 weeks), I returned to a dead battery. I do have a old Montgomery Ward 4amp trickle charger, but am not comfortable using it for that length of time. Is the corse of least resistance to disconnect the battery? TIA

Did you remove the key from the ignition? Later Benz (including my wife's 2004 E320) WILL flat the battery unless key is unplugged. Should restart after 3 weeks if you do. Hers does.

Disconnecting battery may cause you to reset everything.

The answer is a battery tender, not a trickle charger. A trickle charger will just charge and charge regardless but a battery tender only delivers as much charge as necessary and then shuts off. I have 5 cars, a pickup, and a tractor, including 2 classics that sit for 7 months out of the year and a couple of German cars with lots of electronics that love to draw down the power while the car sits. I keep battery tenders on most of them. Thanks to the battery tender, my oldest battery (in a '74 Porsche) is 13 years old and still going strong.

https://www.harborfreight.com/12-volt-deluxe-battery-maintainer-and-float-charger-62813.html

If your battery goes flat in 3 weeks, you need to find out why. See if you have a light staying on or something somewhere that's pulling power. On a modern vehicle with computers and other gizmos that always suck power you should be able to get a couple months of sitting before even worrying about the battery charge state. Older cars without any ECU's or parasitic draws could go for several months.

Keys are never left in car/ignition. Sitting in totally dark garage, no lights are ever left on. If there is a hidden draw, it will have to be discovered at a later date. So, I'm disconnecting the battery. I really don't have anything to loose. Thanks guys!

Occasionally I plug my tapering charger into a simple mechanical timer so that
the charger isn't always on. It wasn't the table light version but the more
robust, outdoor timer which I think was in the 15-20 amp range.