Key left in ignition, battery drained
 

I absently-mindedly left the key in the ignition of my 99 E300 (not sure whether position 1 or 2 but the climate control was definitely off) for about eight hours and was surprised to find the battery essentially drained when I tried to start it. What would drain the battery in that period of time?

The unlock mechanism will do that. It's supposed to time out.

Sales person at MB Dealership where I bought my E320 CDI told me battery left in key holder of W211 will drain battery. Always pull key if not using.

The engine , trans and body computers would be active and maybe anything on the CAN bus. On a gas car the electronic throttle body is active as well so on a diesel with electronic pump control I'd think the rack control solenoid would be engaged.

With climate off and key on, the mono valves receive power ( and get way hotter than you would expect )

Could anything damaged by doing this?

Your pride ?

Nah....safe neighborhood, only had key in to open sun roof to clean rails.

There is a difference between the key inserted and not rotated vs. turned to the running position.

On the 98 W210 and anything later that uses the SmartKey, the act of inserting the key starts the authentication and steering unlock process. This wakes up the CAN buses and the electronics needed to validate the key. Also there is an actuator inside the steering column that unlocks the wheel that draws power. The manual notes that if the key is left in the lock then it will time out and the key will need to be removed and reinserted to unlock the wheel again.

Turning the key to the running position and leaving it there without starting the engine is bad for most vehicles.

My guess is I only had it in the 1 position since that's all that's needed to open sunroof. Recharged battery and everything seems to be fine.

The battery will have a slightly reduced service life if the voltage dropped below 11.7 ish volts. ( this floor for a normal full discharge ) Given you caught it and recharged right away, the reduction will be minimal unless the battery is many years old.

In the old days of ignition points on a gas motor, the coil and points could be damaged. With an electronic controlled diesel injection pump the fuel rack solenoid might be held at full power in anticipation of starting but I don't see that any worse than being on a hot engine.

Overall, I don't expect the car to be damaged.

The battery is an at least six years old (perhaps older...the PO couldn't recall) Everstart. Part of me is looking for an excuse to replace it with a new AGM!

Just put a genuine MB battery in my 98 last winter. Should be good for another 10 years or so...

AGM or Lead Acid?

The only appropriate battery for the 210 is an AGM due to the internal location.

The factory battery is an AGM. Car's at its other usual resting place (CAE parking garage) or I'd get you the p/n.

My local dealer has one for $203....but they say it's lead acid and that it's the only one they stock for the car.

I definitely want AGM when the time comes....but the dealer has me confused.