When i replace the starting battery on my 2011 E350, will the auxiliary battery keep the computer system supplied with power so I don't have to worry about losing codes, etc? Or should I put a back-up power supply onto the power leads while I swap out the battery?

 

No. The aux battery is disconnected from the car when ignition is off.
But what you are worry about? I had my battery disconnected for storage on my 2014 model and don't recall need for any codes? I think only windows need resynchronization and clock might not reset from cellphone.

 

OK, thanks... what battery would you recommend I get to replace the OEM battery?

 

Last time I was battery-shopping PepBoys had best deal on 96-months Bosch flooded for around $100 and if you want AGM, they had it to.

 

In 2016, Consumer Reports rated the Bosch batteries as one of the worst. Read this thread for more info on how to change your battery. I recently changed mine with a Duracell Platinum AGM from Sam's Club for $180.

 

I would close all the glass so that you don't have to worry about retraining the auto open/close. After you reconnect power, only thing you should have to adjust is the time.

I would shop around for AGM batteries and look at coupons, rebates and discounted gift cards to auto parts store. Many folks have used store brand - just make sure it is the correct size and CCA and you should be fine.

 

That's what the guarantee is for.
I bought the Bosch for W210 5 or 6 years ago. Had a coupon at the time, so it cost me about $90. The first battery start failing after couple of weeks and PepBoys gave me another one.
Eventually I sold W210, but kept the battery and presently it sits in our 320 Bluetec. We live in warm climate, so GP don't take lot of current, but presently I played with scanner for almost 1 hr, using battery power with engine off/ignition on and the car started just fine.

 

That's what the guarantee is for.
I bought the Bosch for W210 5 or 6 years ago. Had a coupon at the time, so it cost me about $90. The first battery start failing after couple of weeks and PepBoys gave me another one.
Eventually I sold W210, but kept the battery and presently it sits in our 320 Bluetec. We live in warm climate, so GP don't take lot of current, but presently I played with scanner for almost 1 hr, using battery power with engine off/ignition on and the car started just fine.
Previously I bought Kirkland (COstco) battery with 100 months warranty and it gave ghost at 50th month.
Costco gave me 1/2 price back .