I have a 2012 E350 sport. The battery does hold a charge for very long. If I don't use it for 3-4 days It will not
start. So I have to put a battery charger on it for about 30 minutes. . Is there any special that I need to know before I attempt to change out the battery? On my old 97 S300 I needed the security code for the radio when I went to change its battery?

 

Nope. Just pull out the old and drop in a new one. You may have to reset your windows/sunroof, but I doubt it

 

If it's that weak, you probably need to have it on for a few hours. What's the size of the battery charger? With a dead battery, it took about 4-5 hours with an 8amp charger to charge it up. Nothing special about changing out the battery though. Pepboys has the Champion AGM battery which is a JCI/Clarious battery with a 4 year warranty. Pretty much all the other places like Autozone, Advance Auto are 3 years but they're also JCI/Clarious with AGM batteries.

 

Say, how old is your existing battery??

Disconnecting the main battery on these cars is actually an excellent thing to do.
After reconnecting power, the sleepless SAM computers are freshly rebooted and will stop draining the main battery 24x7 by entering low power normally.
Using a smart float charger is a good insurance to maintain lead-acid batteries.

Nothing special requires a reset code after a reboot.

 

I recently replaced my battery by going to a highly reputable tire and battery place. Did it in 15 minutes and nothing needed resetting and no special codes.
One thing I have learned is that the older a battery gets, you need to drive the car about 20 to 30 minutes periodically to charge the battery. Apparently just letting it run does not actually charge it that much.

 

a rather dead battery needs like all night with a 8 amp charger.

 

2012 MY doesn't have smart charging and I am not sure if the car will remember all the setting.
My polifted 2014 does remember everything, but the system keeps battery undercharged most of the time and constant charging/discharging shortens battery life.
I've got 3 years (short 1 month, so I lucked with warranty) from my last Bosch battery.
Since I don't drive the car every day, for >3 days stay I hook up battery maintainer. After that ECO stops will engage after 1 mile drive, when without maintainer takes several miles for the feature to work again.

 

A discharged battery isn't completely dead. They're usually something like 80-90 amp hour batteries so 5-6 hours at 8 amps is 40-50 amp hours, probably a little less than that.

 

lead acid battery charging is significantly less than 100% efficient. I'm not sure off the top of my head, but I think it can be nearly as bad as 50%, you can put 50AH in and get 25AH out.... ok (googled, skimmed a few articles), thats only in worst cases, overall its more like 85%... so you put 50AH in and get 50*.85 == 42AH out. its worse at very high rates of charge, like a 20A charger into a typical car sized battery.

a smart charger goes through three phases when fully charging a mostly dead battery (which for lead acid hopefully is never below 50% state of charge, because running them lower than that greatly impacts the battery lifetime), first a bulk phase of max amps until you hit max voltage (typically 13.6-13.8V), this takes the battery to 80%, then an absorption phase where you hold the voltage at an elevated level (typically 14.2-14.4V) until the current drops to near zero, this takes 3-4 hours and puts that last 20% in. the '3rd stage' is float, which is just maintaining the battery with a constant 13.2-13.6V.

btw, I'm really happy with my NOCO Genius 10 charger. a lot of smart chargers won't even try to charge a completely dead battery, if they don't see 6-9V up front, they won't even try. The Genius has a manual overide for this which yes, disables some of the 'safety' features, but jams juice into a totally dead battery til it picks up some of a charge, then goes into its normal 3-stage charge cycle like above. It also measures the batteries internal resistance, and only outputs 10 A for the bulk phase if the battery is big enough. The NOCO jump packs rock, too, I have the Boost Plus, which will start most any gas car, and pretty good sized diesels, albeit its not rated to start my 7.3 powerstroke.

 

Not to worry, I'm an electrical engineer. While we call a battery dead, it won't start the car because the voltage is low, but it's not completely dead probably just about half dead. Also as a battery ages, you're not going to get the full rated amp hours out of it. That's why it only takes around 5 hours to charge up a dead battery even though it's supposed to be an 80-90 amp hour battery.