hdsalinas

The other day my cousin had to get his BMW towed to the dealer because his battery ran down.

I wondered why couldn't he just replace the battery by himself or jump start the car. He told me that you need to take it to the dealer because besides replacing the battery, they reprogram the computer.

This is obviously not necessary in regular cars but I was wondering if the day my battery dies, or I need to disconnect the battery, can't just I replace/reconnect it like any other car? Do I need to take it to the dealer?

w204_Generic

Funny that this thread was started. My battery yesterday went out because I was installing LED interior lights. I figured that since LEDs are lower wattage use my battery wouldn't go out....least to my understanding it went out after 20 minutes.

I just jump started the car and let the car run for about 30 minutes for the battery to charge up, it is fine. I see no errors or anything that needs to be reprogrammed...I don't know why your friend got his car towed to the dealer when they are just going to do the same thing I did.


If it had to do with a battery replacement, I don't know if this is right, does not need to be reprogrammed. Sure you will loose the correct time and settings in your car, but you can easily set them in the cars settings.

Dr Matt

Only things I've had to "reprogram" on mine after a battery disconnect is the window and sunroof auto open & close. Instructions for that are in the manual.

Only important thing is remember to disconnect the ground first but hook it up last. That way if you accidently touch your wrench to anything (other than the opposite battery post) you don't create a short and associated cool looking sparks.

C300CA

I am not sure if something like below is on the market:
Get a 12V source, like 8 x 1.5V battery in series, also a 10 ohm resistor in series. Connect this backup battery in before the car battery is disconnected. This way no "programming" is needed. 10 ohm resistor is to prevent too high current going between two batteries. Say one battery is one volt higher than the other, so the "between" current is 100 mA which is ~ a typical standby current. I measured a few cars and got that 100 mA.

NJProTech

Battery can be replaced like any other vehicle. In and out. Easy.

thenew3

On modern vehicles with lots of electronics, you need to supply constant 12v in order to maintain some computer settings.

On my previous VW for example, to replace battery, you had to connect an alternate 12v source, otherwise the electric power steering (among other things) needs to be reprogrammed by the dealer. It even tells you in the manual that to replace battery, you need to connect an alternate power source.

The reason w204_generic didn't have any issues is because his battery wasn't completely dead, it still had enough juice for the computer to run/retain memory, it just didn't have enough juice left to crank the engine. thus a jump fixed that. But I'm guessing the OP's cousin's battery was completely dead, thus the computer and all other electronics lots power as well and had to be reprogrammed.

NJProTech

VW. BMW. Is anyone here listening to the OP. He has a Benz. There is no need to connect a power source when replacing battery.

thenew3

Hello? You are not following the logic here... Since modern VW and BMW's require an alternate power source when the battery is being replaced, it is therefor likely that MB is the same way.
Google around, there are several documents that state that MB require alternate power source when replacing battery or else you would need to have several system re-adapted/adjusted.

AZN Optics

MB doesn't need to reprogram for the battery. Your cousin is right about his BMW though as BMW batteries need to be registered with the ECU so that it recognizes a new battery so to properly charge.

BTW, I've disconnected the battery on the C (for indoor car shows) and plugging it back in with no issues after sitting physically disconnected for several hours. Settings are stored in physical memory and not wiped.

nyca

I'm glad I read this thread - I would never buy a BMW that I couldn't replace the battery on without having it towed to a dealer. That is insane.

NJProTech

Master Tech for Mercedes Benz. 10 years. I guess I'll repeat it one more time. NOT REQUIRED TO CONNECT EXTERNAL POWER SOURCE. Can someone with more factory and dealership experience than myself please back up what I'm saying!!!

thenew3

Master tech doesn't mean you're doing it right. Look at how many "certified master techs" at various dealers that mess up people's cars?

I verified with my uncle who has worked as an engineer for MB for over 30 years that it is recommended to have alternate power source when doing battery changes.

I've experienced first hand with a W203 that would not start after replacing the battery and had to be towed to the dealer for a reprogram.

w204_Generic

Key word in this sentence is "recommended" but not required/mandatory.

C300CA

Could there be differences in design between W203 and W204?

Hish8288

Is there any risk for disconnecting battery for 2 days (c240-2002) on the computer ? Can i just reconnect it and drive safely or i have to take it to the dealer ?