Replacing A BMW Battery

You would think that replacing a battery in a car would be one of the easiest things you could possibly do. Not so for the BMW apparently. I have a 2005 525i E60 BMW, and in my research, for replacing the battery, I came across a tonne of conflicting information.

Supposedly, BMW’s have a smart charging system that determines the best output for your battery, ensuring that its life is prolonged. Honestly, given how expensive everything else is with a BMW, I believed it at first.

I even called a couple of BMW dealerships, and they also told me that batteries need to be installed by BMW. The prohibitively high cost of the battery and installation is a deterrent. I did not know who to believe. I can only imagine BMW probably even advise against changing your own oil.

In BMW’s apparently the battery has to be registered with the car. The computer needs to be told there is a new battery or it will assume the older and potentially faulty battery is still there, possibly overcharging it and shortening its life. Or not charging it appropriately and shortening its life by running it too low.

What really struck me as strange is a charging system that is based off a battery being registered as opposed to reading it in real-time and determining charging rate and level. Considering everything else is so well designed, I refused to believe that BMW would create a flawed charging system based on a new battery needing to be registered.

Then I encountered another potential roadblock. Apparently completely disconnecting the battery from the car is bad for the electronics. Supposedly while you replace a battery, power is meant to be maintained to the car, or bad things can happen.

And on top of that, the car needs to be re-initialised (some reporting the active steering as well).

To replace or not to replace? That is the question.

This put me in an interesting situation. Do I risk replacing my own battery, potentially costing me hundreds or thousands to fix, or do I take it to BMW, get reamed on the price and have peace of mind if something screws up they are responsible?

Then I remembered a while ago I bought the manuals for my car by Bentley Publishers. Two massive books that detail every aspect of the vehicle (including wiring diagrams). In the section that details replacing and swapping the battery, it mentions that it needs to be registered with the car using a BMW tool.

See below for some photos I took of the pages in the book. Click on the images to see way larger versions of them for more detail.

The manual all but confirms that the charging system in the BMW seems to store voltage settings and the state of the old battery. What many thought was a myth or money-grab from the BMW dealerships turns out to be true.

Still conflicted, I did what any reasonably curious hacker would do, I took the risk and bought my own battery. I ended up choosing the cheapest battery I could find at a whopping $269 for the Century DIN92LMF rated at 735 Cold Crank Amps (CCA).

I pop open the boot and look for the panel on the passenger side underneath the boot covering. I then carefully remove the existing battery terminals and lift the old heavy battery out. So far, so good.

One thing I discovered is BMW’s certainly have a lot of tech in them. On-top of the battery, it is a bit of a mission getting the connectors off (there are wires everywhere) and then keeping everything out of the way while you put the new one in.

It might be helpful if you have someone with you handing you the required tools and holding the terminals/cabling out of the way while you remove the old battery and put the new battery in.

Starting the car

The moment of truth. Will the car start, will I get error messages telling me to take the car to my dealer, or will nothing happen at all?

The car starts. There are no error messages, and everything appears to be functioning normally. Heck, I only needed to set the date and time again, and the radio worked fine as well.

Lately, my air conditioning had been doing some weird things. One moment it would be incredibly strong and blowing a tonne of cool air, the next the power would weaken what felt like 50%.

I assumed maybe the AC just needed a service, but since replacing the battery, the AC has been working at full strength. Then I realised that when a battery goes bad, the BMW intelligently limits power usage to ensure you don’t get stranded anywhere with a flat battery.

Conclusion

I conclude that changing your own battery is not the end of the world, but would its life be shortened? Obviously determining this in the short-term would be impossible. You only need to program the battery into the car if you’re changing the type of battery being used, if it is the same battery, then programming is not required.

I am planning on taking weekly measurements of the car battery before I start it and comparing voltage levels. I am not saying that you should do what I did and replace your own battery, but I am saying that I encountered zero issues doing so.

I couldn’t find conclusive proof that my new battery needs to be programmed. Apparently what happens is the car needs to have its recharge voltage rate reset, or it could not optimally charge up your new battery. This seems to go against basic electronics, but whatever, BMW are the experts.

Apparently BMW’s keep batteries around 80% capacity and never 100, so it will be interesting to see if the voltages remain consistent or dip. It is almost summer here in Australia, so probably the best time to find out as no cold weather will hinder temperature values or capacity.

It is also worth noting I installed the same type of lead-acid battery. I considered an Absorbent Glass Mat (AGM) because they last longer and they’re lighter, but also more expensive and definitely would require registration in the car.

Do not change battery types without registering.

If after a few weeks I notice the battery is being impacted, I will take it to get registered. If not, I’ll keep things as they are. You can’t deny the $250 to do it yourself versus potential $450 or more for BMW to do it is considerably cheaper if you’re on a budget especially.

Update October 2017

I sold the car earlier this year, but the car never skipped a beat since I changed the battery myself without registering. I saw no voltage issues or other problems as a result of not registering the battery. So, who knows what registering really does?

It is worth pointing out that I replaced the old battery with a new one of the same type and CCA’s. If you increase or decrease the CCA’s of the battery or put an entirely different type of battery in, things could be different.