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According to BMW dealership the new after-market battery must be registered to the on-board computer, because, otherwise, the car would think that the old battery is still in and it would end up being charged the wrong way.

So this morning out of curiosity I decided to do a test on my friend's BMW X3 that she had been driving for a year with unregistered, after-market battery. Here is what multimeter indicated:

  1. In the morning before even starting the car the voltage was at 12.0V
  2. After starting the car the alternator started to charge battery and voltage increased to 14.0V
  3. I kept driving the car for 30 minutes without stopping it and the voltage across terminals was still at 14.0V (alternator was still charging it)
  4. Then I stopped the car and finally was able to measure the voltage of the battery and it was at 12.4V. I touched the battery and it was cold.

So where exactly is the problem with not registering battery with on-board computer? Some ideas:

  1. Would the car eventually stop to charge battery while it was still on if it was registered with car (i.e. readings in step #3 would have to be <14.0V)?
  2. Also, isn't the morning voltage of 12.0V too low (e.g. three days ago I had to jump start her car, but at that time I chalked it up to the salon fan that was on while engine was off last night.)? Maybe the battery has already started to degrade?

Update #1 Voltages were measured when it was 52 Fahrenheit degrees outside.

  • For fun you can use the "secret menu" to watch the system voltage in real-time if you're curious. Every time I let off the gas the system voltage drops momentarily on my E90, I assume to check the battery condition, but I can't say for sure. – Lathejockey81 Dec 13 '16 at 18:19
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BMW uses a proprietary software to control the charge rate of the AGM batteries in their vehicles. The computer calculates the needed charge rate based on the age and condition of the battery. BMW owner forums are full of comments of owners that just replaced the battery with an aftermarket battery and had no issues. Their evidence may be anecdotal. I would review some comments and make a decision based on the comments that reflect conditions similar to your own.

  • But what kind of advantages this system has – method Dec 12 '16 at 0:04
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    The advanced battery in BMWs are expensive, about twice the cost of a conventional battery. They can be smaller than a conventional battery with the same output. The computer controlled charging theoretically makes the battery last longer, some claim 8-10 years. Fewer dead batteries in the environment and a lighter vehicle. – mikes Dec 12 '16 at 2:28
  • BMW has the IBS (Intelligent Battery Sensor) in several vehicles without AGM batteries as well. It's that sensor which is the reason for recommending registration, because the computer develops a profile of the battery using that sensor. The AGM batteries and the battery sensor are not related, and AGM batteries are common in non-hybrid cars which shut the engine off at stops (i.e. most modern European cars) – Lathejockey81 Dec 13 '16 at 18:16
  • As an anecdotal experiment... I did not register the new battery and in 2 years and 5 months it was not capable to crank the car anymore. O'Reilly battery test proclaimed it faulty. It was a $140 battery though. – Hans Solo May 24 '18 at 22:55
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BMW uses the DME module to charge the battery unlike most cars that depend on the voltage regulator.

On a 2002 and newer BMW the battery needs to be registered so that it is charged properly. Otherise the battery may get overcharged or undercharged.

There are plenty of scanners on the market that will allow you to register a BMW battery yourself. For suggestions and step by step instructions search " BMW Battery Registration by Youcanic"

If you are totally broke and don't want to register the battery, my recomandation is to install a new battery that has as close as possible specs as the original battery. Also don't switch battery type from AGM to Lead acid. All the specs of the old battery are printed on the label on top of the battery.

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