Drivetrain Brake System Battery Discharge Warnings

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Pepi3

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Aug 16, 2021
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My i3 is my daily commuter to work. But last week it was unused for 5 days with a full charge. I tried to drive it after that and the car didn’t respond to the remote FOB. MyBMW app shows that it has issues with the drivetrain, brake system, and battery discharge. It won’t even start. Any ideas before I have it towed for BMW service?
 
Likely a discharged 12 V battery which can result in the setting of many bogus diagnostic trouble codes. I would try charging the 12 V battery before ordering a new one or paying to have it replaced. However, because you commute regularly, the 12 V battery is likely bad and needs replacing. It might hold enough charge to allow your i3 to start so that you could drive it to any BMW mechanic (you wouldn't have to pay a BMW dealer's high prices for a 12 V battery replacement).
 
Thank you for your reply. How do you charge the 12v battery and where can I find this in the car?
 
I decided that DIY’ng the 12v battery will cost me more than just having it towed to the BMW service center, it cost about $750 total to fix but I got a 3 year warranty on the hardware, the old 12v lasted 8 years and 44,000 miles.
 
I decided that DIY’ng the 12v battery will cost me more than just having it towed to the BMW service center, it cost about $750 total

Just an FYI for newbee's seeing this - DIY i3 12V battery change out is usually under $200.

https://www.youtube.com/watch?v=RLZO7pB_uVg

OEM 'AUX 18 L ' batteries are available from:

1. BMW Dealer (often pricy)

2. Local East Penn distribution warehouse - USA and Canada (East Penn makes the "AUX 18 L' OEM battery under BMW, DEKA, and REMY labels).
https://www.eastpennmanufacturing.com/contact/where-to-buy/

3. Online: https://remybattery.com/start-stop-aux18l-auxiliary-battery.html
 
I learn more and more about this car everyday reading this stuff. But I am stuck by the thought of why is a 12V battery even in the car? It seems as if the existing battery pack, and if necessary some type of converter, would be a much better engineering solution.
 
2020i3 said:
But I am stuck by the thought of why is a 12V battery even in the car? It seems as if the existing battery pack, and if necessary some type of converter, would be a much better engineering solution.
A DC-DC converter provides 12 V to the low-voltage system when the high-voltage system is on. However, leaving the high-voltage system and DC-DC converter on at all times would waste a considerable amount of energy. A simple, proven 12 V battery seems to be the solution for all EV's.
 
Thanks for explaining that, but the logical me still thinks one day the main battery pack will be the sole power source. But things probably won't change until Musk throws a tantrum and sends his engineers back on to the floor until a viable solution to do so has been found. Right or wrong view, less points of failure, in my eyes.
 
2020i3 said:
Thanks for explaining that, but the logical me still thinks one day the main battery pack will be the sole power source.

Another thing having a separate 12v power source allows is that certain safety checks can be performed at "low power" before engaging the 390v contactors to the main battery.
 
It still strikes me as odd that BMW's designers chose an ancient lead-acid battery for such an advanced car...

Surely a small lithium-ion 12V pack of similar capacity would have been a better solution!

Chris
 
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