How large a battery is needed to "jump start" (i.e. wake the computer)?

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JohnnyCakes

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Joined
Jun 3, 2023
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There's another thread from a member indicating that his battery died and he had to have AAA come to jump start it.

I've always carried battery jump packs in my vehicles "just in case" I end up with a flat battery. Obviously, those have to be pretty big (cold cranking amps), since they have to crank the ICE. The one I carry in my ICE vehicle has 425 cranking amps.

That doesn't seem necessary for the i3. What is the amps needed?
 
there is no specification for this as when the 12v battery is dead/dying and will not hold a charge, any 12v source you apply to the terminals is going to first go into the battery get turned into heat. A jumper pack must overcome that in order provide enough voltage and current to the car to allow it to wake up so you can power the car on. The OEM battery is rated at 300CCA but since the car does not "crank" it is sort of a meaningless spec.

A dead battery in an ICE vehicle usually means a jump and then charge the battery because "oops I left the lights on" or something.
A dead battery in the i3 means the battery is toast and will not hold a charge.

You might be better off investing in a Bluetooth 12v Battery Monitor for $30 and attach it to the battery terminals. Then monitor it periodically with the BATTERY MONITOR app. I learned what "normal" was over time, and when it started acting "abnormal" it would self-discharge to 12.0 volts, then the car would come on by itself to boost the 12v for exactly one hour. When the 12v on my car did this, I let it continue for 2 weeks while monitoring it before changing the battery so it did not leave my wife stranded. It is a lot easier than hauling around a Jumper Box all the time.
 
A dead battery in an ICE vehicle usually means a jump and then charge the battery because "oops I left the lights on" or something.
A dead battery in the i3 means the battery is toast and will not hold a charge.

Thanks for the reply -- not sure I understand.....

Is it not possible to leave the lights on or some other load/drain on the battery? That's the situation I was asking about. Battery is otherwise fine but runs down due to operator error.
 
Please re-read the last paragraph in my comment.
It not possible to "run down" the battery due to operator error on this car UNLESS the 12v battery is also faulty

added: here is a graph from my Bluetooth Battery Monitor that shows my failing 12v battery self-discharging rapidly over 24hrs and what the car does to compensate. In this case, the car is sitting in my garage for 24hrs untouched.
BpLjkV6.jpg
 
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this is my read out, yep, just replace it when it gets down to 12.2V in my opinion
 

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I've bought a plug in voltmeter for the cigarette lighter socket of my 2018 i3 94Ah. My observations are that immediately after pushing the remote control to open the car, ~14 volts are on the cars low voltage grid. And even after closing the car remotely this voltage remains for a while. In other words it seems that if you have enough power on the car to react to the remote control, it will start and behave normally. If it does not answer, I would suggest to open the door and the frunk manually and supply the battery with 12 volts. As a dead because old battery usually does not take a lot of current to load, any small LiOn or LiPo battery jumper will do. But I would not start the car thereafter pressing the starter button. Instead my first action would be to open it with the remote control, to have the above described situation with the 14 volts on the low voltage grid.
 
IMO you have to be a bit pro-active to enjoy owning a vehicle like or similar to the I3. Otherwise you get to the point where you want to carry a spare tire, a jack, tools, a jump box, spare charger, maybe a spare battery, replumbing the frunk to carry extra fuel. Fear destroys enjoyment.
 
no reason for me to. You do what you want for your own comfort level.
Well, yeah, but I wanted to make sure that I understood your point that based on the technology used in the i3, there would be no use for a jumper box. I *think* that is what you are saying?
 
Well, yeah, but I wanted to make sure that I understood your point that based on the technology used in the i3, there would be no use for a jumper box. I *think* that is what you are saying?
I have driven 2 i3's since 2014, both of which had 12V batteries that failed and were replaced. I never needed a jumper box. Instead, I installed a 12V system monitor like EvanstonI3 and Simoni3 which showed me that the 12V batteries were weakening before they failed. Rather than buying a jumper box, buy and install a 12V system monitor.

BMW warns against jump-starting an i3 because the DC-DC converter might be trying to charge the 12V battery when the jumper box is connected. If the jumper box's output voltage exceeds that of the DC-DC converter, current might flow into the DC-DC converter which might not be good. The DC-DC converter is part of the electric motor electronics (EME) module, the most expensive module in an i3. I would not want to risk damaging it.

In an emergency, a jumper box could be attached to the 12V battery after the high-voltage disconnect is disconnected which would prevent the DC-DC converter from being on, but would also prevent an i3 from entering drive ready state. The jumper box could only charge the 12V battery which might be sufficient for an i3 to enter drive ready state after being charged for a while.
 
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