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Discussion Starter #1
I have read quite a lot about 12v batteries on early cars failing after about 3 years (the UK warranty period).

Is it still the case that these batteries often have a pretty short lifespan or has the OE battery been improved / upgraded in any way in recent times?
 

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The best there is at what I do
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My 2015 model is still running with the original 12V battery so I think they last a bit longer than 3 years.
 

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To maintain the life of any battery it is imperative to control the charging regime - current and voltage. Clearly for the traction battery there is a very sophisticated system on all EVs but from what I have read, the charging of the 12 volt battery seems rudimentary - possibly a fixed voltage with some current limit. Also, the 12 volt battery has a lot of functions. Maybe early models were worse and there has been some improvement in the way the 12 volt system operates?
 

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2020 BMW i3S 120Ah BEV
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It gets charged from the HV battery when drive is engaged. It doesn't need to be sophisticated it just needs to get a charge. I don't know what battery tech it is, whether it's bog standard lead acid or AGM or something. I guess you can pull the battery state from the diags port?
 

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Yes it needs to get charged but if the charging system is rudimentary then the life of the battery will suffer. The best stand-alone chargers have several steps of constant voltage, constant current etc. This isn't practical on a vehicle but some basic control is important. The i3 battery provides a lot of power to auxiliary services so the management of this and the charging regime matters. Maybe BMW got it wrong on early vehicles?
 

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Kona 64
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I would say that most if not all manufacturers have got it wrong. You will find plenty of threads on here for other makes. It is a problem affecting maybe 5% of EV’s. EV’s are generally fitted with high CCA (cold cranking) batteries needed to chuck out loads of amps, batteries designed for ICE.cars. An EV needs volts not amps to trigger the relays, once the voltage drops you are stuffed.

I no longer leave the headlamps (LED) on Auto but turn them off before turning ignition off.it is scary to see the voltage drop as you turn the ignition on (simple voltmeter plugged into ciggy lighter) and once it goes below about 11.5V thats it. Not able to start car.

i trickle charge the 12v battery every month.
 

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I was going to ask whether sticking it on a Smart Charger every so often will prolong battery life. Realise it probably does not matter to a lot of people on here getting an i3 on a 2 or 3 year lease - but hopefully I'll be keeping mine longer (that is, when I get it!! Apparently it docks at Southampton tomorrow - which I guess is where it will sit a while...….)
 

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... I don't know what battery tech it is, whether it's bog standard lead acid or AGM or something. ...

The 12v batteries in the i3 are AGM, glass mat, and are designed for start-stop vehicle applications.

Yes it needs to get charged but if the charging system is rudimentary then the life of the battery will suffer.

I don't know how "rudimentary" the charging system is, but no matter what, the i3 takes a toll on the 12v battery because it's constantly in use for just about everything. It can get depleted quickly, I'm sure the i3's BMS normally has to fast charge it.

Question is, does the i3's 12v get fast charged all the time, or are there times of lower demand when it slow charges? I can't imagine how, since once the i3 is parked, the HV battery is cut off from the 12v battery,

Heck, even in my wife's car with a start-stop engine, the battery gets a break whenever the motor is running, so has the opportunity to be charged more leisurely.


I was going to ask whether sticking it on a Smart Charger every so often will prolong battery life.

I wonder. BMW says not to, but I don't know their reasoning. There are battery conditioners with an AGM mode, making me curious if there's something more that can be done, yet I regard half of them as snake oil with outrageous claims. But yet I realize that the 12v battery gets loads more abuse through constant cycling than your run-of-the-mill ICE 12v battery, so I'm tolerant of the typical relatively short 3-4 year lifespan.
 

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2020 BMW i3S 120Ah BEV
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In principle the i3 can charge its 12V battery any time it likes, whether the car is in Drive Ready mode or not. I imagine it chooses not to unless there's a risk to the condition of the 12V battery though. I suspect they have battery condition monitoring on the 12V side too so the system is aware of the battery condition.

You can probably fit your own battery state monitor as well if you want, but batteries are ultimately consumable, particularly with the degree of gadgetry which runs after switch-off of the main drive train on modern cars. You consume the chemistry of the one you've got, then you pay for a replacement and your old one goes to be recycled into new ones (assuming I remember to take it out the shed one day..).
 

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In principle the i3 can charge its 12V battery any time it likes, whether the car is in Drive Ready mode or not. I imagine it chooses not to unless there's a risk to the condition of the 12V battery though. I suspect they have battery condition monitoring on the 12V side too so the system is aware of the battery condition.
You're right, my mistake. I always think of the 12v system being the "gatekeeper" to the HV battery, but forgot that connecting a charging cord to the charging port can bring both the HV and 12v batteries back to life. So there's obviously some 2-way negotiation incorporated.

And as a follow-up to the earlier conversation w/ Andrew re: 12v capacity, I found a reference in the 2014 training guide that specifically calls out a 20Ah 12v battery. So at some point was something higher introduced? Because 20Ah is the sole battery I see available.
 

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When I bought my 2014 REx last year, I asked the dealer to check the battery statuses and have the output from the BMW Integrated Services Technical Application. It says that the last battery change was at 0 km, the current battery charge state was 98% and mentions that the startability limit (set by BMW testing?) is 38%.

So, batteries can last a long time and the startability limit is quite low (before a battery replacement is required?).
 

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That seems suspicious since it's not rare for the 12v to fail after 3 years, but to be at 98% after 5 years doesn't pass the sniff test. I'd bet it was replaced but not registered.

I wonder (out loud, again) if the BMW has this 38% serviceability threshold in mind, why can't this car monitor that and provide an appropriate warning before it's time to replace?
 

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You're right, my mistake. I always think of the 12v system being the "gatekeeper" to the HV battery, but forgot that connecting a charging cord to the charging port can bring both the HV and 12v batteries back to life. So there's obviously some 2-way negotiation incorporated.
Acually I do believe that unless the car is plugged in the 12V battery runs the computer and it is indeed the computer that is required to invoke the activation of the HV battery pack and the voltage converters required to start charging the 12V battery. So yes, if the 12V battery fully depletes then I suspect you won't be able to start the car no matter how much charge is in the HV pack ..

Perhaps this is why they recommend leaving the HV battery with at least 80% charge if you're going away for a while and not able to leave the car plugged in. My assumption is that the computer will tick on the 12V charging circuit periodically to keep the LV system topped up, but I've not seen this happen yet.

I was once asked to jump start a Prius at an airport car park. 12V system was dead, but the HV pack was fine (as was the fuel tank). In that case it was one step further - the petrol engine had to be running to charge the 12V system, it wasn't sufficient to put the HV battery live.
 

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I was once asked to jump start a Prius at an airport car park. 12V system was dead, but the HV pack was fine (as was the fuel tank). In that case it was one step further - the petrol engine had to be running to charge the 12V system, it wasn't sufficient to put the HV battery live.
IIRC for Toyota hybrids (at least 2012 onwards), the 12V battery is directly charged by the HV. Once in ready mode (HV online), the HV will do the 12V charging and start the petrol engine. The petrol engine would start immediately, if the HV battery was low or the heating was set to on.
 

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Well .. the guy had issues that he didn't fully understand, so he found he had to leave the petrol engine running because otherwise the dash would die (12v went flat). His HV reserves were fine. I had no interest in learning any more so left him to it!
 

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... if the 12V battery fully depletes then I suspect you won't be able to start the car no matter how much charge is in the HV pack ..

This is the section of the i3 Electrics Training doc that talks about bringing the 12v battery back to life via the charging port. Nice, crafty little fail-safe they built in!

p.52 https://www.leschroniquesdegoliath.com/wp-content/uploads/2018/01/Mercedes-01_I01-Product-PresentationGB.pdf

4.2.4.Emergency charging

It is not possible to operate the vehicle if the high-voltage battery and 12 V battery are not charged.

In order to put the vehicle back in to operation, the charging cable for charging the high-voltage battery must be connected. If necessary manually unlock the charging socket cover and connect the charging cable. Emergency charging is started automatically. The charging status is not displayed.

The 12 V battery is charged somewhat after a few minutes. The 12 V vehicle electrical system is ready for operation again. The high-voltage system can be started, the high-voltage battery can therefore be charged. The charging status of the high-voltage battery can be displayed again. Continue to charge the vehicle until the state of charge for the desired range is reached.

An additional AC/DC converter is located in the electrical machine electronics (EME) for the emergency charging function. This supplies the electronics of the EME if the12 V battery is discharged, thus enabling the start-up of the high-voltage system.With an active high-voltage system the 12 V battery is charged using the DC/DC converter in the EME by the high-voltage battery.
 
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