Mito
Posts: 27
Joined: Sun Apr 09, 2017 11:55 pm

BMW i3 auxiliary battery sizes

Wed Apr 26, 2017 1:09 pm

Having had the auxiliary battery fail on the i3 I wanted to fit the upgraded 40ah version rather than the standard unreliable 20ah Version.

The standard black EPM 20ah battery S 61 21 7 623 736 is listed at about £70 but the bigger 40ah battery 61 217 635 788 is only about £50. So a no brainer to get the bigger better battery for less money you would think?...

However the battery strap and tray do not seem to accommodate the larger battery and I am told by the dealer that the larger battery is not a UK option however the Etkbmw.com parts site list it and call it a " Warranty " version and BMW in the UK will supply it to part number so presumably it fits a 1 series or something.

Does anyone else have the larger translucent 40ah battery and if so what country are you from?

alohart
Posts: 1494
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: BMW i3 auxiliary battery sizes

Wed Apr 26, 2017 2:16 pm

Why would you want a larger, heavier, flooded-cell battery rather than a smaller, lighter absorbent glass mat (AGM) battery? With no 12V starter motor, why would you need a 40Ah capacity when 20Ah is apparently sufficient for all new i3's?
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

jadnashuanh
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Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: BMW i3 auxiliary battery sizes

Wed Apr 26, 2017 3:57 pm

The charging routine for an AGM battery is different than one for a typical lead-acid battery. When BMW sweated every gram of weight on the car's design, there obviously were some compromises, but the vast majority of people get by fine with the stock battery. That new battery may not have the same life since an AGM typically uses a slightly higher charging voltage that would shorten the life of a typical one. On some BMW models, you can adjust the charging routine depending on the battery installed, but I do not know if they are accessible with the right software on the i3.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

Bawareca
Posts: 48
Joined: Wed Apr 12, 2017 6:40 pm

Re: BMW i3 auxiliary battery sizes

Wed Apr 26, 2017 7:05 pm

jadnashuanh wrote:The charging routine for an AGM battery is different than one for a typical lead-acid battery. When BMW sweated every gram of weight on the car's design, there obviously were some compromises, but the vast majority of people get by fine with the stock battery. That new battery may not have the same life since an AGM typically uses a slightly higher charging voltage that would shorten the life of a typical one. On some BMW models, you can adjust the charging routine depending on the battery installed, but I do not know if they are accessible with the right software on the i3.


My car came with a completely dead battery and that upset me a bit (10k miles). Browsing REaloem I found the upgraded 40Ah battery and instead of buying another laptop battery (the original one :P ), I bought the upgraded without doing much research. At the end I had to modify the battery tray and I had to change the position of the windshield washer reservoir.
There has been many cases when the main battery shuts down completely at low state of charge and the small "laptop" battery will be sufficient for only a few minutes before it is completely exhausted, let alone to start the REX. The 40ah battery has some real energy stored in it and if one needs it only 1 time it is worth it (for me).
From what I see there is no "charge routine". The moment the car wakes-up 14-14.5V from the main battery thru the DC/DC inverter is supplied to the 12V system, to the moment the car goes to "sleep". Actually the original AGM battery is very fragile when fully discharged (partially because of the low capacity), lead battery should fare much better in these conditions.

jadnashuanh
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Location: Nashua, NH USA

Re: BMW i3 auxiliary battery sizes

Wed Apr 26, 2017 7:51 pm

AGM batteries are much better at deeper discharge than conventional lead-acid ones. An AGM battery IS a lead-acid battery, it's just that the electrolyte is different. The logic to determine if the battery is fully charged and how to do it is different between conventional chemistry and AGM batteries.

When the i3 needs the REx, it uses the main batteries, not the 12vdc one, to start it, although, the fuel injection and ignition system is (I think) still run off of the 12v battery. IOW, there's not a huge load on the battery at any time.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

Bawareca
Posts: 48
Joined: Wed Apr 12, 2017 6:40 pm

Re: BMW i3 auxiliary battery sizes

Wed Apr 26, 2017 10:35 pm

jadnashuanh wrote:AGM batteries are much better at deeper discharge than conventional lead-acid ones. An AGM battery IS a lead-acid battery, it's just that the electrolyte is different. The logic to determine if the battery is fully charged and how to do it is different between conventional chemistry and AGM batteries.

When the i3 needs the REx, it uses the main batteries, not the 12vdc one, to start it, although, the fuel injection and ignition system is (I think) still run off of the 12v battery. IOW, there's not a huge load on the battery at any time.


Obviously there is no charging logic here. The original 12v battery is so small that if it is leaved without charge for 5 minutes(with the car running), it will be completely discharged.
The AGM batteries are by presumption "Deep cycle", but there are many documented cases of dead i3 12v batteries, and they are mostly 1-3 years old. OTOH I have recovered many AGM non-AGM batteries from 0 volts. In the case of the i3 the problems are related to the size/capacity of the battery and not to the type of the battery. This is my personal, non-scientific opinion.

alohart
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Re: BMW i3 auxiliary battery sizes

Thu Apr 27, 2017 1:37 am

Bawareca wrote:The original 12v battery is so small that if it is leaved without charge for 5 minutes(with the car running), it will be completely discharged.

If that happens to your i3, it has a serious abnormal 12V load that needs to be fixed, or the 12V battery is bad. For a healthy 20Ah battery to be fully discharged in 5 minutes, the load would need to be 240A! But if the 12V battery is unable to hold a charge, is partially short-circuited, etc., it could discharge quickly under the conditions that you describe. But that's not normal behavior for an i3.

Bawareca wrote:The AGM batteries are by presumption "Deep cycle", but there are many documented cases of dead i3 12v batteries, and they are mostly 1-3 years old.

I suspect that many of these failed 12V batteries were allowed to discharge completely by normal parasitic loads while the car was sitting unsold somewhere. A complete discharge can permanently damage a lead-acid battery.

Bawareca wrote:In the case of the i3 the problems are related to the size/capacity of the battery and not to the type of the battery.

Our Honda Insight has a 16Ah AGM battery that is 7 years old and still going strong. That battery has even less capacity than the i3's 20Ah battery. Like the i3, out Insight does not use a 12V starter motor, so the 12V battery powers only its computer modules and accessories and is able to do so without being seriously discharged due to a lack of capacity. So all evidence suggests that a healthy 20Ah battery is sufficient for an i3.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

Bawareca
Posts: 48
Joined: Wed Apr 12, 2017 6:40 pm

Re: BMW i3 auxiliary battery sizes

Thu Apr 27, 2017 8:27 am

alohart wrote:If that happens to your i3, it has a serious abnormal 12V load that needs to be fixed, or the 12V battery is bad. For a healthy 20Ah battery to be fully discharged in 5 minutes, the load would need to be 240A! But if the 12V battery is unable to hold a charge, is partially short-circuited, etc., it could discharge quickly under the conditions that you describe. But that's not normal behavior for an i3.

That was just an example. The fact is that many i3 owners have used the emergency charging port opener, and that means that the 12v battery was flat. In my opinion this will not happen with the 40ah battery. This fact alone for me is enough to justify 7lbs more for a 40ah battery.

alohart wrote:Our Honda Insight has a 16Ah AGM battery that is 7 years old and still going strong. That battery has even less capacity than the i3's 20Ah battery. Like the i3, out Insight does not use a 12V starter motor, so the 12V battery powers only its computer modules and accessories and is able to do so without being seriously discharged due to a lack of capacity. So all evidence suggests that a healthy 20Ah battery is sufficient for an i3.

Not sure what both cars have in common. With i3, if your 12v battery is dead, your car is dead. You may have 100% charge on the HV battery, but no way to operate or drive the car.
My 1988 BMW 325i will start right away after sitting for a months and the battery is only 45Ah. Most modern BMWs, and most modern cars for that matter, would be dead in the water after a month sitting, and they have much bigger batteries.

jadnashuanh
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Location: Nashua, NH USA

Re: BMW i3 auxiliary battery sizes

Thu Apr 27, 2017 2:49 pm

Apples and oranges...a 1988 MY vehicle has nowhere near the computers and electronics that more modern cars have! When you shut one of those off, it's essentially off except for maybe a clock. That's not true with most modern cars.

The i3 has a transport mode they use when shipping from the factory that essentially does shut pretty much everything off in the vehicle. Once it gets to a dealer and is prepped for sale, that mode is typically disabled, and all of the electronics are now running. If the car sits there for long enough, the 12vdc battery can become discharged. Throw in the possibility that the car may have been stored when the temperatures got quite low, and that just makes a low battery more susceptible for damage and potential failure. AN AGM battery is no less prone to damage when cold than any lead-acid battery.

FWIW, a typical lead-acid battery does not like 14.5vdc going into it...most AGM batteries will work fine with that. A normal lead-acid battery much prefers slightly less. On my other BMW, there are service menus that must be set to both tell the car the size, type, and install date so that the computer can adjust how to treat the battery to maximize its life. I'd be really surprised if the i3 didn't copy that logic. An older battery, in that scenario, is treated differently by the charging logic than a newer one, as are those of different capacity and type.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

alohart
Posts: 1494
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: BMW i3 auxiliary battery sizes

Fri Apr 28, 2017 2:12 am

Bawareca wrote:That was just an example. The fact is that many i3 owners have used the emergency charging port opener, and that means that the 12v battery was flat.

Maybe, maybe not. It could also indicate that the latch mechanism has failed (assuming that this latch mechanism requires 12V power to function).

Bawareca wrote:Not sure what both cars have in common. With i3, if your 12v battery is dead, your car is dead. You may have 100% charge on the HV battery, but no way to operate or drive the car.

That's one thing that our Insight and our i3 have in common: they both require sufficient 12V power to boot their computer modules so that they can enter a drivable mode, even if their propulsion battery packs are fully charged. But their 12V batteries do not have to have enough capacity and cold cranking power to allow the car become drivable as do cars whose engines are started by a 12V starter motor. So an i3 and an Insight do not need large, heavy 12V batteries to function well.

But if you want to modify your i3 so that a large, heavy 40Ah 12V battery will fit, that's your choice. My point was that doing so isn't necessary and, in my opinion, would be a waste of money and effort to solve a non-existent problem. You obviously disagree which is fine.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

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