07-13-2015, 10:17 AM | #1 |
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Why two batteries?
The new X5 M (F85) has two separate car batteries.
Does anybody have a clue why -- and which battery does what? What happens if the left or the right one, or both, fail? Do all F15 have two batteries, or only the F85 ? |
07-13-2015, 11:12 AM | #2 |
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X5m has 2 batteries, 90mah + 50mah. X50i has a single 90mah while X35i has a single 105mah (which is strange, but I bet the X50i has lithium vs standard - the maintenance scheduled of the X50i is less frequent on the battery compared to X35i). X35d has 2 batteries as well.
I'm very confident that it has to do with e-CAT (electronically heated catylic converters). During cold start, the first battery is used to power the starter (smaller 50mah battery) while the larger battery heats up the e-CAT and all else electronics. This reduced the car's stress from heating up the e-CAT after the voltage is drained from the single battery design, which requires the car to run at the higher RPM to re-charge the battery from a cold start. The F80 doesn't have an e-CAT. During cold start-up, the turbo wastegates are wide open and the engine RPM goes up + rich fuel to heat up the CATs. So the emission is worst on the F80 during a cold startup compared to an F85. This is also why the F85 sounds so much better at cold starts compared to my last F80. I wouldn't imagine that if one fails, the other will need to be replaced as well. They're independent batteries tied to the charging grid.
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Last edited by JNoSol; 07-13-2015 at 11:20 AM.. |
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07-13-2015, 02:35 PM | #3 |
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Thank you, JNoSol, for the really solid info!
On the X5 M (same as for the former X5 50i) on a cold start the engine revs higher, about 1100 rpm for some 20 seconds as long as you do not start rolling. I always wondered why, maybe it is just to reduce the heating e-CAT battery drain as you mention above -- if so, it may pay to wait for the higher rev routine to finish, to conserve battery charge. |
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07-13-2015, 09:21 PM | #4 |
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I think it also has something to do with having enough juice to safely run cooling fans for up to several minutes after the car has stopped. There are threads around here that this was important for this generation of high output turbocharged engines bmw has been using recently.
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07-13-2015, 09:29 PM | #5 | |
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Coming from an F80 M3, I'm shocked how long the F85 fan runs after I parked. The M3's fan didn't run much, if it does at all after it parks.
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07-15-2015, 08:54 AM | #8 | |
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BMW usually have a single large fan behind the radiator. The X35i and X50i guys are claiming the fan runs long after stops as well. So the 2 batteries theories for the fans might not apply as they have a single battery.
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07-15-2015, 12:07 PM | #9 | |
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http://f15.bimmerpost.com/forums/sho....php?t=1126569 This is why I think the bigger battery is about the energy draw from the routine need to drive cooling fas after engine shut off. |
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07-25-2015, 12:32 AM | #10 | |
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It's all about the cooling fans |
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08-25-2015, 11:37 AM | #12 |
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Yep. X5 35i / 35d / M all have 2 batteries. Oddly enough, the X5 50i doesn't have 2 batteries but just one large battery instead.
After observing the X5M DPs, it's not an eCAT which requires the battery to heat it up during cold start. It's due to the fan running 10-15min after shut down. The primary battery is not used for the fan cool down so there's enough amp to start the car.
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08-25-2015, 03:03 PM | #13 | |
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08-25-2015, 05:28 PM | #14 | ||
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10-28-2019, 10:37 PM | #15 |
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Yes, same here... My 50i has two batteries as well... Perhaps this link will explain why two batteries... I think you need to have a combination of the following options?
https://www.realoem.com/bmw/enUS/sho...diagId=61_5900 Last edited by ~ViO~; 10-30-2019 at 04:32 PM.. |
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10-30-2019, 03:15 PM | #16 |
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Two batteries are there if you have add'l 12V sockets!
https://f10.5post.com/forums/showthread.php?t=1575215 Last edited by menelaos; 10-30-2019 at 03:29 PM.. |
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10-30-2019, 04:33 PM | #17 | |
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01-15-2020, 10:17 PM | #19 |
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It was 20 below and my f85 sat outside all day. I took a short trip to the hardware store and got a "battery drained, start car and drive for a long distance" or something like that.
Went home and hooked it up the the c-tek for the night. No other issues. Does this mean the battery is shot, or just a combo of sitting outside at -20 with minimal driving in between trips for the last 3 days? At home, it is in a heated garage. Do you know at what voltage the warning is triggered?? Thanks
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11-14-2020, 02:03 PM | #20 |
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BMW X5 completely dead
I'm running through a dead car situation and I was looking for info about the batteries.
yesterday the car went through what I think is a shortcircuit situation and does not start. I'm trying to diagnose the situation first the big battery for my shows 5.5 volts. I think that is wrong. Second the piece in the middle of the two batteries has one of the connectors broken, that could be the cause of my problems? third can I disconnect the battery to see if the 5.5 is because something else is broken?? |
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11-14-2020, 07:39 PM | #21 | |
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The PCU controls power to the smaller aux battery through a cut-off relay.
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Become a BMW CCA member! Click HERE to join and feel free to use my BMW CCA member #191509 as a referral. Last edited by Qsilver7; 11-14-2020 at 08:12 PM.. |
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