How much battery drain should I be seeing?

fortythirtyfive · 12-01-2023, 04:32 AM

I swapped my battery 2 years ago as the old one was failing to hold charge, the car would randomly throw faults like no power steering or the dash would go off. The worst of all came when it finally shut the car off whilst I was driving.

Anyway I came to the conclusion it was the battery being 6 years old at the time and I was correct.

However I now appear to be fighting against charge levels. I have an MMI box installed plus an ambient lighting kit, both of which use the OEM plugs as they essentially piggy back or fully replace the old counterparts (in the case of the ambient lighting kit). Even though these shut off when the car sleeps I have fully unplugged them for now just to be sure, yet I am still loosing a few percentage overnight.

I then suspected my headlight modifications as they run a separate 12v module controller, I designed these myself and made sure that the relay only opens when the car is unlocked, I can see this doesn't draw anything as intended when metering it, however just to be sure I pulled the fuse on this too.

Realistically how much should I be expecting to see the car battery drop over 5-7 days of no use? This seems to be the biggest issue as I often have weeks where the car is sat for 5 days, when I get in I find the battery has bottomed out at 50% which throws a bunch of under voltage errors for the DME and gearbox.

Billfitz · 12-01-2023, 10:12 PM

Much depends on the temperature, but on average you shouldn't see more than a half volt loss over 5 to 7 days. More than that indicates either a weak battery or parasitic power drain.
You should never see the car shut off while driving, as while driving all the power the car needs and more is provided by the alternator. If it did that it's extremely unlikely that the battery was solely at fault.

CPSI90 · 12-09-2023, 04:41 PM

I've a 2015 F32 435iX. Battery is over 9 years and still going strong. I haven't use a charger until the 7th year. Touch wood it is still going strong. I'm in a cold country and without using it for 10 days it will drop around 20%. It is usually at 70-80% level so at 50-60% I never have any problem or warning.

Billfitz · 12-09-2023, 05:00 PM

We just had our first week of temps in the teens F every night. Over six days without being driven my nine year old battery went from 12.5v to 12.3v.

12-01-2023, 04:32 AM	#1
fortythirtyfive Major 641 Rep 1,382 Posts Drives: 435i MS Join Date: Sep 2016 Location: North iTrader: (0)	How much battery drain should I be seeing? I swapped my battery 2 years ago as the old one was failing to hold charge, the car would randomly throw faults like no power steering or the dash would go off. The worst of all came when it finally shut the car off whilst I was driving. Anyway I came to the conclusion it was the battery being 6 years old at the time and I was correct. However I now appear to be fighting against charge levels. I have an MMI box installed plus an ambient lighting kit, both of which use the OEM plugs as they essentially piggy back or fully replace the old counterparts (in the case of the ambient lighting kit). Even though these shut off when the car sleeps I have fully unplugged them for now just to be sure, yet I am still loosing a few percentage overnight. I then suspected my headlight modifications as they run a separate 12v module controller, I designed these myself and made sure that the relay only opens when the car is unlocked, I can see this doesn't draw anything as intended when metering it, however just to be sure I pulled the fuse on this too. Realistically how much should I be expecting to see the car battery drop over 5-7 days of no use? This seems to be the biggest issue as I often have weeks where the car is sat for 5 days, when I get in I find the battery has bottomed out at 50% which throws a bunch of under voltage errors for the DME and gearbox. __________________ 2015 435i - 401whp Scorpion catless dp, VRSF 6.5" HD FMIC, MST V2 inlet, NGK 97506, MHD Stage 2+ 102, xHP Stage 3, M Performance Exhaust (modified for valved straight thru)
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12-01-2023, 10:12 PM	#2
Billfitz Lieutenant General 8256 Rep 16,101 Posts Drives: '15 328iX GT Join Date: Jan 2015 Location: New Hampshire iTrader: (0)	Much depends on the temperature, but on average you shouldn't see more than a half volt loss over 5 to 7 days. More than that indicates either a weak battery or parasitic power drain. You should never see the car shut off while driving, as while driving all the power the car needs and more is provided by the alternator. If it did that it's extremely unlikely that the battery was solely at fault.
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12-09-2023, 04:41 PM	#3
CPSI90 New Member 3 Rep 11 Posts Drives: 2022 G82 M4 Join Date: Jun 2021 Location: Vancouver iTrader: (0)	I've a 2015 F32 435iX. Battery is over 9 years and still going strong. I haven't use a charger until the 7th year. Touch wood it is still going strong. I'm in a cold country and without using it for 10 days it will drop around 20%. It is usually at 70-80% level so at 50-60% I never have any problem or warning.
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12-09-2023, 05:00 PM	#4
Billfitz Lieutenant General 8256 Rep 16,101 Posts Drives: '15 328iX GT Join Date: Jan 2015 Location: New Hampshire iTrader: (0)	We just had our first week of temps in the teens F every night. Over six days without being driven my nine year old battery went from 12.5v to 12.3v.
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