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Is anyone able to help. My 840ci wont start unless I jump start it i need to do this every time has anyone had this problem before?
 

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M sport 335i lci
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Is anyone able to help. My 840ci wont start unless I jump start it i need to do this every time has anyone had this problem before?
Have u tried trickle charging the battery, or does it still go flat
 

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Discussion Starter · #3 ·
Trickle charge works but i don't want it constantly trickle charging.

I've replaced the battery and alternor works fine.
 

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Discussion Starter · #9 ·
I only changed one the other was fine and working been advised it could be the general module
 

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I re-configured the wiring in the boot and now only use one battery (the RHS), no problem at all. A mismatch between the batteries will give you endless trouble. Even more so if it's not a daily driver.
 

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I only changed one the other was fine and working been advised it could be the general module
Always change both batteries even if one appears good. Signs of general module trouble can be battery drain, clicking relays in the boot, even windows not working or in some cases working on their own even if the car is not in use. I would say change the other battery for a brand new one of the same brand as the one you already replaced.
 

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The car was designed as a GT with the possibility it would be driven and then parked for long periods - hence the need for two batteries in parallel to provide the increased energy storage.
 

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Timm, that still doesn't make the use of two batteries in parallel the right option. Two big 6v batteries in series would have been better and kept the weight distribution 'even'. Has anyone out there tried that?
Also, wasn't there an option for a fuel burning heater for realy cold environments?
 

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The aim was to increase energy storage so that the 50mA draw didn't discharge the batteries so quickly - and that is better achieved (at the time of manufacture) with two 12V batteries in parallel. The weight is distributed with one battery on each side of the boot (trunk). Having batteries in parallel doubles the CCA and Ah rating of the battery combination.

Batteries in series decreases CCA by a factor of two but retains the same Ah rating as a single battery!
 

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"Batteries in series decreases CCA by a factor of two but retains the same Ah rating as a single battery!"
Balderdash old boy! I have been trying to think up a mechanism as to how that can happen and failed. Same CCA on each battery and double the voltage. Rather than compose an argument a quick search on the interweb came up with :-



 

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Yes, you are right, double the voltage and twice the ISR so the same cranking Amps. Also, 6V batteries of the same dimensions would have twice the CCA. So all in all the same voltage and CCA as the 12V alternative.

12V Lead-Acid are ubiquitous which is probably why BMW chose to use them instead of the 6V alternative.
 
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