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Battery Discharging to 45%

2K views 7 replies 7 participants last post by  Doug Huffman 
#1 ·
I have a weird charging issue on my '15 435i, CPO.

After sitting idle a few days, I get a warning message that the car's battery has been continuously discharging while sitting. I checked the charge % with the battery charger and it was at 45%. Is this normal for a battery to be only half charged. (while on the charger, the battery will charge in a few hours to 100%). I believe this is the original battery.
This has me concerned but hasn't left me stranded yet...

I would appreciate any advice on this....
 
#3 ·
Your BMW's batttery continuously discharges. The computers / modules are on frequently even while the car is 'off'.

The charger may be estimating the charge level based on what it thinks is the no-load voltage, but the connected BMW battery is always under load supplying the computer / modules.

Your BMW's ECU maintains a proper more accurate register of State of Charge by comparing the number of electrons into and out of the battery through the Intelligent Battery Sensor. Your BMW's ECU also maintains a careful State of Health register of comparisons of battery capabilities now to when it was new.

Your battery charger estimates end of charge by its voltage to achieve final charge current. As your battery reaches its End of Life its nominal voltage increases and the charger will see that as 100% capacity end of charge.

One ampere-hour of overcharge electrolyzes 0.335 grams of electrolyte water to 400 cubic-centimeters of oxygen and hydrogen gas that is vented off and lost to an already starved plate battery. As the electrolyte volume and level decrease, so does its actual AH capacity. As the electrolyte water is lost (as gas) the acid becomes more concentrated raising the battery voltage.

Lead Acid Battery Technology: Fundamentals, Materials and Technologies is available on Scribd. The '0.335 grams' comment is on page 65 IIRC.
 
#4 · (Edited)
The battery may have been at a less than optimum state of charge when the engine was shut down. In town driving with auto stop start engaged could easily do that.

FWIW my car with 67k miles gets periodically stored for over 2 weeks with no error message. A fair part of my driving is on the highway for 100 plus miles.
 
#5 ·
The smart charging system in the car only keeps the battery charged to around 70% or so. Your battery is around 4 years old now, so it***8217;s not going to hold as much of a charge as a brand new battery. If the messages keep up it might be time for either a new battery or an electrical system inspection.
 
#7 ·
Also you need to use the proper type charger for the battery.
If you drive short distances and use things like lights, seat heaters and steering wheel heat, you won't keep up charging with discharging.
Place a charger on the battery once a month over night.
 
#8 · (Edited)
I charge weekly with a CTEK 4.3 MUS for about 5 AH - 10 AH summer and winter.

BatteryFAQ.org makes the point that an average State of Charge ~ 70% correlates to the sweetspot in AGM battery throughput versus cycle life at 1,000 cycles of full nominal discharge and recharge. An average SoC of 100% is a fully charged but never used battery with unlimited lifetime of uselessness. An average SoC of 0% is a dead battery, also useless, and susceptible to sulfation.

BatteryFAQ.org has also a list of battery manufacturers around the world and their brands.
 
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