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· Premium Member
13,595 Posts
Discussion Starter · #1 ·
With all the battery posts of late, and my recent bout of a weak battery, thought I'd check out how the onboard system actually does it's job of "maintaining" the battery ... using my existing battery as the guinea pig :p

1) Initial conditions
  • battery (3rd OEM from Benz installed 04/11/18) tethered in AGM maintenance mode 12/22~12/28
  • stopped maintenance charging on 12/28
  • observed voltage periodically 12/28~01/14 (car dormant 12/28~01/20 no door switch activity)
  • as expected the battery level decreases with all the telemetry systems always running
  • drop was 12.76V to 12.63V

Rectangle Slope Font Parallel Pattern

2) First drive
  • 7.5mi trip over ~23min
  • with systems "on" before ignition 11.9V (dropped from ~12.6V)
  • ignition point 10.4V (cold engine dormant for 23 days / coolant 52F / ambient 53.6F)
  • after ignition full charging profile during trip / never <14V

Rectangle Slope Font Parallel Plot

3) Second drive
  • 7.5mi trip over ~27min
  • with systems "on" before ignition 12.4V
  • ignition point 12.2V (warm engine easier start / coolant 145F / ambient 77.0F)
  • after ignition full charging profile during initial drive time
  • until software "adapted" to battery's state of charge for remaining time (INTERESTING to finally see it !!)
  • resting voltage at engine idle 12.6V

Rectangle Font Parallel Slope Pattern

4) Comparison with new (unconnected) battery
  • charged up the new H7 after purchase
  • stopped charging left battery dormant not connected
  • observed OCV (open circuit voltage) over time
  • from 12/28~01/21 drop was only 12.87V to 12.83V (during same 12/28~01/14 period)
  • self discharge is nice and slow

Rectangle Slope Line Parallel Pattern
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