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Why is the Battery Management System (AP9921X) reading negative ohmic values?


Issue

My AP9921X is displaying negative ohmic values.

 


Product Line

  • Battery Management
    • AP9921X


Environment

  • All serial numbers
  • All firmware versions


Cause

The battery management system will not display a valid ohmic value until it has seen a discharge event long enough to scan the entire battery string. 


Resolution

It takes a discharge of five seconds at a minimum of 10amps to trigger the discharge test.  Once that is triggered, the system begins scanning at four monitoring points per second until:

  1. a positive current flow indicates that the discharge has ended, or
     
  2.  the user defined low voltage is reached. At that point, the BMS goes to a "hibernate" mode, simply checking for a positive current flow on a regular timed cycle, and shuts everything else off. When a positive current flow is detected, the system wakes up and begins monitoring again.
The BMS has to be able to scan the entire battery string one time during the discharge in order to provide ohmic data in the Ohmic Value field in the display.



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