@SoS
Once a cell shorts, the input to the BMU for that particular cell will show a short circuit. How the BMU responds to that will depend on what it thinks is the cause of the short circuit.
Right, and the BMU will hopefully disable the charger.
For this scenario, the parasitic source - main bat bus -> switch -> nav light -> switch -> APU bat - remains, feeding the APU bat.
Another possible parasitic connection reported is direct connection of Main bat and APU bat. If so, scenario applies to Main bat as well.
Scenario is
- APU bat cell short
- BMU detects cell short
- BMU shuts charger down and alerts
- APU bat voltage is reduced by up to 4V
- But parasitic source remains at above 28V (4V x 7)
- Source voltage to be taken up by the remaining 7 cells
- Ideally (if BMU not already disabled), the BMU trys to balance against parasitic charge current to prevent 7 cell overvoltage.
- BMU fails because current too large (> 3 A)
- Multi-cell overvoltage
- Cascading cell failures
- Runaway
Edit
As I explained in earlier posts, short may be transient, and hard to detect, unless BMU sees cell voltage transients.