787 Batteries and Chargers - Part 1

[saptzae]

@fizz57

Concur, by connecting to NAV lights from two sources, such as main bat bus and APU battery, there may be a parasitic current path into APU battery, bypassing the intent of the design.

When any cell is fully charged, a very small net current (mA!) into the cell, would lead to overvoltage of the cell, unless the BMU balancing manages to act against it and compensates it out, to keep cell voltage down. I am not sure if parasitic bypass current would be high enough to cause overvoltage. Depends on the bypass voltage, which would have to be well above 32V, and also on how good BMU is at balancing.

Any overvoltage will deteriorate cell, thus lead up to Primary failure.

Once a cell shorts (Primary failure), and battery voltage drops, the parasitic current path allows larger current to flow (a few A), which also would be too large to be balanced out by the BMU.

That would set battery up for Secondary failure of multiple cells by overvoltage.

When multiple cells fail, Tertiary failure is just a matter of time.

It may be a lead, its not all of it. A parasitic current path into main battery remains to be found.