A Shorted Monitoring Circuit and a Thermal Runaway Precursor

A shorted monitoring circuit caused a Max↑ Min↓ cell voltage pattern at one of our client facilities. Left undetected, this cascade does not just take a node offline — it creates thermal runaway precursor conditions.

Reading the signature

The pattern itself is the diagnosis. Maximum cell voltage climbing while minimum cell voltage falls, within the same module, is not what ordinary degradation looks like. Ordinary degradation moves the whole distribution together and slowly.

A simultaneous divergence at both ends points to something draining a specific cell while the balancing system compensates elsewhere in the string — which is what a shorted monitoring circuit does.

Why it escalates rather than settles

The dangerous property is that the fault is self-reinforcing. The affected cell sits at a lower state of charge than its neighbours, so it is worked harder on every cycle to keep up. It heats more than its neighbours. Heat accelerates the very degradation that created the imbalance.

None of the individual readings need to breach an alarm threshold for the trajectory to be bad.

Why a threshold alarm misses it

Pack-level voltage stays in range. Pack-level temperature stays in range. A monitoring configuration built around absolute limits sees a healthy system, because the information is not in any single value — it is in the relationship between the maximum and the minimum, and in the direction that relationship is moving.

Catching it requires the EMS to watch cell-level spread as a first-class signal, and to treat divergence as an event in its own right rather than a precursor to one it will report later.