Page 20 - PEN eBook July 2023

Smart Energy

manufacturers can use one or all features in one system, depending on the required safety level.

The thermal cutoff kicks in when the battery pack reaches a certain temperature level. In high-

voltage systems like EVs, this feature is normally activated by a digital processor, whereas in low-
voltage applications, it is possible to implement this protection to be triggered by itself based on
a predefined threshold.

In environments where humans can be harmed, protection against fire and explosion is especially
important. Therefore, a digitally triggered pyro fuse comes into play. The fuse is connected to the
high-voltage path on either minus or positive or on both. The fuse gets triggered as the last defense
line to prevent significant damage to the battery.

In special environments like trucks, where service continuity is important, we tend to use more elaborate
and expensive solutions. A circuit breaker based on back-to-back SiC MOSFETs is one possible way to
protect the battery pack system from damage in case of a short-circuit. The drawback of this solution is
the price and size. The functionality is the same as a pyro fuse, but it can be turned on after the event.

Overcurrent protection
As explained earlier, the cells get balanced with the help of a current flow. Depending on the

charge capabilities of the battery, this current flow is in the range of 100 mA and 500 mA. The
overcurrent protection is a specific current limit that the balancing IC shall not exceed. Mostly,
this limit can be set individually and helps to protect the battery against irreversible damage,
fire or explosion.

The current consumption depends on the ambient temperature, which should be considered when
defining thresholds. Furthermore, it is necessary that the current-level limit is set below the real
battery current-draw level. Normally, the level is charged with an additional safety factor of 2 to 3.
In case of small current fluctuations, false triggering of the BMS overcurrent protection can occur.

To prevent the system from doing this, some BMSes have a feature called hysteresis and digital
filters.

Thermal runaway protection
Batteries can support temperatures as high as 60°C based on the used chemistry. The temperature
of a hot cell can spread to the neighboring cells and the entire battery pack can heat up in no time.
The heat triggers a chain reaction that can set the whole battery pack on fire through different

chemical reactions that can release inflammable gas.

The thermal runaway protection gets triggered when the predefined temperature threshold is met.
It shuts down the battery and prevents the battery from going into the thermal runaway.

20 JULY 2023 | www.powerelectronicsnews.com