The purpose
The use of (and the number of) sensors depends on many parameters, which all interact somewhat. In the end, the purpose of the measurement counts:
- Keep the cells within their working temperature.
Heat generation
First of all it is important to know how much the cells heat up due to the use in the application.
In high power applications cells with a lower internal resistance will be used, again to reduce the heating.
If during continuous use and at a high ambient temperature the cells become too hot, the design of the package is not adequate.
Then placing more or fewer sensors will not help either.
If the heat dissipation is sufficient, the temperature measurement will have to protect the cells, which have deteriorated due to ageing, and thus become warmer.
Heat conduction and distribution
When the heat can only be dissipated through the air, cells will influence one another little.
When one cell heats up a lot, its neighbour will hardly warm up extra.
In that case, it is theoretically best to provide all cells with their own sensor.
Unfortunately, this is not practically feasible. A better solution is to thermally couple the cells (possibly in groups).
Each group will then have its own sensor.
In most packs, however, cells are not thermally coupled and more cells are used in parallel.
When one cell deteriorates, the neighbouring cells absorb most of the current and thus prevent the strong heating up of the deteriorated cell.
In this way, the average temperature of an area can be measured without major risks.
Usually, an area of about 4 by 5 cm is measured with one sensor.
Rule of thumb
As a rule we can say:
- More cells in parallel require fewer sensors (NTCs).
- More cells in series require more sensors
In formula:
NTCs = β x number_of_S / number_of_P
See also our example of a good BMS for lithium batteries.
A short telephone call will tell you a lot more.
