2.3 Temperature Limits

This page helps you set the temperature limits of the BMS from the datasheet of your cells: when charging or discharging must stop because the battery is too cold or too hot, when the BMS switches the path on again, and how the board and FET temperatures protect the BMS itself.

Applies to

Boards: BMS1820, BMS3610, BCU2002, BMS1601, BMS1810.

2.3.1 How It Works

The cell limits act on the external temperature sensors of all modules. Every sensor is compared with the limits on its own; one sensor past a limit is enough to act. An input without a sensor is ignored.

Fig.2.3.1: Temperature Limits of the Charge and the Discharge Path

Charge path OFF ON OFF CHG. U.T. Protection CHG. U.T. Warning Over Temp. Warning Over Temp. Protection CHG. U.T.P. Release Over Temp. Release Discharge path OFF ON OFF DCHG. U.T. Protection DCHG. U.T. Warning DCHG. U.T.P. Release Over Temp. Release -30 -20 -10 0 10 20 30 40 50 60 70 80 Temperature of the coldest or hottest external sensor (°C) Factory default values; choose your own from the cell datasheet. Orange: warning band, the path stays ON.

  • Charge under temperature: a sensor at or below CHG. U.T. Protection switches off the charge path. Most lithium cells must not be charged in the cold, so this limit is higher than the discharge limit.
  • Discharge under temperature: a sensor at or below DCHG. U.T. Protection switches off the discharge path.
  • Over temperature: a sensor at or above Over Temp. Protection switches off both paths.
  • Timing: a warning or a protection acts when the sensor has stayed past its limit for 1 s. The path switches on again when every sensor that tripped is back past its release value for 1 s. Each trip counts towards Error Recovery Attempts (3.1 Protection and Recovery).
  • Temperature delta: when the difference between the hottest and the coldest external sensor stays at or above Temperature Max. Delta for about 30 s, the BMS raises Max. Temperature Delta Protection. This is a CRITICAL event: both paths switch off until the BMS restarts.
  • Board limits: PCB O.T. limits act on the temperature sensor of the BMS board and switch off both paths. FET O.T. limits act only on boards that switch the current with on-board FETs (BMS1601, through its board sensor); other boards read 0 °C here and never trip. Cell Balance O.T. Warning stops balancing while the board is too hot (2.2 Cell Balancing).

2.3.2 Settings

Table.2.3.1: Temperature Settings (LiMon™ Temperature tab)

Setting What it does Unit Default
CHG. U.T. Warning Warning when a sensor is at or below it. °C 3
CHG. U.T. Protection Charge path off at or below it. °C −5
CHG. U.T.P. Release Charge path on again at or above it. °C 0
DCHG. U.T. Warning Warning when a sensor is at or below it. °C −5
DCHG. U.T. Protection Discharge path off at or below it. °C −20
DCHG. U.T.P. Release Discharge path on again at or above it. °C −10
Over Temp. Warning Warning when a sensor is at or above it. °C 55
Over Temp. Protection Both paths off at or above it. °C 70
Over Temp. Release Both paths on again at or below it. °C 60
Temperature Max. Delta Highest allowed difference between the external sensors. °C 10
PCB O.T. Warning Warning when the board sensor is at or above it. °C 66
PCB O.T. Protection Both paths off at or above it. °C 81
PCB O.T. Release Both paths on again at or below it. °C 71
FET O.T. Warning Warning when the FET temperature is at or above it (BMS1601). °C 65
FET O.T. Protection Both paths off at or above it. °C 80
FET O.T. Release Both paths on again at or below it. °C 70
Cell Balance O.T. Warning Board temperature at which balancing stops. °C 60

The LiMon™ reference describes every field of the screen: 7.3 Temperature Settings.

The defaults are a starting point only. Set the cell limits for your cells before the first charge; keep the board and FET limits unless LiBat™ advises otherwise.

2.3.3 Choosing the Values

Take the charge and discharge temperature ranges from the datasheet of the cell. Keep the values in this order for each limit:

Protection < Release < Warning for the under-temperature limits, Warning < Release < Protection for the over-temperature limit.

  • CHG. U.T. Protection: at or above the lowest charge temperature of the cell. Charging below it damages the cells for good, so do not set it lower than the datasheet allows.
  • DCHG. U.T. Protection: at or above the lowest discharge temperature of the cell.
  • Over Temp. Protection: at or below the highest temperature of the cell; use the lower of the charge and the discharge limit, because this protection switches off both paths.
  • Warnings: far enough from the protection to give the system time to react, for example to reduce the current or to start a heater or a fan.
  • Release values: a clear gap from the protection limit. Temperature changes slowly; a gap of a few degrees prevents the path from switching on and off at the limit and the trips from adding up.
  • Sensor placement: the limits only see what the sensors see. Place the sensors on the cells that get the coldest and the hottest, not only in the middle of the pack.
  • Temperature Max. Delta: above the spread that your pack shows at full load and in the sun or the cold. Too low stops a healthy battery with a CRITICAL event; too high lets a hot spot or a loose sensor go unseen.

2.3.4 Checking the Result

  • Live View shows each external sensor of the selected module, the highest temperature and the temperature delta. Every sensor that you connected must show a plausible value.
  • Warm one sensor by hand: its value rises and the others stay; the delta follows.
  • Events › Event Status shows every warning and protection with its time.

2.3.5 Common Mistakes

  • CHG. U.T. Protection set like the discharge limit. The BMS then allows charging in the cold. Use the charge temperature range of the datasheet, which is narrower than the discharge range.
  • A sensor that is not connected. An input without a sensor is ignored, also when a wire breaks. Check in Live View that every sensor reads a value after installation and after service.
  • All sensors in one place. The warmest cell in the centre of the pack or the coldest cell at the wall is never measured. Spread the sensors over the pack.
  • Release value too close to the protection limit. The path switches on and off at the limit and the trips reach Error Recovery Attempts. Leave a gap of a few degrees.
  • Over Temp. Protection above the cell limit. The cells overheat before the BMS acts. Set it from the cell datasheet.
  • Temperature Max. Delta too low. A pack in the sun with one side in the shade, or a sensor near a busbar, reaches the delta in normal use and both paths switch off until a restart.