the discharge current limit, a state of charge is specified where to begin reducing the discharge
current limit along with a value of amps per percentage state of charge. For most applications,
this feature is not used and should be disabled to prevent errant SOC calculations from altering
the usable range of the pack unless there is a specific reason for enabling it. This feature may
be required, however, if the battery pack must be kept within a certain state of charge.
3. Cell Resistance - The BMS reduces the current limit to ensure that, if a load or charge is
placed on the battery pack, the load or charge would not cause the cell to exceed the maximum
cell voltage or drop below the minimum cell voltage. This calculation uses the internal resistance
of the cell and the open circuit voltage of the cell. This can be thought of as an ohm’s law
calculation where the BMS is solving for the maximum possible amperage that will still keep the
cell voltage inside the safe range. This calculation preemptively keeps the cell voltage within
specifications and also results in a 0 amperage discharge or charge current limit in the event a
cell voltage drops below the minimum or goes above the maximum voltage respectively.
4. Pack resistance - If enabled, the BMS performs the same calculations as in point 3, but for the
minimum and maximum pack voltages and reduces current limits to maintain these values.
5. Cell Voltage - In the event that the above calculation were to ever be inaccurate due to incor-
rect data such as an incorrect cell resistance or incorrect open circuit, or if the current limit is ig-
nored by the external device, the BMS contains a backup algorithm for reducing the current lim-
its if a cell voltage limit is exceeded. If the BMS measures a cell voltage above the defined max-
imum cell voltage or below the defined minimum cell voltage, the BMS will cut the respective
current limit by one fifth of the current limit at the time the out of range cell voltage is measured
in an attempt to restore the voltage to a safe level. If this fails to bring the cell voltage back to
within the defined range, the BMS will again cut the current limit by one fifth of the maximum
continuous amperage and try again. This will happen very rapidly up to a total of five times. If
the voltage is still outside of the range, the BMS will have reduced the current limit to zero amps
which prohibits all discharge or charge (depending on if the cell voltage was too low or too high
respectively.) This ensures that under all circumstances, if a cell voltage is ever above the max-
imum limit or below the minimum limit, the BMS will always have a zero amp charge or dis-
charge current limit which prohibits all charge or all discharge respectively. This ensures that the
charge enable, discharge enable and charger safety enable outputs are all off if a cell ever ex-
ceeds a maximum cell voltage or drops below a minimum cell voltage.
6. Pack Voltage - If enabled, the BMS performs the same calculations as in part 5, using the pack
voltage limits rather than the cell voltage limits. Total pack voltages are calculated based on the
sum of the individual cells. For best reliability, pack voltage limiting should only be used when it
is necessary to restrict the pack voltage more than the individual cell voltage restricts the pack
voltages. For example, if a pack has 10 cells and the cell voltage limits are 2.5v and 3.65v, the
pack voltage is already inherently limited to 25v to 36.5v.
7. Critical Faults - In the event that the BMS detects a critical fault relating to the ability of the
BMS to monitor cell voltages, the BMS will go into a voltage failsafe condition. The specific pos-
sible causes of the voltage failsafe mode are defined in the “Understanding Failure Modes” of
this manual. If one of the critical faults that cause a voltage failsafe condition occurs, the BMS
will immediately start gradually reducing both the charge and discharge current limits to zero
which prohibits all charge and discharge. The gradual reduction allows a vehicle time to pull
over and safely stop. The speed at which the limits are reduced is programmable in the BMS
settings. The relay outputs will be turned off only after the gradual de-rating has occurred.
