Orion BMS Operation Manual
11
able range of the pack unless there is a specific reason for enabling it. This feature may be re-
quired, 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. In firmware 2.6.5 and earlier, the total pack volt-
ages are measured using the total pack voltage sensor. Starting in versions 2.6.8, 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 indi-
vidual 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 ar
e 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.
With firmware versions 2.6.5 and newer, diagnostic information is provided from the BMS in the live text
data tab in the utility as to which of the above reasons the BMS is limiting current.
Orion BMS Operation Manual
12
Selecting Current Limit Settings
The Orion BMS utility has data for many common cell types already pre-loaded into the utility. These
can be accessed by using the Profile Setup Wizard in the BMS utility. For cells which
are not listed, or if
custom settings are required, the following guidelines may be helpful for selecting proper values.
Maximum Continuous Amperage Setting
- The continuous maximum amperage should be set at or
below the maximum allowable continuous amperage as specified by the cell manufacturer. In some
cases, it is desirable to use a lower value than what the manufacturer specifies in order to extend the
lifespan of the cells. In some cases the manufacturer will specify a “C” rate. To convert a “C” rate to an
amperage, simply multiply the C rate by the amp hour capacity of the cell. For example, a 100 amp
hour cell with a 2C continuous discharge rating is has a maximum continuous discharge rate of 200
amps.
Pulse Current Limit Settings
[firmware 2.7 and newer] - This feature should be disabled unless the
cell manufacturer specifies a pulse charge or pulse discharge current limit. Never assume a
cell can
handle pulse currents. Pulsing may lead to internal damage and plating of the cell, which can lead to a
fire risk. If a cell manufacturer specifies a pulse limit, calculate how much over the standard continuous
amperage limit the pulse limit is as a percentage. For example, if a cell has a 50 amp continuous limit,
but a 100 amp pulse limit, the pulse limit is 200% of the standard. This value should be entered into the
utility. The time limit specified by the cell manufacturer for maximum duration of the pulse must also be
entered into the utility.
Rest Current Limit (after pulse)
[firmware 2.7 and newer] -
This feature allows the battery to “rest”
after a pulse
charge or discharge occurs. Many chemistries require a “rest” period after a full pulse has
occurred in order for the cell to thermally recover so as not to cause permanent damage to the cell. The
rest period and rest amperage are defined by the battery manufacturer. The rest amperage is pro-
grammed in as a percentage of the maximum continuous amperage.
Current Limit Temperature Settings
- All cell manufacturers specify a minimum and maximum oper-
ating temperature for charge and discharge. Often times the temperature range for charging is usually
more restrictive than the temperature for discharging. Some cells are not permitted to be charged below
a certain temperature. For example, many iron phosphate cells cannot be safely charged below freez-
ing. Additionally, it may be desirable to further limit the amperage at low or elevated temperatures since
high charge and discharge rates at such temperatures may reduce the lifespan of the cells.
Temperature limits must ensure that no charge or discharge is permitted below the minimum or above
the maximum temperatures. For both charge and discharge settings, select a temperature at which the
maximum amperage should be reduced. This value should be programmed into the BMS utility, and an
amps per degree Celsius value should be calculated to ensure that the slope of the line intercepts zero
amps at the desired cutoff temperature. This should be done for both high and low temperature limits
for both charge and discharge current limits. Warning: If the temperature de-rating line does not inter-
cept zero, the BMS will not protect for over or under temperature!
In a very limited number of applications, it may be necessary to allow a minimum charge or discharge
value at all temperatures. If this is the case, the
“Never reduce
limit below xx amps for temperature