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Voltage drop under load from an uncompensated high impedance busbar (blue)
Maximum voltage swing can be calculated if the maximum cell voltage, resistance of the busbar / cable,
and maximum amperage are known using the following formulas. Please keep
in mind that the
resistance of the busbar or cable must include the resistance of the terminal and any crimps. Headroom
must be left for possible small increases in resistance due to eventual corrosion, etc.
Peak_voltage = Vmax_cell + (Rbusbar * Imax_charge) (Peak_voltage must be < 5v)
Lowest_voltage = Vmin_cell - (Rbusbar * Imax_discharge) (Lowest voltage must be >0v)
In the above example, if the cells have a maximum voltage of 3.65v (Vmax_cell) and a minimum of 2v
(Vmin_cell) and a maximum amperage of +/-200A (Imax_charge and Imax_discharge), the peak
voltage with the given 2 mOhm cable would be 4.05v and the lowest voltage 1.6v. These voltages are
within the limits and busbar compensation could be used.
Skin effect issues due to AC currents
The vast majority of motor controllers available on the market provide some amount of filtering on the
DC bus in order to limit radiated and conducted emissions. While most motor controllers generate high
levels of noise, the noise generally does not lead to a significant AC component on the DC bus. A small
minority of motor controllers lack adequate internal bulk capacitance and filtering and actually produce
a high frequency AC component on the DC bus.
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The Orion Jr. is extremely resistant to electrical noise (EMI) and has been tested in real world situations
to operate successfully with this excessive noise. However, a strong high frequency AC component can
create a “Skin Effect” on the cables connecting the batteries.
Skin effect occurs when eddy currents
form within the cables, causing only the outside portion of the wire to carry current and effectively
increasing the resistance of the wire.
Although the Orion Jr. has been tested extensively to operate and measure voltages properly in these
extremely harsh noise environments, the BMS may correctly measure unpredictably changing
resistance values since the effective resistance of the cable is changing considerably with respect to
amplitude and frequency. The Orion Jr. BMS bases many calculations on the measured resistance of
the cells, including open cell voltage calculations which are used for determining maximum current
limits and determining when cells are weak. If a significant skin effect is present, it can introduce
inaccuracies with some calculations even though the Orion Jr. BMS continues to operate.
The overwhelming majority of motor controllers have adequate filtering to reduce conducted emissions
to levels that prevent a skin effect from forming. For the small number that do not, the skin effect can be
mitigated somewhat by using suitable rectangular busbars or straps rather than round cables. In an
environment where a skin effect forms, any round cables should be wired such that they fall between
cell groups since the BMS cannot effectively compensate them out. Preferably, it may be possible to
add external filtering to the motor controller to suppress the conducted emissions generated by the
motor controller to a tolerable level. Extremely high amounts of AC noise can also cause back EMF
from stray inductance in the cabling. If this occurs, the emissions will need to be limited to a safe level.
Transients
Transients on the battery pack must be limited to reasonable levels. This is important both for the
protection of the BMS and the protection of the attached lithium cells. Lithium cells exposed to extreme
transients may short and enter thermal runaway. Extreme transients are often generated by the
combination of rapid changes in current and stray inductance. These usually occur when bulk
capacitors in motor controllers or chargers are switched into circuit or by other rapid changes in current
such as from a blowing fuse or sudden connection of current. Transients due to normal switching from
contactors can be limited by using proper pre-charge circuits and limiting slew rates for motor
controllers. The use of multiple chargers with series diodes is not recommended for use with the Orion
Jr. BMS as these configurations can cause damaging transients.
Excessive transients may cause damage to the Orion Jr. BMS unit or to connected lithium ion cells. In
the event that excessive transients have occurred, the Orion Jr. unit and lithium cells should be
immediately inspected or tested for damage as damage can lead to safety risks.
If a unit has been damaged by transients, it must be disconnected immediately from service and
repaired or replaced. Never continue to use a damaged unit.
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Wiring Errors
Wiring errors can cause serious damage to the Orion Jr. BMS that is not covered under warranty. The
Orion Jr. has internal fuses on each cell tap wire to protect the BMS and connected wiring from
excessive currents. The Orion Jr. is designed to withstand certain minor wiring errors such as
accidentally swapping the order of 2 cells or even wiring cells in the reverse order. The internal fuses
are designed to blow with voltages more than +/- 24V for Rev A units, +/-48V for Rev B units, and +/-
60V for revision C units (though there are some rare situations where fuses can still blow at lower
voltages and some situations where they won’t blow until vol
tages are higher). The Orion Jr. can
withstand wiring errors only for a short period of time and repeated or extended exposure to wiring
errors may cause permanent damage which may alter the accuracy of the voltage measurements. If an
Orion Jr. unit is wired improperly, small currents may flow through the BMS, and the BMS unit
may heat. An incorrectly wired unit should not be left connected to the battery pack for more
than 5 minutes as it will eventually drain and can destroy cells connected to it. If there is question
as to whether the harnesses are wired properly, do not leave them connected to a unit for any length of
time. The Orion Jr. units may report an
“open wire
fault
,”
“wiring fault
,
” or “low cell fault” if incorrectly
wired. Revision C units with firmware 2.0.2 or newer may also rapidly flash the red LED to indicate
incorrect wiring that is damaging the unit. The absence of a rapidly flashing red LED does not
necessarily indicate correct wiring. If the red LED begins to flash rapidly, immediately disconnect the
cell tap harness and inspect the wiring. If a unit has been damaged, do not continue to use the unit.
Voltage tap harness lengths
Standard voltage wiring harnesses are available for the Orion Jr. in 6 foot lengths. The length of the tap
harness slightly affects the accuracy of the voltage readings by the Orion Jr. The length of cable does
not significantly alter the accuracy of the voltage measurement.