SNT is working with (an Electric Vehicle Manufacturer) to test and
re-manufacture battery packs that were replaced under factory warranty.
The battery packs SNT receives need to
be graded for the simple fact of knowing how good or bad they are, and to know
where the weak cells are. To test the battery, I used an NI chassis
and 6 acquisition cards. I built the LabVIEW program that checks the health of
the battery section and grades the section for capacity. The program required
two sub-tests. A power supply, ABC150, was used to cycle the battery and charge
and discharge the section.
The first sub-test was a load test
to see how stable the section is at near maximum output. The section was
discharged at 300 amps, then the sections voltages were checked to make sure
they were within a given specification. To do this test the section first had
to be at a standardized initial state. A subvi was created to ‘Precondition’
the section for the load test to verify each cell of the battery section were
within a given requirement and there is no more than twenty millivolts
difference between any two cells. Once the Precondition subvi was passed the
battery would then go through the load test. This test discharged the battery
at 300 amps for 30 seconds. To do this, initialization of the remote channels
of the ABC150 were established and -300 amps was requested at a specific time.
A subvi was created to communicate with the ABC150 so that when current was
requested it would poll the output and not continue until the requested current
was received. Safety checks were added to the test in addition to an emergency
stop button, so that the battery would safely make it through all tests.
Internal thermistor voltages were constantly polled so that if the temperature
of the battery ever got out of hand it would kill the test. After the battery
section successfully made it through the 30 seconds at 300 amps the test would
then poll the cells to discern if the cells were within a required voltage and
had an acceptable voltage difference between the cells. A bad voltage
difference, whether before or after the test, indicates a bad cell. There are
two types of cell failure, an imbalance between the cells and a faulty or bad
cell. When viewing the graph of the cells it is easy to discern whether the bad
voltage difference is due to a bad cell or an imbalance: a bad cell will start
higher (or even) with the other cells, but drop much more rapidly than the
other cells, this shows that the cell has a lower capacity than the other. One
would see the bad cell cross the voltage level of the other cell. In the
situation of an imbalance, an unbalanced cell will stay parallel with the other
cells, but be higher or lower than the rest of the cells during the whole test.
If the battery section makes it through the 300 amp test with an acceptable
voltage level and difference then the program will continue to the next test.
The proceeding test would grade the
section to determine the overall health and capacity of section. The test
starts out by charging the section to full overall capacity through several
steps of decreasing current. The battery would first be charged at 90 amps
until the voltage tops out, and then 45, then 20 and so on until it was only
charging at 2 amps to the maximum section voltage. The reason the section must
be charge at these steps is to guarantee the section top voltage is attained.
When the voltage limit is hit pushing 90 amps into the section the battery is
far from fully charge. This is because the energy going in boosts the voltage
of the section and will make the section report a higher voltage than it will
stabilize out to be. For this reason the test sets a constant current and lets
the voltage drive up until the top limit is achieved and then repeated with
incrementally smaller current. After the battery section is fully charged the
section then is required to discharge 45 amps until it hits its lowest
allowable voltage. This step is internally timed to show its capacity. The time
(in hours) is then multiplied by the amount of current (45 amps) to reveal the
sections capacity in AH (amp-hours). This grade tells the user how much current
the battery can continuously provide for one hour. The AH value is then sent to
a reference table so that a grade can be assigned. The battery section then
goes through the grading test again to verify a correct reading. The capacity
of the section must be within 2% of each other to pass the grading test. Once
these tests are complete the battery section is reconditioned to have a safe,
storable voltage.
Custom harnesses for quick
disconnection and transitions between tests were also built so that one does
not have to open up the acquisition cards to change out the battery. These
harnesses were also made universal so that the other tests could be ran on the
NI Chassis with minimal harnessing.
This test is being currently being
used in production, after the disassemble of the packs. The samples are sent to
a CSV file and filed away with the serial number of the battery, all
automatically. The user only has to scan the serial number of the battery and
hit GO.
