With detailed data from OBDII (see this post) I've started thinking about ways to track the health of our HV batteries. But first, some of the data available from the Battery Energy Control Module (BECM) is:
- Battery Age: Mine is 23 months - sounds right for car built in April 2013.
- Battery voltage: Runs about 282V. A 5 minute run gave min/max of 264V/303V.
- Battery Current: Varied from -114 amps (charge) to 94 amps (discharge). Limits (see below) should be about +/-120 amps.
- Battery Temperature: My 5 minute run had values from 62 to 66F (mid 30s OAT).
- State of Charge (SOC): Both actual and displayed values in %. Fitting a straight line to the data gives: Actual SOC = 0.38545*(Displayed SOC) + 26.4%
- Charge Limit: Reads a constant 35000 watts. At 282 volts that's 124 amps.
- Average Cell Voltage: My min/average/max for this PID was 3.48/3.71/3.99 volts
- Minimum Cell Voltage: My min/average/max for this PID was 3.44/3.70/3.97 volts
- Battery Cell Variation: My min/average/max for this PID was 0.00/0.02/0.07 volts. On a statistical basis the % of times each value was registered was:
0.00=0.3% 0.01=58.8% 0.02=24.3% 0.03=10.1% 0.04=6.0% 0.05=0.4% 0.06=0.1% 0.07=0.02% (only once!)
- SOC Module Variation: Reads a constant 1.62% - not sure what this means. Will it go up?
- In Car Temp: Mine read 52 or 54 F the whole time (we Virginians are tough!). Of course this is probably the inlet cooling air, so with 30s outside and recirc heat likely on up front, the air reaching the battery could well be that cool (but then we don't sound so tough).
- Fan Speed: Read 0 during this run - I guess the HVB wasn't warm enough.
So back to health, here are some possibilities.
- Battery Cell Variation: With age, this might go up. Right now I have a maximum of 0.07 volts. You must be careful though because the maximum values appear to depend on the maximum current. Will try and set up a "standardized" test where the battery is run through heavy charge/discharge cycles of, say, +/-100 to 120 amps.
- Battery Cell Average & Minimum Voltage: Might be useful but the above "variation" may be enough of an indicator.
- Battery Resistance: This value must be calculated. Using the average pack voltage, voltage variation with charge/discharge and battery current, the resistance of my battery appears to be about 0.10 to 0.15 ohms. Again the above test should give a more consistent value. Resistance may vary with SOC and temperature and is expected (?) to go up with time.
- Battery Age: For reference
- Total Miles: For reference (and maybe lifetime EV miles as well)
Sure wish I had numbers from when The Enterprise was new - and from other cars out there!