How to measure traction battery capacity

This post describes what is needed to measure the health and status of a Prius traction battery. It applies to all Prius but obviously the older Prius owners (and new owners of older Prius) may have more interest.

REQUIREMENTS

• Prius - any model
• Prius traction battery aware OBD scanner
  • SOC
  • current
  • min block voltage
  • min block number
  • max block voltage
  • max block number
  • time-stamped, record keeping pad or data file
• 500 ft / 160m hill with a parking area within 0.5 mile or 1 km of bottom and at least 5 mi / 8 km run to top

THEORY

After driving the Prius for at least 20-30 minutes, normally, you descend the hill using "D" to put a peak charge on the traction battery. Then using the battey aware, OBD scanner, you will park the car with the rear wheels against a curb and put it in "R". Use the battey OBD metrics to measure the raw data showing the Ahr as a function of traction battery SOC. This will be used to calculate the Ahr capacity of the traction battery as well as look for evidence of weak pairs.

Once you park the car and put it in "R", record the following:

1. time, current, SOC, min pair volt, min pair number, max pair volt, max pair number
2. record a second set of step 1 data
3. wait 2-3 minutes
4. repeat 1-3 again, when SOC reaches 45%, shift to "P" and let car recharge until engine stops

Take the recorded data home and enter into a spreadsheet. Average the pair of data observations for each 2-3 minute period. Calculate the amps/hour as a function of %SOC. Once the rate of SOC change per amp/hour is calculated, use the observations to project the 100% Ahr value.

Use the max and min block values and numbers to identify the weakest set. If the lowest set changes over the different observations, the pack is in good shape. If the top set changes, your pack is in excellent shape. If the same pair show up as both top and bottom, WOW! Sleep the rest of the blessed.

Another useful metric is are the four traction battery temperatures. One, the inlet temperature, will be lower than the other three. If they are within 3-4C, no problem BUT this only is a general rule of thumb. More than 4C and it suggests there may be something not good going on.

There is a code thrown when the voltage difference is greater than 0.3V. However, it isn't clear if this has to be for some 'given period of time.' I've seen such differences in the past with single observations suggesting it takes some "n" samples before the code is thrown.

Although it is tempting to think charging would show a similar pattern, in fact, charging is not at a constant rate. As the traction battery SOC increases, the current decreases. If there is automatic data recording, it can be used but the "charge rate" influences the Ahr capacity. The discharge rate will be constant.

Bob Wilson

 

I'm done for now . . . need to take a nap.

Bob Wilson