Steve K, who we are not affiliated with in any way, used the Prolong battery reconditioning process to recondition a weakening 2008 Prius hybrid battery and documented the following before and after comparison:

All images & content below courtesy of Steve K. Used with permission.


Car: 2008 Prius, 143K miles. Purchased and operated its entire life in the Phoenix, AZ area. Original owner vehicle & battery.

Owner complained of deteriorated mileage. Mileage had dipped from the low-40s to the mid-35s. When the owner made a concerted effort to get good mileage, about 42 was the best he could achieve.

Techstream reported very consistent IR of .023-.024 and a typical min/max block voltage difference of 0.2V. Driving charge/discharge tests didn’t show anything alarming. The only outward signs of an issue were the battery meter dropping pretty quickly and reduced fuel economy.

Baseline tests were conducted as follows:
Force charge to max SoC.
Drive to test location (about a mile)
Conduct discharge test via reverse and slipping the brake

Chart of output:

Capacity was computed by the average current between each time point and represents the area under the red curve. The average current of all readings was 15.3A. The total capacity used was 625mAh. The range of SoC was then used to extrapolate the total capacity of the pack. In this case the SoC went from 73 to 38.5 or 34.5% total. So 625/.345 = 1813mAh extrapolated total capacity of the pack – about 27.9% of the rated 6500mAh.

Prolong charge/discharge process performed:

  • Force charge with car to max SoC
    Charge for 24 hours with battery cooling fan running at all times.
  • Discharge with 2x 200W bulbs in series to 170V (current ranged from 1.7-1.4A)
  • Discharge with 2x 25W bulbs in series to 85V (current ranged from 0.17-0.12A)
  • Charge for 8.5 hours with battery cooling fan running at all times.
  • Reset 12V and car charge to max SoC
  • Reset 12V again and car charge to max SoC.

Conducted another discharge test that I was unable to export for some reason, but here is the Techstream chart:

It looked improved, but with a lot of tedious effort in transcribing the data, I couldn’t quantify the results in the same way. Additionally, the engine restarted at just under 50% SoC, which led me to believe that the ECU did not yet have an accurate representation of SoC.

Drive 70 miles over two days. Repeat initial discharge test. It went much better. I’m able to maintain higher currents more consistently. Here’s the chart:

As you can see after the first minute or so, the current climbs, and I’m able to maintain 35-45A discharge throughout most of the test. The average current for this test is 25.5A – significantly higher than the original.

Computed Capacity: 1,679mAh
Extrapolated Capacity: 5,895mAh or 90.7% of rated.
DeltaV also dropped from a high of 0.58V before to 0.38V after.

In addition, the performance feels great. I was able to easily drive the vehicle at speeds up to 30mph in EV mode before the ICE kicked in (1.6mi). Mileage of 48mpg+ is attainable with only minimal efforts to “drive it like a hybrid.” Getting less than 43 is tough even when I’m accelerating at full throttle to freeway speeds and deliberately avoiding regen (putting it in “N” when off-throttle and coming to a stop).

Concerning the results, the owner is thrilled to report his wife has seen the mileage climb to 48 with her normal driving.