The stuff of nerds…

Hybrid Battery de-Construction

A Hybrid battery is essentially one big battery with many smaller batteries inside. In this example we will breakdown the 2004-2009 (2nd Gen) Prius battery. The battery and it’s cells are manufactured by the reputable Panasonic brand. Each hybrid battery contains 28 replaceable Nickel-metal Hydride (NiMH for Short) modules. But, it doesn’t stop there. Each of those 28 modules contains 6 more individual cells inside (non-replaceable). If your good with math that means that your 2nd gen Prius hybrid battery has a total of 168 individual cells inside. Each of the 6 cells in a module is wired in series, and all 28 modules are also wired in series (Series wiring increases voltage). Therefor, on the cellular level the nominal voltage is 1.2 volts. Thus the total nominal voltage of the pack is 201.6 volts (1.2v x 168 total individual cells). From here on out we will be referring to the 28 replaceable modules, as those are the ones we are targeting for repair. Since we know that each of the 28 cells have 6 individual cells inside and additionally we know that each individual cell has a nominal voltage of 1.2v. Each of the 28 replaceable cells should have a nominal voltage of 7.2v (1.2v X 6 = 7.2v). For more technical battery details check out this article from hybridcars.com Whew, now that we got that out of the way…

The Hybrid Battery Repair Process

1. Step one is to disconnect all the modules in the battery pack (that is, after the pack has been removed from the vehicle and case cover has been removed). All modules are connected in series via copper bus bars. The copper bus bars are removed and placed in an acid based cleaning solution to clean off the years of corrosion they have gathered. They are thoroughly rinsed and a mild base is used to ensure acidity doesn’t remain. Then they are lightly coated with a dielectric grease to help prevent future corrosion.
2. Step two is to do an initial voltage test of all the modules. The approximate operating voltages are anywhere between 7.2v and 8.4v. These are the values the car maintains. The first pass measuring module voltages, typically will indicate which modules have failed. These modules typically register voltages of a 10%+ variance from the other modules in the pack. During this step each individual module is also inspected for any cracks or damage. Those modules are marked for removal and we go on to step 3.
3. Load test the individual modules. A second sweep of the modules under load testing will reveal if any of them are not performing as they should. The modules are load tested with a 1.5a load and aim for a depletion of .5v. The time/capacity of depletion is monitored and anything outside of normal is marked for removal.
4. Replace all the failed modules with modules that share a similar capacity and resting voltage.
5. Balance charge all modules to equal the same voltage + or – .01v to the second decimal place (Ex. 7.64v).
6. Reassemble the modules in the battery pack.
7. Complete 3x Discharge/Charge cycle of the entire battery pack. Each cycle aiming for a deeper discharge of the modules. This step is done to regain potential lost capacity.
8. Re-Check the balance charge of all the modules, balance them accordingly if necessary and reassemble the battery pack.
9. Real-World Testing. The completed battery is then placed in a test vehicle for a 10 mile test drive. All voltages are carefully monitored under full load of the vehicle (roughly 120A) and full charge (roughly 60A) to ensure the best performance of your new battery pack!

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