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Battery repair equipment & theory

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Hybrid vehicle battery reconditioning, repair and maintenance equipment for individual | fleet servicing

What is an HEV battery module?

Modules are nickel metal hydride (NiMH) cells connected in series. NiMH cells are typically rated at a nominal voltage of 1.2 V. A Ford Escape module has 5 cylindrical NiMH cells in series. A Toyota Prius module has 6 prismatic NiMH cells. The full traction pack battery consists of many modules configured in series. A Ford Escape has two 25-module traction packs. A Prius has a single 28-module pack.

  cells/module module V type modules/pack pack V
Ford Escape 5 6 cylinder 25 150 V
Prius 6 7.2 prismatic 28 202 V
Honda 6 7.2 cylinder 26 187 V

What does “out of balance” mean?

The driving cycle of HEV battery modules permit only a shallow depth of discharge that utilizes no more than about 10% of the battery capacity. Unused electrode material becomes electronically separated, and essentially unavailable with time. This reversible failure mode (i.e., module-to-module variation of amp-hour (Ah) capacity) results in an “out of balance” battery pack. This is often referred to as the “memory effect”.

What is battery cell balancing?
Cell balancing is also referred to as “reconditioning”: The cell balancing process (CBP) requires (1) extraction of the battery pack, (2) removal of high voltage cable, (3) parallel connection of modules to a charge discharge system, (4)  rapid identification and switch-out high resistance modules (e.g., > 10 mΩ for a 6 cell module) and finally (5) subjection of modules to deep, split-current, cycling (i.e., the CBP). Cell balancing must be done with modules in parallel because individual module voltages cannot be controlled when connected in series. The first step in the CBP is to level all modules to a common state of charge (resetting). After resetting, the modules are subjected to an automated CBP with charging currents tapering down as the end of charge voltage is approached.

What tool is used for battery conditioning / cell balancing?
The EVc-10 and EVc-30  are fully automated battery cyclers that balance (or recondition) modules that have undergone reversible failure modes such as “memory effects” due to repetitive shallow depths of discharge. EVc units not only execute CBPs, but also output detailed state-of-health parameters on each and every module processed. An accompanying analysis package, PackBuilder, organizes module inventory data and suggests selection of modules for pack assembly. PackBuilder requires use of a QR code reader. EVc units now include the Code Reader 900FD.  

EVc-30 hybrid vehicle battery reconditioning unit connected in parallel to a 28-module Prius pack

How long does it take to recondition a hybrid battery pack?
Cell balancing can take 12 to 72 hours depending on your choice of the end of discharge voltage (VEOD). NiMH modules have system parameters specific to the form factor (e.g., prismatic versus cylindrical) and number of cells in series (e.g., Ford Escape: 5; Prius prismatic: 6; etc.).  These parameters are the VEOD and the reset current (IRESET). For the Prius pack, the historical value VEOD is 6 V.   IRESET is the highest current possible to reset (discharge) to VEOD with a relaxation (i.e. bounceback) of less than 3%. Resetting is step-one in a CBP.  While NuVant is establishing IRESETa reasonable value for now is 100 mA.  Using an IRESET(100mA) results in a CBP exceeding 40 hrs.  This can be shortened by increasing VEOD, for example to 6.8 V.  However, the higher the VEOD, the lower the quality of the CBP.  A “family” vehicle with less than two years of lifetime may benefit from a shorter CBP for lower warrant costs.  CBP protocols and their automated implementation are covered in the NuVant battery reconditioning course.  The business model is not extract-balance-reinstall  on a per customer basis, but rather development of an inventory of characterized modules for instant replacement of customer packs. This requires a variety of VEOD  values depending on warranty options.

What battery conditioning data does the EVc provide?
The EVc gives internal resistance, amp-hour, watt-hour capacity and real-world nominal voltages for every module. The separate PackBuilder package uses this data to rank modules for pack rebuilding. EVc output files are shown below for some popular traction battery packs.

Toyota Highlander (click on figures to expand)

EVc-30 provides complete data for every module reconditioned.

EVc-30 data: Toyota Highlander pack before reconditioning.

Note the uniformly low internal resistance of the EVc-30 reconditioned pack.

EVc-30 data: Toyota Highlander pack after reconditioning.


Ford Escape (click on figures to expand)

Before reconditioning there is wide variability in the amp hour capacity in internal resistance.

EVc-30 data: Ford Escape pack before reconditioning


Pack reconditioning removes variability in Ah capacity and internal resistance.

EVc-30 data: Ford Escape pack after reconditioning



Toyota Prius (click on figure to expand)

There is wide variance in Ah capacity and internal resistance prior to reconditioning.

EVc-30 data: Toyota Prius pack before reconditioning

Reconditioning removes Ah and internal resistance variability.

EVc-30 data: Toyota Prius pack after reconditioning


Battery-sorting software module

Critical battery parameters are calculated

Real time display of comparative battery parameters and charge/discharge curves










The PackBuilder database analyzes the above data and suggests modules from your QR coded inventory for assembly of packs.  Addition information concerning the PackBuilder will be provided in the short courses.





How do I learn more about battery repair prior to purchasing an EVc?
The hands-on hybrid electric vehicle battery reconditioning short course allows participants to connect Prius pack modules to an EVc reconditioning unit for cell balancing. During the reconditioning process, the terminology required for understanding cell balancing will be thoroughly explained.   On the second day, results of the overnight process are thoroughly discussed along with data processing by the PackBuilding analysis package.  

Testimonials from existing EVc users

Click here for specifications and prices.



Max Load Voltage  10 V/channel (Customized up to 20V)
Max Load Current 5A per channel
Number of channels  10, 30 or 40 channel systems
Applied Current Accuracy 0.5% of the Applied Current
Applied Current Resolution .3 mA
Potential Measurement
Measured DC Potential Ranges 10 V/channel (Customized up to 20V)
Resolution  .6mV
Accuracy 0.08 or 0.03% of FSR
Current Interrupt Resistance 
Data Acquisition Speed 12.5 kS/s
Serial Resistance Precision 0.5 %
Measurement logging time 1 second


What if I need additional training after an EVc purchase?


New owners of EVc reconditioning equipment receive a complementary 5-hour EZtime ticket. This complementary on-line (TeamViewer) training provides sufficient time for remote reconditioning of your first 2 packs simultaneous with the suggested training agenda.  

  • On-line training provided after customer receives purchased EVc unit. Training includes:
    • Unpacking and set-up of EVc unit.
    • Connections to prismatic or cylindrical modules
    • Use of EVc to identify high resistance modules
    • Module reconditioning
    • Database module sort and pack build-up.
  • Additional 5-hour EZtime tickets can be purchased for $175.


Call 219-644-3231 for more details