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Hybrid battery reconditioning overview.

What is a hybrid battery?

The hybrid electric vehicle (HEV) traction battery is for start-up from a stationary position (e.g., a red light) and regenerative breaking. Most HEV batteries are nickel metal hydride (NiMH) cells configured in series as modules. The fully charged negative and positive electrodes are the metal hydride and nickel oxyhydroxide (hereafter the “nickel electrode”) respectively. The left image is a NiMH module composed of 6 cells (1.35 V each) in series. A Prius battery is 28 modules in series (right side) at about 210 V. The Ford Escape and Honda have cylindrical modules with 5 and 6 cells respectively.

Toyota Prius module with six cells Toyota Prius battery removed from vehicle enclosure

What is an unbalanced battery pack?

The HEV driving cycle subjects the battery to repeated shallow depths of discharge of less than 10%. This causes a reversible reduction of battery module amp-hour (Ah) capacities. The modules seem to have a “memory” of the reduced capacity demand.  Module Ah capacity loss does not occur evenly across the battery. When a battery has modules with substantially variable Ah capacities, it is “out of balance” the vehicle warning light is triggered. When an out of balance battery discharges, the module with the lowest Ah capacity expends its “nickel” electrode material prematurely.  The neighboring modules then force the spent cathode (nickel hydroxide) voltage to shift negative of the metal hydride electrode (reverse polarity). Hydrogen is evolved from the reverse polarized nickel electrode. Further discharge of the battery can expend the metal hydride electrode. The metal then shifts so positively that the rare earth metals are oxidized. After that, oxygen gas evolves from the metal oxide. At this point, the module is in danger of exploding.  One fully reverse polarized module can take down an entire battery.

Batteries, like people, breath.  Electrodes expand and contract with charge and discharge. If your breathing is chronically shallow, your lungs lose capacity.  The solution, deep breathing exercises, recovers lung capacity.  Battery deep breathing is deep depth-of-discharge cycling.  There are additional tricks employed by battery doctors that involve pulsing, high power discharge), etc. We call the sum of these processes reconditioning.  

How is a hybrid battery reconditioned?

The battery is removed from the vehicle and the high voltage cable is removed. Hereafter, all reconditioning steps must be done with all modules in parallel. The voltage of individual modules cannot be controlled when they are configured in series.  Parallel processing affords complete voltage control over each module. Reconditioning is time consuming because the charging currents must taper down as full charge is approached. This is referred to as the CCCC method or controlled current-controlled current method. Charging can start at several amps. As full charge is approached, the charging current must taper down to below 500 mA per module to prevent excessive oxygen evolution at the nickel electrode. This oxygen must diffuse to the metal hydride electrode for recombination back to water. This causes module over-heating at higher currents. The deep charge-discharge process (including pulses) reintegrates, or homogenizes electrode material. This is reconditioning. The time required for proper reconditioning is 20 – 24 hours of tapered charge discharge cycling and Ah capacity analyses.

How are hybrid battery modules balanced?

After modules are reconditioned, they must be positioned within a battery in such manner as to minimize voltage differences between modules during vehicle operation. The first step is to select 28 Prius modules such that the standard deviation of the average is less than 5% (the smaller the standard deviation, the better). After modules are selected from inventory, they are paired in blocks. The highest capacity module is paired with the lowest capacity module.  The next block is again assembled with highest and lowest capacity modules.  This is continued until 14 blocks are assembled. The blocks then assembled into a battery. The high voltage harness is attached to the balanced battery for vehicle installation. Not all modules can be reconditioned: There are irreversible failure modes.  Modules that have experienced full polarity reversal, have vented substantial amounts of water due to overheating, or are leaking electrolyte, cannot be reconditioned. Most modules, from an original battery, can be reconditioned.

What tools are required?

The required tools are (1) a menu driven charge discharge cycler, (2) a database for storage of amp-hour capacity, midpoint voltage and internal resistance of every module in inventory, and (3) a QR code reader to manage inventory.

About the author:  Dr. Eugene Smotkin is the founder of NuVant Systems Inc. and a Professor of Chemistry  at Northeastern University. He is an affiliate faculty member in the Chemical Engineering Department and teaches Energy Systems for the mechanical engineering department. He has published over a hundred peer reviewed papers and has 14 patents.  He authors the “Electrochemical Energy Storage: Current and Emerging Technologies” chapter in the Handbook of Industrial Chemistry and Biotechnology.