Introduction to traction battery rebuilding/refurbishment

Introduction

If rebuilding a traction battery, let's go over the basics, instrumentation, and tools. I had bought a failed traction battery in 2007 and a serviceable salvage battery that summer. The failed battery modules were used in rehydration experiments. But eventually I replaced my perfectly fine, NHW11 battery with a Re-Involt battery to gain the advantages of reduced internal resistance and stronger terminal seals.

Theory

An NiMH battery discharges by transferring hydrogen ions (aka., protons) between a nickel electrode, the potassium hydroxide electrolyte and the interior spaces of a metal matrix. The hydrogen ion gives up an electron to the nickel electrode and joins with nearby, potassium hydroxide ion in a saturated water solution. This in effect bounces a hydrogen proton on the far side that gains an electron from the metal hydride matrix and suddenly becomes an atom. But the local pressure forces it into the matrix where it is stored. The metal hydride can store hydrogen by itself if put under pressure and at one time was considered to be a way to store hydrogen for other purposes.

To charge the battery, a hydrogen atom in the matrix loses an electron as it is forced into the potassium hydroxide electrolyte. Unbalanced, a hydrogen ion on the far side joins with the nickel electrode, a nickel hydride. But along the way two bad things can happen, heat and electrolysis, a small fraction of the water converts into O{2} and H{2}.

Charging is exothermic which causes everything in the battery to expand, the internal pressure increases. But the trace O{2} and H{2} seeks a way out. The case is a solid piece of the thermal plastic except at the terminals. There is a soft, rubber "O" ring between the nickel terminal and the plastic case that at ordinary, room temperatures, is quite effective. But as the temperatures increase, the "O" ring can melt and let a little of the O{2}, H{2}, and electrolyte escape. This highly conductive, potassium hydroxide seeps out and can form a weak current path to the case . . . the ground fault detected that causes the P3009.

But over time, the escaped H{2} and O{2} causes the potassium hydroxide to convert from a saturated, conductive solution to small, potassium hydroxide bits which no longer can pass hydrogen ions between the nickel electrode and metal hydride matrix. This forces all transfers into smaller areas through a plastic fiber separator. This increases the heat at points and eventually, it melts a small hole, a permanent short, and the cell fails. So far, it sounds as if you may not have a failed cell, yet, but the clock is ticking.

Now the 38 modules are connected in series. Every pair has a voltage sense wire, 19, that goes to battery controller to measure the voltage pairs. Nominally at 7.2 volts, the voltage pairs are at ~14.4 volts but depending upon state of charge can range higher or lower. But the potassium hydroxide is wicked stuff, drain cleaner and it 'eats' copper, the stuff the sense wires and buss bars. It is not uncommon for folks rebuilding a traction battery to find one or more sense wires break in the insulation, a break hard to see but easily detected with a different code and the module pair voltage being off.

Now the traction battery is very heavy, nearly 100 lbs, so it makes sense to carefully remove it from the car and put it on a bench. The case comes off easily enough and removing the buss bars including the one with the sense wires is easy enough. The bottom of each module has two screws holding it in the case. The end compression plates hold the modules whose surfaces interlock.

Now it is very important that the modules be at the same state of charge and voltage level along with similar amp-hour capacity. If one module is charged higher than the rest and the traction battery is charged that one module will get hot and generate gas. Unchecked, it can and will burst. Overcharge the traction battery and it will destroy the compression rods and case.

On the other end of the scale, an undercharged module, upon discharge the voltage can get so low that a cell ceases to work as a battery. Reverse charging an NiMH cell converts it into a useless resistor. The literature discusses 'activation' but so far, I've not found a clear description of this protocol and home experiments have not found it either.

So to summaries the goals of rebuilding:

• don't break the voltage sense wires - this will cause another fault
• do remove all external, leaked electrolyte
• do balance the charge, the voltages of each module
• do use same Ahr capacity modules (if you have an inventory)

Tools

Critical is an OBD scanner that can read the information from the battery controller. This helps identify failed or failing modules, temperature issues, and which sense wire broke. Ones I've used are:

• ScanGauge II - ~$150, it requires button programming XGAUGEs to read out traction battery metrics and shows only four values at a time. Still, the 30, programmable XGAUGEs are enough to see what is going on. It is a little tricky to connect to the OBD port but this really is a primary instrument for any NHW11 owner.
• AutoEnginuity - ~$500 with your laptop, this has everything needed to maintain a Prius. It can also record some sets of data, one ECU at a time.
• Graham Miniscanner - no longer in production, about 300 exists in various states of repair. It covers 50 critical values and has a serial line that can dump any six data values in a one second period. Quirky, it has been my first and remains my first tool for engineering studies.

There is a TechStream-Lite similar to the AutoEnginuity but expensive, $1,200-1,500. You can find Chinese knockoffs with pirated software. I have no experience with either.

The amp-hour capacity can only be measured by a discharge-charge cycle. This can be measured manually with a load resistor, power supply, volt-ohm meter, and clock. But it is mind-numbing boring and if not watched carefully an over or under charge destroys the module. So I use a computer controlled, RC hobby charger, an MRC 989. At $150, it is not cheap but it reliably and safely can cycle discharge/charge a module and report the value.
A digital, VOM is assumed and I have no preference.

Recommended Protocol

1. Using OBD scanner, measure the 19 module pair voltages. Module pair 1 is adjacent to the control electronics. Look for the lowest and highest pair.
2. Drive the car for at least half an hour checking the four temperature probe values. Three are in the battery pack and one measures ambient air temperature. Look for a difference of 3C or greater, the tell of a failing module. The 19 voltages tell which pair.
3. Remove the battery taking care not to crush the external wires or connectors and put it on a bench.
4. Survey the 38 modules measuring each voltage. Any modules off by 1.2V from the others has a bad cell and must be replaced.
5. Carefully remove the buss bars inspecting for corrosion, the tell of electrolyte leakage.
   1. If just addressing the P3009, clean everything with a mild acid and wash throughly with distilled water. Dry and if you have a high-voltage, MEGER, verify it is above 1 megaohm.
   2. You have the option of shifting weaker modules to pair them with stronger ones based upon voltage.
6. Remove any failed modules and if you have inventory, the modules on either side that were exposed to excessive heat.
7. Use a computer battery charger to measure one of the existing, module Ahr. Then select from the inventory of spare modules ones that match to replace the failed modules. Normalize all modules (aka, charge them to the same voltage.)
8. Reassemble without breaking the voltage sense wires.
9. Reinstall in Prius.
10. Test.

The reasons for going with the Re-Involt pack:

• Inventory of replacement modules - they survey them so matched modules make up the new pack.
• Nickel plated buss bars and wires - this protects them from KOH attack.
• NHW20 modules - these have stronger terminals to resist leakage and half the internal resistance to reduce ohmic heating and local heat concentration.
• Professional assembly - they've done this before . . . probably hundreds of times.
• One year warranty.

However, it is perfectly OK to give rebuilding a hack and even taking the risks of some shortcuts. After all, every rebuilder had to do it first, once. <grins>

GOOD LUCK!
Bob Wilson