Introduction

This web page is less and essay than a collection of photos, graphs and text describing my 2003 Prius (model NHW11) battery experiments. For now, consider it a journal or notes for a future chapter.

Spare Battery

By happy accident, a salvage traction battery came available on Ebay:

Eventually I will instrument and refurbish this battery for advanced testing. However, my current battery is quite healthy in part because I take pains to keep it from over heating.

MPG After Battery Replacement

Patrick Wong had a warranty battery replacement in his former, 2001 Prius. Earlier data from the Dept. of Energy fleet studies had suggested that a new Prius has a slightly better mileage performance in the first 3-4 months. Patrick's data suggests replacing the battery does boost mileage for a short period of time:

Battery Compression Plates

Unlike a standard, steel cylinder NiMH battery, the thin plastic sides can not contain any significant pressure from gas formation. This means we need compression plates with holes for the interlocking plugs and nipples. In my case, I used thin plastic sheets from a hobby shop to build the battery interface sides. But I built up layers of plastic and aluminum foil to make a capacitive, load measurement cell. The capacitance increases as the batteries swell and push against the compression plates:




This shows a module under test using an MRC 989 battery charger and the capacitance tester measuring the stress as the battery is charged and discharged.

Comparing NHW20 and NHW11 Modules

There were substantial improvements in the NHW20 modules over the NHW11. The case visibly has more plastic and the terminals are slightly lower and out suggesting the cronic leaks have been addressed. These changes also mean we can not mix NHW20 modules with NHW11 but it may be possible to replace all 38 NHW11 modules with NHW20s:






Unrecoverable Shorted Cell

In December 2007, another owner of a 2003 Prius decided to pay for his out of warranty, battery replacement. This made sense because the $4,500 was much less than trying to buy a replacement, 45 MPG car. I bought his old battery pack knowing the modules would be worn out and I could use them to experiment with battery refurbishment. Sure enough, of the 38 modules, one had an unrecoverable, shorted cell. No amount of water replacement can correct a cell whose separator has melted and shorted the electrodes:

I soon discovered that gas generation was pretty aggressive even using the relatively low currents, 6 A., from my MRC 989. In this photo, bubbles of electrolyte after water replacement are bubbling out:

This is a very caustic, KOH solution that will eat paper, cloth, eyes or other soft tissue. It feels slipery but you do want to wash it off and keep it away from cuts and open sores.

Inside of Module

Upon realizing this module had a permanent short, I decapitated it to understand the internal structures and get parts for further testing:



Alternate Sealing Mechanisms

Knowing these modules lose water due to electrolysis and heat, I investigated using stainless steel "tee nuts" as water fill access port:


Subsequent failures of the "tee nuts" suggest that one-sided, heat sealing is not sufficient. It may be possible to put a plastic cover over the top of the tee nut but different thermal expansion rates make this desirable modification somewhat impractical ... darn it!

Electrode Elements

One by-product of decapitating the module was a collection of electrode elements: (1) nickel metal sheeting, (2) metal hydride electrode sponge, and (3) plastic fiber separator. It is melting of the plastic fiber separator that leads to a permanent shorted cell:


Battery Electrode Activation

One critical aspect is to find out if water replacement can restore battery capacity. It does but only after the electrodes are reactivated by a series of charge-discharge cycles:



We also learned that heating the battery accelerates electrode activation. But the most important lesson is we don't have to do a full charge-discharge cycle. A partial charge-discharge seems to be just as effective allowing many more cycles and less power to be used.

Don't Use Manual Charger

One of our Prius Technical Group members put a charger on his NHW20 pack without an automatic charge shutoff system:

I have found using an ordinary lead acid battery charger, even with current limiting resistors, can do the same. At a minimum, your battery charger should have a dV detection as well as a temperature probe backup. No, you can not promise to "be good," either use a automated shutoff or destroy some batteries and then use an automated shutoff.

In Situ Testing

The Graham scanner is able to report the traction battery current and voltages of the lowest and highest pair of modules. By using the trend function over a set of data, the slope of the line should be proportional to the internal resistance of the weakest and strongest module pairs. In this case, the slope of the trend lines are nearly identical suggesting the differences are not too far off. However, we don't have new cells to compare this data to figure out the relative capacity of the modules:

Forced Charge Heating

One of the earliest observations is charging NiMH batteries leads to module heating, it is an exothermic reaction. The Graham scanner will report the lowest and highest module pair temperatures:


A paper titled "Thermal Behavior of Small Nickel/Metal Hydride Battery during Rapid Charge and Discharge Cycles" Takuto Araki, Masato Nakayama, Kenichi Fukuda, and Kazuo Onda" has figure 10 charts showing temperature response of an AA sized battery under charge and discharge. This excellent paper includes a model including the chemical reaction heat functions and matches these graphs. They had excellent agreement between theory and measurement:
These charts show 2C charge going from 60%, nominal Prius battery SOC, to 80% forced charge limit versus discharge from 20% to 40% discharge, the ratio is:

If we take the forced charge data from my April 28, 2008 exerperiment and plot how many seconds it takes per 1C change, we find charging has much shorter intervals as a function of Ahr versus the longer times of similar magnitude discharge:
The larger size of an NHW11 module compared to AA batteries means there is some thermal lag. The NHW11 thermistor probe is located on the top of the battery, as about 5x further from the center of battery mass than an AA battery.

Module Voltages In Marginal Traction Packs

Two users have provided either individual module voltages or Toyota reported voltages for each module pair. What this shows is the end modules, those most likely to be coolest, seem to be healthiest. In the first case, two modules from another salvage pack had replaced failed modules:



This data suggests one strategy would be to take every other end module and swap it with every other middle module. This would have the effect of equalizing the charges in a marginal pack BUT this would only delay the inevitable.

Hill Descending Heating

The other primary source of charging currents is to descend a hill at high speed using regenerative braking versus using "B" or engine braking. As is clearly evident, using "B" does a much better job of avoiding charge current heating of the traction battery:

In Car Module Refurbishment

It had never occurred to me that modules could be removed from the traction battery assembly in the car:

In this case, the JB Weld seals were a little inconsistent. Properly mixed, JB Weld should be the same color as the battery module plastic: