Balancing and Reconditioning a Gen 1 Insight Hybrid Battery Pack part 1: The Challenge
One problem I consistently have is that no project of mine is ever finished, and as such I can never get around to writing up a finished project report. I've decided to start chronicling these projects in smaller, incomplete steps, with searchable tags, so that you can find and follow progress in detail as it happens and in the future.
This is the first post regarding the work I am doing on reconditioning the hybrid battery pack in my Insight.
First, a little background: The First Generation of the Honda Insight (ZE1) was introduced in 1999, and was one of the first hybrid cars to be sold in the U.S. It is also, to this day, the most fuel efficient vehicle ever sold in the U.S., continuing to outperform newer hybrid models including newer Insights by a wide margin. Why haven't we made a better one yet? That's a good question. You should ask automotive company execs.
The bottom line is that thanks to the car's exceptionally light weight, good aerodynamics, and small displacement inline 3 motor with a lean burning mode that allowed it to run with an Air-Fuel Ratio as high as 20:1, it can achieve fuel economy of upwards of 70mpg completely unmodified, with some hypermilers squeezing up to 100mpg out of it without any major changes to the drive system.
The downside is that due to the car's age, it uses relatively archaic power systems, including old Nickel-Metal Hydride battery technology, which has fallen out of favor (and support) since the emergence of superior lithium-based battery technologies. As a result, all of these battery packs are 20+ years old and aging out, with cells falling out of balance, losing capacity, or outright failing. Almost nobody uses NiMH cells for anything anymore, and so nobody produces tools for maintaining or servicing these old battery packs.
NiMH battery chemistry is difficult to work with using tools intended for Li-Ion and similar chemistries, because it behaves differently, and operates at much lower cell voltages. In fact, while many Li-Ion chargers claim compatibility with NiMH batteries, most of their features are not available for NiMH cells, and will not function in that context due to the nominal cell voltage of 1.2v being below the minimum safe operating voltage of the Li-Ion cells they are primarily designed for.
The ZE1 Insight uses a battery pack comprising of 20 'sticks' of 6 roughly "D" size cells each in series, strung end to end, for a total of 120 series cells, with a nominal voltage of 144 volts. Because the cells are all in series and do not have any active balancing, some cells will charge and discharge at slightly different rates, and over time their relative voltages and capacities will drift apart. This becomes a problem eventually as cells with relatively lower voltages can be over-discharged when the battery pack is drained, resulting in loss of further capacity and degradation of performance of those cells with each charge/discharge cycle. The lowest performing cells can actually become inverted, permanently damaging the cells and burning them out. As cells drop out, the total voltage and capacity of the pack is reduced, and the remaining cells need to absorb more charge, and cannot be fully discharged. The resulting self-reinforcing cycle of deterioration will eventually cause the battery to lose most of its capacity, and the IMA (Integrated Motor Assist) computer will throw a code and stop using the battery, resulting in the loss of the much-needed low end power/torque that the IMA provides to keep the car running efficiently, particularly in stop and go traffic.
While modern Li-Ion batteries use active cell balancing and monitoring circuits to keep the cells in balance, NiMH packs do not have this. In most cases, NiMH packs do not require it, due to the fact that NiMH cells, unlike lithium cells, are not damaged by slight overcharging at low current, and as such, each time the battery is 'topped off', the highest voltage cells bleed off some of the excess charge as heat, allowing the lower voltage cells to catch up. However, due to the long service life and limited operating ranges of the hybrid battery, these charge corrections don't happen frequently enough, and the pack health deteriorates, until it can no longer function without frequent "grid" charging (a modification to manually charge the battery and top it up while the car is parked), and eventually even that will fail to maintain the pack, and it will die.
The typical 'fix' for this problem is to simply buy a new or refurbished battery, but as supplies dry up for new cells, and aftermarket suppliers stop supporting this aging and increasingly rare battery pack, the situation becomes increasingly dire and expensive for ZE1 Insight owners.
Some particularly handy individuals will even break down the packs and test each 'stick' of 6 series cells, replacing just the bad sticks to eek some extra life out of the aging packs. This is a more efficient way to extend the service life of older packs, however, it wastes the increasingly rare and precious healthy cells in a partially failed stick.
The goal, therefore, that I set out to achieve, was to rebalance and recondition each cell of each stick as best as possible, and replace only the failed/failing cells of each stick as necessary, to bring the battery pack back to a healthy functioning state. To do this, it was necessary to re-invent some forgotten wheels... the tools for balancing and maintaining NiMH series battery packs.
So far, this endeavor has included the design and construction of 3 different devices to augment the available tools for the job:
- A whole-pack Adjustable DC Load Discharger (to supplement a whole-pack grid charger for complete cycling of the pack)
- A 6-series passive NiMH Cell Banlancer and voltage monitor (for use with a basic charger to charge and balance individual sticks)
- A single-cell NiMH load tester (to assess the health of individual cells)
A breakdown of the design and construction process, as well as practical results for each will be coming in future posts. For now, here's a teaser of a couple finished units:
