Reconditioning NiMh hybrid car battery packs is time-consuming—you cannot rush it! However, if you have the patience, reconditioning them yourself can save you more than 90% of the cost of having a car a dealer do the job.
Last fall, I bought my friend’s dysfunctional 2003 Civic Hybrid. It was not something I needed, but it did have only 110,000 miles on the odometer and some nice custom features—and it was cheap! Plus, it had been well-maintained: All the dealer checkups and service had been properly performed.
But besides the car’s computer spewing some error codes, it also had performance issues suggesting expensive battery problems. The car was very sluggish and its mpg had dropped off—from more than 50 mpg to 35 to 40 mpg. It stalled easily; the auto-stop function—which turns off the IC engine when the car comes to a full stop—did not work; and, of course, the “check engine” and integrated motor assist (IMA) warning lights wouldn’t shut off.
The Honda dealership wanted $3,200 to replace the battery pack with a factory-refurbished one (new cells are not available for a Civic Hybrid of that vintage). Replacement battery packs are made from dead packs dealers return to Honda, which recovers the remaining good subpacks for reuse. These refurbished packs include a one-year factory warranty. There is at least one third-party that claims to do the same thing for $1,250, but you have to sell him your old battery pack first, which he uses for salvageable parts. The few online bloggings from DIYers who tried reconditioning their own packs were not very insightful. However, by doing a lot of “reading between the lines,” I felt I learned enough to try my hand at refurbishing.
The root of the batteries’ underperformance had to do with the nickel metal hydride (NiMH) cells getting warm from discharging and charging. After thousands of cycles, the fine nickel powder inside starts to form larger nickel crystals, which eventually impede the electrons from flowing. If the crystals get big enough, they can puncture the separators between the positive and negative sides of the cell, shorting the cell. Reconditioning helps break up these crystals. Although it won’t make the cell like new, it’ll result in a better-functioning cell.
All you need to recondition IMA battery packs are a few basic tools: a good digital multimeter; the right charger/reconditioning machine (there are several to choose from online); a #30 Torx driver; standard hand tools; and, most importantly, the will and time to do the work right. Because this project can take several weeks of charging, you may need a second car to drive until the project is completed.
Hybrid car blogs published plenty of warnings about getting electrocuted from the batteries. Since this was new territory for me, I took this advice seriously—but there is only one place on a battery pack where any real hazard exists: from voltage across the full pack, which can be up to 180 VDC.
That single location is easily found with a multimeter, and once you turn off the IMA pack’s built-in circuit breaker and start unbolting the individual subpacks, the risk drops significantly since the sticks are only about 8 V each. Honda designed the battery pack in an almost idiot-proof way. The terminal bolts and other fasteners are arranged so that it is easy to avoid touching two ends of any higher-voltage terminals at the same time. Common sense coupled with basic mechanical skills meet most everything else you come across during this task. But working carelessly can still hurt, as well as cause you expensive additional repairs.
Start by parking the car in a place it can stay for a month or two. Finding that you need to move the car after you’ve removed the battery pack is a real bummer, since the car will not move under its own power without a working battery pack.
Next, remove the rear seat cushions (three 10 mm bolts and some plastic clips). Then take off the small cover over the battery box’s circuit breaker (two bolts, #30 Torx), remove its red plastic safety cover and turn off the circuit breaker. The 180 VDC electrical hazard is now gone—the breaker separates about 40% of the sticks from the rest of the pack. Nonetheless, be careful as you work. To get the main cover off, you will need to break off the little black plastic safety snap/pin to the right of the circuit breaker with a large, flat screwdriver.
Remove the metal main battery box cover (six bolts, #30 Torx) and slide the entire cover up an inch or two to release the two metal fingers on the extreme right and left of the cover. You may need to have someone hold the seatbelts to one side to do this.
Once the cover is off, you will find a bewildering array of wiring and connectors. If you disconnect the 12 V battery (yes, there is a standard car battery, too) in the engine compartment, even that small hazard is eliminated. To do this, disconnect the cable at the negative battery post and insulate the end completely with electrical tape or a similar insulating material. This is safer than disconnecting the positive side and less likely to create a short to the chassis. In any case, continue to be careful not to bang up anything since replacing broken terminals or repairing cut wiring is a major and unnecessary chore.
Inside the battery box, remove the three 10 mm bolts that hold the two main IMA power wires and ground wire for the battery pack. Unplug the five sensor connectors from the pack. Then remove the four 12 mm bolts that hold the battery pack in the metal box (two on the top of the pack and two on the bottom). Gently bend any wires out of the way and lift out the battery pack. It weighs about 50 pounds and you will be stooped over in an awkward position, so protect your back and consider having a friend help with the lift.
Use the existing carry straps to lift it, being careful not to get tangled up with any wires. Take a few photos as you go along so you remember where everything goes in a month (or two) after completion.
I set up my work area in the living room, since it is climate-controlled and well-illuminated. Find an organizable work area, because there will be no less than 90 assorted bolts and screws holding the pack together. As you take apart the pack to access the subpack sticks inside, you’ll need to keep a close count of all the fasteners—use a spill-proof container!
You cannot disassemble the sticks—they are welded together in groups of six cells and tightly wrapped in yellow plastic. These yellow sticks are what you will be “reconditioning” with your new charger/reconditioner machine. Remove each stick from the plastic holding frame to allow for easy judging of how warm each cell gets as reconditioning proceeds. Each stick has a “square” end and a “hex” end. The positive side is the square—mark it as such to avoid confusion. If you do connect the charger in the wrong polarity, it is smart enough to scream at you and display “connection reversed” before any damage can be done to the cells.
Be careful of the small sensor wires attached to three of the sticks—they pull apart easily and are a major pain to put back again. These three sticks can only be removed from the frame by firmly pushing them from right to left (looking at the pack as it would be in the car). The sensor wires are not very long, so carefully bend them so the sticks they are attached to are not pulling on them. Remember (and take photos of) how the wires are routed around the pack and where their little holders snap together. They will only fit in one way without making trouble for you when reassembly starts.
Although NiMh cells were once thought to be “memory-proof,” all nickel-based batteries develop memory problems; NiMhs are just much more resistant to the problem than NiCd cells.
Deep cycling has always been the method of minimizing, and somewhat repairing, memory issues. This is what your charger/reconditioner does, in a very sophisticated way. Its computer brain monitors the cycling cells to optimize breaking up the microscopic nickel “crystals” that form as a cell ages with use. The reconditioning is not perfect and will never make the cell factory-fresh again, but it does go far in making the cell work as well as it can.
Follow the charger manufacturer’s instructions for setting up your reconditioner for NiMh cells. Be sure you understand the instructions. I used an iMax B6 ($55) and the instructions had to have been written by someone whose English was not great—it was difficult to understand at first.
Do not despair if the instructions are too difficult to decipher. The default settings for each type of battery (which are already programmed into the charger) seem to work well enough. But I changed the minimum cell state on mine to 0.8 V per cell, just to get a little deeper discharge and, hopefully, a more functional battery in the end. Do not discharge them too far, which can cause cell polarity reversal, which means you end up with a dead stick!
Number the sticks with a permanent black marker to avoid mixing them up. Have a pad of paper and pen handy to record the values the screen displays at the end of each programmed cycle to track how each stick performed. Believe me, after a month or more of this, it is a hopeless task remembering it all.
Fully charge each stick before starting the discharge/charge reconditioning cycles. But before the lengthy cycling process starts, set the machine for a minimum of three discharge-charge cycles. My iMax can do up to five cycles automatically, but you don’t really need to spend that much time on each stick to sort the good from the bad. With the good sticks, more than three cycles does not necessarily make the stick perform significantly better. If a stick is somewhere “between” good and bad, then the cycling process often straightens it out nicely.
With the iMax B6, each cycle takes at least 10 hours to complete. Multiply that by 20 sticks and you can quickly see that reconditioning the entire pack will take 25 days of round-the-clock reconditioning. Try to time your “starts and stops” so that the charger alarm doesn’t wake you up in the middle of the night, though the alarm can be turned off.
The iMax B6 is made for small, radio-controlled batteries, and with light loading to keep the cells from getting too hot, the IMA conditioning process is lengthy. But it does get the job done with IMA sticks too, and it is less expensive than similar devices. Other reconditioners use much higher discharge rates and cycle faster. However, the additional heat may also cause otherwise-OK sticks to be heat-damaged.
If the stick is working right, it will feel slightly warmer than room temperature and all the cells will be the same temperature during the process. It should also charge to about 6,400 mAh when full and do it in a bit more than one hour from a minimal state of charge (0.8 V per cell). If all goes well, the final numbers in the display will be about 6,400 mAh charged, and 5,700 to 6,100 mAh discharged. If you have any sticks that vary greatly from these values, treat them with suspicion and mark them as problematic.
It is best that all the sticks in your pack discharge to about the same level. Very odd discharge characteristics in some sticks will drag down the better ones, causing the dreaded IMA warning light to come on sooner than it would otherwise. However, a 100 to 400 mAh difference will not present a significant problem to the car’s computer. None of the sticks will have exactly the same discharge value no matter what you do.
At the end of a month of reconditioning, six bad sticks—out of the 20 in the pack—showed up, which explained a lot as to why the car functioned so poorly. Each failed stick was either excessively slow to charge or only at a partial charge after the machine said it was done. During cycling, poorly performing sticks will also get much warmer than correctly performing ones. Heat is one of the items that triggers the IMA warning light on the dashboard.
The next step was finding replacement sticks. I had to replace 30% of my battery pack and the most cost-effective way was to buy a junkyard battery pack and put it through the same charge-discharge process to find its good sticks. This took another 25 days, but was well worth the effort—the car now runs like new, gets 50 to 55 mpg on the highway again, and all the computer errors have vanished. I also have 12 good spare sticks for the next time I need to do this—probably in three to four years, judging by how long the original battery lasted before the computer errors showed up.
Failing IMA packs can also cause seemingly unrelated computer errors, like oxygen sensor failures and a 12 V battery error. The 12 V battery charges from the IMA pack through a DC-to-DC converter—my best guess is that the car’s computer uses it as a reference voltage to judge how well some of the car’s other sensors are working. In any case, all of the original error codes went away after the IMA pack was refurbished.
Michael Lamb s a professional handyman who loves tinkering with electrical things. After 10 years working at the Energy Efficiency and Renewable Energy Clearinghouse as a consultant, he retired to tinkering full-time.