Working on Hymotion Battery Packs, possible or not advised?

Hi Ron,

There are a couple of possible causes for what you are experiencing. One is loss of cell capacity. It seems unlikely to me that this is your problem, but I can't rule it out completely at this point. What I think is more likely is that you have a cell or two that has higher self-discharge than most of the rest of the pack. There are a some simple ways of figuring out which condition you may have.

Balance after pack stops charging. The balancing algorithm basically charges the pack at full blast until any single cell hits 3.6V. If there is severe imbalance, balancers may be turned on during this period, but A123's discharge curve is so flat in the middle, that the difference in SOC doesn't usually become apparent until you get to the tails of the curve. Once there, the BMS cuts back charge current (in a couple stages, like you are seeing) so that other cells can continue to charge while the high cell can be bled off to a steady max of about 3.62V. The balance resistors only draw about 300 mA, so you can see that once any cell hits 100% it can be a long time before any stragglers catch up (this is the bottleneck when you are seeing ~3 W consumption). Problem is, if the balance target is not achieved within a certain amount of time (I'm not sure how long exactly, but I think ~8 hours is the max for a full charge cycle) the pack gives up and shuts down. ALL YOU HAVE TO DO TO KEEP BALANCING FURTHER IS UNPLUG AND RE-PLUG THE PACK AFTER IT HAS STOPPED CHARGING. You could set up the pack with a timer to do this automatically. Remember that this is a slow process, but is almost surely faster than any self-discharge you may have happening so it's not hopeless. If imbalance is your problem, this technique should gradually reverse the problem. Depending on how imbalanced you may be and how often you are re-plugging your pack, full recovery could take days to weeks so be patient.

If you have this condition, it will get worse if the pack goes unused for very long. We have a Hymotion Prius in the office, and I have driven it probably more than anyone else for the last year and a half. It sat idle for about three weeks once, and I noticed that the top capacity bar on the display wasn't lasting as long as it had been. After using the technique described above, the problem resolved itself fully after about a week. The pack in my own car came from a crate of scrap test packs. The pack had been sitting in storage for probably a couple of years and there were two cells that were well below the rest. I manually balanced them before attempting to let the pack do it automatically. It would have taken weeks to balance just by plugging the pack in. I still use the re-plug technique on that pack a couple times a week to keep it well-balanced. I haven't measured, but based on how fast that top bar goes away, I believe I've seen the apparent capacity fade after a week or two if I don't do this.

As far as trying to fix one of these packs yourself, I really can't recommend it. The biggest problem (in the past, anyway) is that A123 doesn't know who you are or what your skills are, so they have no way of being sure you won't kill yourself trying anything inside there--and the potential definitely exists. Even if you have the knowledge to pull off a repair safely, you surely don't have the tools. Maybe you can figure out where to buy a replacement BMS, for example, but you probably don't have the firmware to flash into it, so it's useless. You probably don't know what to torque everything inside to either, and this is absolutely critical in any battery pack, especially where there is HV present. An undertorqued HV connection can be disastrous, as you are surely aware. Individual cell replacement is impossible in these packs due to the module design. The smallest replaceable block is a 1/2 module which contains 44 26650 cells.

The downside of all this is that servicing a pack involves significant expense and hassle for the user because the pack has to be removed and shipped (as Class 9 hazmat, no less, which is ridiculous, but that's another discussion). But here's the thing--even if the Green Chip dealers could have been trained to repair the packs, how many do you think would be willing to shell out $125,000 for the Aerovironment ABC-150 they would need to re-test the pack where they're done with their repair? Then add another $10,000+ to get it installed and set up with a PC to control it. That's a lot of coin to service a handful of battery packs.

Anyway, if the re-plug trick doesn't work, you probably are seeing loss of capacity, but with only 900 charge cycles this seems highly unlikely--at least to the extent you describe. Have you noticed decreased mileage from the pack? How much?

Hopefully this will help some folks here get more, better use of their aging hymotion batteries. Please do try this out and let me know how it goes for you.

 

Interesting. I can see how steady use without getting to top of charge on a regular basis could look the same to the pack as just being idle for a long period of time. In your case I would definitely use a timer with your charge setup at home. After thinking a bit more, I would set it to run 6 hours on, 10-30 minutes off (whatever the resolution of your timer will allow, but at least ten minutes off), then 6 hours on again. Let it do this every night. It will probably take several days to weeks for you to balance yourself back out of the hole but you should see slow, steady improvement until your capacity starts to look more like what you started with. Once there, back off the re-plugging to the point where it's just enough to keep you there. This will give you the best balance possible between calendar life and capacity/performance.

Plug-ins should be plugged in whenever possible, IMO. With present balancing schemes being what they generally are, this turns out to be critical. People who can't be bothered to plug in their plug-in cars (believe it or not, these folks actually exist) are usually disappointed in the result.

Good luck!

 

Hi again,

Sorry to hear about your job and the lack of success with the pack. At this point, it seems most likely to me that you have some cells in the pack that are fading faster than normal. You're probably not in a place right now where you want to fix that right away. You can continue to use the pack with no consequences to the remaining healthy cells and may want to consider replacing the bad module(s) at some point. If/when that option interests you, this could make a good test case for a field replacement. Is there a Green Chip dealer near you?

 

Hi Ron,

It will be perhaps another week or two before anyone is ready to get into defining details of a potential service program for Hymotion. I consider your case to be one in which there may be an opportunity to allow owners or local repair shops to service a pack without dealing with the hassle of shipping the pack to Holland. Sybesma's would still be the source for parts.

For better or worse, the short-term fate of A123 will be sealed some time next week. Hopefully at that point, people can start talking about the things they need to talk about for it to make sense to begin fleshing out details of how this program might work. I believe you previously expressed an interest in working on your pack yourself, yes? If you email me at [my moniker] at komkast.net (change the k's to c's) we can go into more detail if you like.

Best,

 

Just got word this morning that CFIUS approval for the deal with A123 has been granted to Wanxiang. Still a couple details to take care of, but I would say we now know who will own A123.

 

Please Hymotion have no support,customers, and new customers . The city of Houston uninstalled the Hymotion L5 Prius because of the charging issues. The demise of Hymotion L5 has come and gone and was overpriced
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