GBS Cells - Defective Safety-Valve Design?
A year ago, I purchased a Current C124. The first few of these scooters used Thunderskys, but changed over to GBS LiMnFePO4 cells - which my scooter is equipped with. The switch over to GBS was supposedly because of better performance, but I suspect another reason is because their supplier, Elite Power Solutions, discontinued selling the "thundersky" cells - actually Thundersky Ningbo. I had some experience with unsatisfactory performance (internal resistance way too high), which I don't see with the current Thundersky incarnation, Winston cells, but that is another subject.
The cells are mounted turned on their narrow sides. This should be OK, as these are not "flooded" cells. Only a small amount, if any, non-absorbed electrolyte is in them. The performance had been satisfactory until last month, when one cell simply refused to come to full charge voltage. Current sent a new cell which I installed. There were no visible defects, but the bad cell seemed to be lighter in weight than the other cells. It was a few days later that I just happened to press down on the safety-vent cap and discovered it was not snug against its seat - and I detected the sweet-alcoholic smell of the volatile electrolyte (dimethyl carbonate). Then a few days later, another cell started acting the same way. There was a delay getting a new cell shipped to me, and I kept riding the scooter - and the cell's maximum charged voltage continued to fall. Then, upon arriving home from a trip on some rough roads this afternoon, I discovered that voltage of this cell had gone to near-zero. I plugged in the charger, noticed some electrolyte smell, unplugged the charger, disassembling some bodywork and removed the cell cover - which revealed the safety-vent cap was completely off.
This experience confirmed what I suspected. GBS cells only rely only on some plug-friction to keep the safety valve's rubber seat sealed. There is no spring to keep the valve seated like other cells I've seen. This is a very poor design. I see no way that such a "friction-plug" vent is NOT going to become unseated from temperature change, elevation change, and especially mechanical vibration - particularly if the cell is mounted on its side. This will allow the electrolyte to evaporate and moisture to enter the cell - ruining it.
Now, the possibly is that what I actually experienced was the valves performing as they should from some kind of abusive event that heated the cell up. This is what Current may offer as an explanation - but I know of nothing that could have done this the cells have not been over charged or over discharged. They are charged at a just 8 amps, and discharged at no more than 3C or so.
Ideas? Anyone else have experience with GBS cells?
All I can cotnrigute to this safety valve issue is what Elite Power Solutions write themselves in their comparison of GBS with other cells, on the subject of the safety valve design. Bold format in the quote is by me:
Competitors use sandwich structure of a threaded cap/spring/metal plate as the safety valve. The design does not allow for reliable opening under extreme conditions, which may result in fire and explosion. Spring and metal plates may corrode over time and lose effectiveness.
GBS cells use a patented latch design safety valve which pops open when internal pressure reaches a designed threshold. The reliability and durability of the safety valve is much improved. The cells do not catch fire or explode even in worst case extreme conditions, such as a steel rod piercing through the cell causing a full internal short.
Which to me indicates that GBS look at the safety valve with a strong emphasis on the safety bit, not so much on the valve bit. So their valve will always readily pop open (unlatch so-to-speak) but then never close again. Not really good in my opinion, and possibly an important reason to only orient the cells vertcially?
All I can contribute...
WHERE IS THAT EDIT BUTTON?????
Wonder if it is more of a problem for cells mounted on their sides?? I have 3600 miles on my GBS cells with no issues; but then again, my cells are mounted right-side up within a study battery box.
I picked up two more cells from a local GBS cell supplier to do a booster pack (which I still haven't gotten around too) and they asked me if I was having any venting issues. I thought it was odd question at the time and wondered what they were doing wrong. Now, you mention a venting issue. I should contact the vendor again to see how the troubled cells were mounted.
Most likely defective BMS design over-stressing the cells.
And one other point. (I wish they had an edit function too. Being vertical will not guarantee the caps will stay sealed. The caps fit semi-snug in their recesses, so if partly unseated, cap will not fall back to the seated position.
But being vertical mainly gas will escape from the valve, not liquid electrolyte. That is quite a fundamental difference as to cell longevity.
My understanding is that Current decided on their sideways cell arrangement after a lot of effort at finding a pack that would physically fit on the frame, and could be installed and removed from the scooter as a unit. The safety valve should no depend on the cell being vertical.
This is correct.
But one thing for sure, they are starting up full production of their scooters, have jacked up their MSRP to a stratospheric $9995 for the 24-cell version and $11,400 for the 30 cell version, and if this is a problem, they better jump on it.
I have made mention of this possible problem to the Current engineers, but I have found them to be pretty resistant my pointing out design issues, which is frustrating, as I thought this is what being a Current motor "Test Pilot was supposed to be about.
I'm sorry you're frustrated - we do take input from everyone and we do act on it. What we do is gather data from several sources and look for identifiable trends and common failure modes before developing an appropriate plan of action. This still results in a quick turn-around.
There are a couple of different reasons behind the price increases - some to do with new features and some to do with the realities of producing more bikes (production price is initially going up before it can come back down). It is unfortunate - but it is a necessary step for us. We still feel that we offer best value in this segment (and until such time as there are independent side-by-side tests the jury will remain out on that).
Thank-you to all of our Test Pilots for their feedback!
Oops, John H. replies to this thread but leaves no comment at all concerning the original question about the safety valve on GBS LiFePO4 cells?
PJD,
Nope, nothing bought from elite power systems. I bought the cells from Evoleelectrics.com; about $52/cell. Very convenient, as their shop is about 13 miles from my house. I don't know when I will get around to making the booster pack or making the other mods. I tell myself that I'm too busy, but the real truth is this bike meets my current needs and why mess with it if it isn't broken. I'll get around to those mods but it is such a low priority.
Oops, John H. replies to this thread but leaves no comment at all concerning the original question about the safety valve on GBS LiFePO4 cells?
That's because as far as I know the safety valve design is OK and I don't have any more data to add to that discussion. We've had cells vent and pop the valve. We've not seen valves pop or become loose without cause.
Thanks for the reply, John :-) So the question here is: WHAT is the cause for PJD's troubles with his cells? Something MUST be amiss... And I certainly do not want that to happen to my GBS cells!
We have seen GBS cells vent, but only when over charged. I do not know why PJD's experience is different. I would very much like to find out what's happening there.
FWIW, I have personally seen Thundersky cells where the vents did not work. In once case the pressure got so high the cell split apart. At least the GBS vents work! :-)
So could it be a defective BMS - blancer board or what not - that SHOULD have interfered with the charger when it's cell got overcharged, but didn't? All unnoticable by PJD?
I have ALSO experienced cells being damaged by BMS failure! It seems that the manufacturers of some (Most?) BMS circuits FAIL to protect them from Radio-Frequency waves and/or fast risetime "Transient" voltage spikes, which cause internal shorting of the BMS, and continual discharge of the affected cell(s)!--All circuits used on a vehicle should be tested to be sure they are immune from the above, as there is great liklihood that at some time, such circuitry WILL be in a HIGH R.F. radiation area, perhaps by driving by another vehicle with a high power transmitter, or driving by a radio/TV or military transmitter site! These circuits need R.F. chokes, bypass capacitors, shielded wiring, etc. to make them safer.
Good, I'm glad to hear that there isn't a mystery problem with the GBS cells.
Our BMS is reasonably tolerant of RF energy and "spikes". But as PJD notes, it is not at all tolerant of mistakes when working on the pack. It shares that weakness with the Goodrum-Fechter design and with many high-dollar "big company" BMS's. Like every part of EV's, BMS are a continually evolving science, or is it art. We are working on new BMS designs that are more fault tolerant, and that do a better job of reporting events to the main processor and the user, so there can't be "silent failures".
PJD, we'll be happy to repair or replace your BMS, regardless of the cause of this problem.
I wonder how many electronic systems/devices used in vehicles can survive, and operate properly, in the RF fields produced within 15 feet of another vehicle using a 1,000 watt transmitter, operating on frequencies from 1.8 thru 148 mhz? How about installed within the SAME vehicle? (Equipment should survive exposure to PULSED, CONTINUOUS, FM, AM, or SIDEBAND modulations of the Radio Frequency radiation) In the Washington, D.C. area, there are places where vehicles regularly drive within one block of very high powered radio, TV, and other transmissions.-I often operate a 1,000 watt transmitter from my HONDA Goldwing motorcycle, in very close proximity to other vehicles, sometimes less than 10 feet away! (HONDA seems to have done a good job with R.F. protection of the electronics on my vehicle, Ihad no major problems.)
MERollier and IB Scootin,
Thanks for your comments.
I am familiar with GBS' safety argument - but the Li-metal-phosphate chemistry is already inherent safe. And, even in the rather unlikely case of a cell bursting it is unlikely to start cause injury or start a fire unless there is already fire or sparks present (the dimathyl carbonate is comparable to alcohol in flammability). At a rate, spring-loaded relief valves are a well-established design for all sorts of safety applications. So, I am not convinced by their argument.
I think the real reason for their safety cap design is simply because it is a cheaper solution, and the safety argument is an after the fact justification.
My understanding is that Current decided on their sideways cell arrangement after a lot of effort at finding a pack that would physically fit on the frame, and could be installed and removed from the scooter as a unit. The safety valve should no depend on the cell being vertical.
But one thing for sure, they are starting up full production of their scooters, have jacked up their MSRP to a stratospheric $9995 for the 24-cell version and $11,400 for the 30 cell version, and if this is a problem, they better jump on it.
I have made mention of this possible problem to the Current engineers, but I have found them to be pretty resistant my pointing out design issues, which is frustrating, as I thought this is what being a Current motor "Test Pilot was supposed to be about.