XM-3500

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PJD
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Re: XM-3500

Henry,

So, your may have had 50 miles on a charge? It appears you may have simply ran the scooter down to a damaging level of discharge. Other cells may be damaged too. This is how LiFePO4's behave if completely discharged, the voltage plummets from a plateau to a damaging low level.

A low voltage cutoff is the absolute minimum for any LiFePO4 pack. If you don't have one, strictly as a temporary measure, you need a voltmeter installed on your instrument panel, and don't let the pack voltage sag below 55 volts. Normally the motor controller has a LVC - but this only cuts off based on the the entire pack voltage, not individual cell voltage, and may be set too low anyway. Maybe this scooter doesn't even have that!

PJD
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Re: XM-3500

Sparkie where are you? There's more good info on the BMS to piece together....

There is a good BMS available here:

http://www.tppacks.com/proddetail.asp?prod=EBKE%2DBMS%2DKit

Gary is temporarily out of stock, but will hopefully have more soon.

For 20 cells, you need two boards - one of them serving as a slave board, with various components deleted from it.

jdh2550_1
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Re: XM-3500

Oh joy, more good news! There indeed appear to be two "Thunderskys".

This is supposed to be their registered trademark and these "Thunder-sky Ningbo" cells are counterfeits

Are they counterfeits or are they being made after Ningbo bought or licensed rights from Thundersky? On another forum the intrepid Tracy is also on the trail of these two Thundersky's and his post includes this:

Dear Sir,
i would like to mention to you that Thunder Sky Ningbo is not our company. They bought from us some patents and they are allowed to use our brand name, but actualy is a completely indipendent company

Tracy, you'll probably be pissed in some way but for what it's worth - thank you for this info - John H.

PJD, before pouring more gas on the fire can you confirm these guys are counterfeiters? If so, based on what info?

The bike does have a low voltage cutoff. Henry did not ride for 50 miles on one charge - he rode the bike a total of 50 miles so far and has recharged once before hitting the 50 mile mark.

It is also possible that Henry simply has a bad cell and that cell will get replaced with a working one. That's what the warranty is for. Henry - let us know how X-Treme responds.

I'll recheck the voltages on my bike. I hit the low voltage cut off at the end of my 48 mile ride.

As for Dr. Marco Loglio it is also possible that he's trying to encourage sales of his product. Or he might not. I've often encountered folks who want to point out in alarmist terms terrible deficiencies in products so that they can sell their own merchandise.

I grant you this is very confusing and not particularly encouraging news. But please can we not all jump off the deep end? (Well, if you want to jump off the deep end I can't stop you - but at least look around and do some fact checking before you leap).

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

PJD
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Re: XM-3500

I'd still recommend putting a voltmeter on the bike - they make self-powered versions, like this one here:

http://www.mouser.com/Search/Refine.aspx?Msid=58020000&Mkw=DMS-20LCD-2-DCM-C

JavierCentenera
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Re: XM-3500

That IS a shocker! - The cells have the blue and red Thundersky globe/lightning bolt logo, correct? This is supposed to be their registered trademark and these "Thunder-sky Ningbo" cells are counterfeits - they copy their logo, and even their their slogan "one percent progress every day"

Here is the real Thundersky:

http://www.thunder-sky.com/

Here is the counterfeit:

http://www.thunder-sky.cn/

The real Thundersky has been in business since at least the 1990s, Ningbo has been in business since only last year.

And I was very close to buying some spare cells from a US dealer called "elite Power Solutions" which I now recall called their cells "Thundersky ningbo"

Every time I think I've plumbed the depths of unethical behavior on the part of Chinese businesses, something like this come along.

Of course, that cell at 1.3 volts is ruined. Even one excursion below about 2.1 volts - under load, ruins any LiFePO4 battery.

Take a glance to this link http://www.thunder-sky.cn/product/20070322.pdf You´ll see the web address "printed" on the page #1 is www.thunder-sky.com So,....well!, who's is lying? who is the manufacturer for the LFP Batts? same organization ... what do you think? :-)

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Re: XM-3500

Hi jdh2550_1 I agree with You a lot of people play the business game in a no fair play way...It's a pity!!!

PJD
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Re: XM-3500

The original company, in business since the 1990s, was Thundersky.com. The rep that Henry contacted, stated clearly that Thundersky-ningbo is not related to Thundersky in any way. Unless thundersky ningbo is making Thundersky cells under a strict contract, how in the world would Thundersky let them use their registered trademark, slogans, and even lift in entirety their users manual?

Xavier, A agree a lot of people don't play fair, and it seems to be part of the new, cutting-edge version of Chinese capitalism. I mean, their government at all levels is crooked and doesn't play fair - so their businesses are just following their leaders. The USA is following close behind...

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Re: XM-3500

Not to divert too much attention away from an exciting game of "Will the real counterfeiter please stand up" but...

...am I the only one here who found this nugget from the good Dr. intriguing:

We, have already in the market a similar motorcycle with all the safety items including special charger with themperature sensor, BMS with screen on the dashboard, interacting with the charger and the controller to give longer life and safety to the battery pack.

Anyone know what bike he's pimping?

~ scott
My Blog: <a href="http://www.petrozero.org">PetroZero.org</a>
My Bike: 2007 Lashout Electric Scooter (12mph/12miles)
Considering: xm-3500Li

boyelectric
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Re: XM-3500

The original company, in business since the 1990s, was Thundersky.com. The rep that Henry contacted, stated clearly that Thundersky-ningbo is not related to Thundersky in any way. Unless thundersky ningbo is making Thundersky cells under a strict contract, how in the world would Thundersky let them use their registered trademark, slogans, and even lift in entirety their users manual?

Xavier, A agree a lot of people don't play fair, and it seems to be part of the new, cutting-edge version of Chinese capitalism. I mean, their government at all levels is crooked and doesn't play fair - so their businesses are just following their leaders. The USA is following close behind...

I assume you mean, http://www.thunder-sky.com... http://www.thundersky.com is a multimedia pictures company.

jdh2550_1
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Re: XM-3500

Petro - please, divert all you want! Look at this page: http://www.thunder-sky.com/products_en.asp?fid=71&fid2=73 - not terribly earth shattering. However, I'd like to own the company that makes the base bike - everyone seems to be using it! (Erider, EFun, Thundersky, others?)

PJD - In another communication from "the original Thundersky" a company representative says just that - that "the new Thundersky" is entitled to use their brand and patents. The company representative above doesn't sate that the new TS is doing anything wrong with regard to name and patents - he just doesn't like their products and prefers his own.

I do very much agree - fitting that DVM (or similar) would be a nice addition - you could put it in place of that silly looking bike symbol ;-) Remember that the fuel gauge is simply an analog volt meter with no scale - but a DVM will give you more info.

However, I'm looking forward to sparc5's GreenBMS which promises to be a thing of great wonder and beauty at a great price. Go sparkie, go!

Mikie - thanks for that link to the vendors of the "ningbo" cells. IMO, it remains to be seen if they're sub-optimal or not. I hope they're at least adequate - because those single cell prices are very attractive (which, I grant you, raises the specter of "you get what you pay for").

What's an early adopter, EV rider to do???? At times it all just makes my head hurt!

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

PJD
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Re: XM-3500

John,

I already have done some business with Elite Power Systems and I have already mentioned them several times in this forum. Their rep, Jennifer Wenren seemed pretty knowledgeable, but now that I know that they are knock-off cells made by a firm that has been in business only a year, using their products seems risky. It took several years for the real Thundersky to get their quality to the point where most will admit that they aren't entirely junk. (at least the LiFePO4's).

As far as my business with them, I was going to order a couple spare cells from them, but they were out of stock until at least mid-September. So I canceled the order.

reikiman
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Re: XM-3500

I'd still recommend putting a voltmeter on the bike - they make self-powered versions, like this one here:

http://www.mouser.com/Search/Refine.aspx?Msid=58020000&Mkw=DMS-20LCD-2-DCM-C

Additionally the Paktrakr can be used with Li-ION batteries.

- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
- Electrified Electra To

CivicSky
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Re: XM-3500

This is my first post here on VV and I have been keeping my full attention on the XM-3500Li for the last few weeks and hoping for positive review on this scooter. Comments on this scooter are mostly regarding the actual speed and battery care. I am sure those are critical for us (those who are thinking of buying this scooter) I also worry the batteries quality, so I did an extensive search on the web about Thunder Sky (Ningbo) and Thunder Sky (Shen Zhen). I believe they are both under the same corporation management. (check out this link and it will show you how they are related to each other --> http://www.thunder-sky.com/Base.asp) Although they may not mention anything on their website, the factory based at Shen Zhen (the so-called original TS) is their corporate headquarter near Hong Kong. Thunder Sky Ningbo Ltd is TS's sub-division located at northern China near Shanghai. Both have their own assembly lines. In addition, they also have production lines at Beijing and ZhiLam. Except the company logos and products are idenical, the flags in front of their buildings tells me a little something about the background of this company (from right to left: Company flag with their logo at the corner, China flag, and Hong Kong district symbol). I hope I am not confusing you guys here. I am from Hong Kong and I know how things work overthere. Economy and manufacturing model are similar to the relationship between USA and Mexico. The lower the manufacturing cost the better. Hmm... I'm afraid I maybe driven away too far from the point: whether TS Ningbo or TS Shen Zhen produces the "real" Thunder sky batteries? If they were the same company and share battery technologies, I would think that they produce the same quality battery. It also makes more sense XM-3500Lis are using the TS from Ningbo because Mr Mt Chen's factory happen to locate closer to Ningbo factory. I hope I am right on this.

V

Mik
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Re: XM-3500

Hub motors are wonderful in their simplicity and reliability, but it will be hard to make them safe at highway speeds. I'm sure that's why the Vectrix doesn't use a hub motor.

Waddayacall this baby, then????
PhotobucketClick here for a higher resolution picture.

Mr. Mik

This information may be used entirely at your own risk.

There is always a way if there is no other way!

JavierCentenera
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Re: XM-3500

This is my first post here on VV and I have been keeping my full attention on the XM-3500Li for the last few weeks and hoping for positive review on this scooter. Comments on this scooter are mostly regarding the actual speed and battery care. I am sure those are critical for us (those who are thinking of buying this scooter) I also worry the batteries quality, so I did an extensive search on the web about Thunder Sky (Ningbo) and Thunder Sky (Shen Zhen). I believe they are both under the same corporation management. (check out this link and it will show you how they are related to each other --> http://www.thunder-sky.com/Base.asp) Although they may not mention anything on their website, the factory based at Shen Zhen (the so-called original TS) is their corporate headquarter near Hong Kong. Thunder Sky Ningbo Ltd is TS's sub-division located at northern China near Shanghai. Both have their own assembly lines. In addition, they also have production lines at Beijing and ZhiLam. Except the company logos and products are idenical, the flags in front of their buildings tells me a little something about the background of this company (from right to left: Company flag with their logo at the corner, China flag, and Hong Kong district symbol). I hope I am not confusing you guys here. I am from Hong Kong and I know how things work overthere. Economy and manufacturing model are similar to the relationship between USA and Mexico. The lower the manufacturing cost the better. Hmm... I'm afraid I maybe driven away too far from the point: whether TS Ningbo or TS Shen Zhen produces the "real" Thunder sky batteries? If they were the same company and share battery technologies, I would think that they produce the same quality battery. It also makes more sense XM-3500Lis are using the TS from Ningbo because Mr Mt Chen's factory happen to locate closer to Ningbo factory. I hope I am right on this.

I thing so too...You Are in the right way...

jdh2550_1
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Re: XM-3500

but now that I know that they are knock-off cells made by a firm that has been in business only a year, using their products seems risky.

Right, that's where we differ - I'm just not sure how you come to the conclusion that these are "knock-off cells". I'm not saying they are or they are not of similar quality. At the end of the day it would be great to get a side by side comparison of these cells.

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

jdh2550_1
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Re: XM-3500

This is my first post here on VV and I have been keeping my full attention on the XM-3500Li for the last few weeks and hoping for positive review on this scooter.

I will be doing a side by side test of the XM-3500Li with the EFun-D in the next couple of weeks.

I hope I am not confusing you guys here. I am from Hong Kong and I know how things work overthere. Economy and manufacturing model are similar to the relationship between USA and Mexico.

So, if I understand your post correctly this could be like the difference between a Ford plant in the USA and a Ford plant in Mexico. Both would be making the same product, but cost and quality could very well be different from both plants. Ford of Mexico strictly speaking is a separate company from Ford in the US (and Ford of Europe is different again) - clearly though they are the same company.

Did I understand you right?

Can you offer any insights into Dr. Marco Loglio's commentary sent to Henry? He appears to want to distance himself from the Ningbo operation and to sell more product from his own company. If, as you suggest, both Thundersky's are owned by the same top level corporation then this seems strange to us with a Western background. For example if Ford of Europe were to say "buy our engine because the one's made in the US are cr*p" you can bet that the person who wrote that wouldn't be working for Ford for very much longer!

Given your background does this intra-company rivalry seem strange, or normal to you?

Many thanks for your post and the info and your different perspective.

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

jdh2550_1
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Re: XM-3500
Hub motors are wonderful in their simplicity and reliability, but it will be hard to make them safe at highway speeds. I'm sure that's why the Vectrix doesn't use a hub motor.

Waddayacall this baby, then????
PhotobucketClick here for a higher resolution picture.

Mr. Mik

I call it big. And beautiful. But don't listen to me I'm strange... ;-)

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

CivicSky
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Re: XM-3500

To be honest, it is kinda strange to hear someone giving negative comments about their own. Since I don't know the whole story of what Dr. Marco Loglio actually said to Henry, I don't want to jump to the conclusion that one Thunder sky facility really makes better batteries than the other. Different level of quality control maybe applied but I don't think that could make hella big different. They are a huge corporate company in China and wouldn't do anything stupid to ruin themselves. But there is one thing I am sure, batteries installed on Mountain Chen's scooters are not counterfeit. I believe Chen bought batteries from Ningbo factory was mainly because of their geographic location. Both factories are somehow related to each other (TS Ningbo is a limited company). What Dr Loglio said really created a stir on how we look at this new bike. On the other hand, TS Ningbo uses the charger not recommented by Dr Loglio does need our attention.

One more thing, in the eariler post I mentioned about the relationship of USA and Mexico and some business model thing, I was trying to say Thunder Sky was financially supported by "wealthier" economy from Hong Kong which similar to Sony USA opens up production lines across the border at Mexico (no offence). Both factories are located within Chinese border anyway. Hope this clearify my point from the eariler post.

V

Henry42
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Re: XM-3500

The batteries were charged to 69.0 volts. I used a dome light bulb from a car (previously used it to drain cell phone batteries, so it already had wires soldered to it.) to drain four of the batteries, which were at 4.25 volts, the rest were at 3.33 volts. I charged the batteries again, this time the charger shut off at 71.0 volts. After a few minutes the voltage dropped to 70. One thing I noticed is that as soon as one battery reaches ~4.25 volts the charger shuts off.

I relocated the battery charger connector from under the seat to under the small door in front of the seat. It seems easier to charge the battery and removes what I consider a hazard. Anyway, I open the small door instead of opening the seat to charge the batteries.
DSCF1454.jpg

I rode s total of 30.4 miles (10.5 miles, stopped for a few minutes, then another 19.9 miles.) The throttle was mostly wide open, except for having to stop about four times. The voltage was 65.5 at the end of the ride. All batteries had 3.29 except #11, which had 3.25 volts.

This time I did reach 55 mph according to the speedometer; however, the real speed (as measured by a car) was 44 mph. If the scooter is really detuned, maybe I can adjust it back to 63 mph and get it to go 55 mph. Tomorrow I plan on riding it to work (the reason I purchased it), which is exactly 10 miles. I plan on checking the accuracy of the odometer at that time.

Mikie
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Re: XM-3500

Henry-- thanks for all of that information, you are a jewel. I think somewhere floating around one of these blogs is some info on how to take out a circuit? of some sort (if my memory serves me right) that will then give full access to the fuel tank. These manufactures all seem to do things the same way so if somebody knows what I am talking about then speak up, it should be a similar simple fix. It would be great if we could at least get a few more mph out of these babies, especially here in the peaks of seattle where I need all the running start I can get. Lets keep on this improvement of their product guys, we are doing good and I bet the word is getting back to the manufacturers one way or another. They work for us and remember we have what they want $$, and good happy customers make more $$....Mikie

mikie

PJD
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Re: XM-3500

Up until just a couple years ago, Thundersky cells had a terrible reputation among EV builders. Only recently have they come to be considered even fair in quality. So I would be suspicious of cells that come from newer manufacturing facility - lots of bugs to shake out.

As as someone who work in construction QC, I assure you, QC is everything.

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Re: XM-3500

Henry, thanks for all the info. I can say your cells seem wildly out of ballance and going below 2.5 and also up to 4.25 is way out of tollerance for these batteries. A grouped cell BMS as described earlier will only work if the cells are within a close tollerance.

Your cells do not seem to hold a close tollerance. The cells need to have a charge capacity within 1-2% of each other. 3-5% will cause issues and anything over that could make the pack self destructive.

Now this also means if you hurt any one cell by taking it below the 2.5 limit you also then risk taking the cells capacity ratios well above the acceptable capacity levels. This will then cause a cascading effect to the other cells capacity levels and quickly deteriorate the entire pack.

With this in mind it is best then with this type of ballance algorythm to break in the battery pack. While you would do this at your own risk an acceptable methodology, considering 40 mile real world, would be to make three 4 mile runs charging between each (10%). Then two 10 mile runs (25%) and then two 20 mile runs (50%). Lastly at least two no more than 30 mile runs with charges inbetween (75%). Remember it is important to use the included charger to fully charge the pack between each run.

This breakin period should, at least theoretcially, force an equalibrium between the cells on their capacity charge levels. I would especially not trust the pack is fully charged out of the box with all cells to the same capacity level. And if you take it out and let the voltage cutoff kick in without proper cell levels you are certainly placing some cells at dire risk.

Remember also with lithiums take heed to the voltage cut off. once it is kicking in you are most likely placing some of the cells in danger by continued use even from all cells being at a true equal level. Again making oppertunity charging, especially at or below 25%, your best bet for continued use.......

awilensky
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Re: XM-3500

Is Mountain Chen related to Mr. Alan Chen of Green-E scooter of Santa Clara CA? If so, Hi Alan!

Hello folks, my name is Alan Wilensky - I was CTO for an Israeli EV company that tanked, and I owned a PEV scooter store in Northampton MA, back in 2000-2001. Im not in the business anymore, but I have been following your thread with much interest, as I thought that the 3500 was vapor ware.

I have a great deal of EV systems experience, and I also worked with a Taiwanese factory for production. Things can get very interesting with these smaller, non-brand name electrics that come from China! Many of their engineers can't get their proper specs into the 'as built' product. As you see with BMS, it is seen as an option to expedite production.

Not in my opinion.

I hope to hear many good things about your new XM-3500's and if anyone near boston area needs help with EV troubleshooting, I can help. Let's all hope that the Japanese Mfrs get religion and that we see a Honda, suzuki, or Yamaha, or KAw electics soon.

My Oxygens have been very reliable/ I am sure we have some current and past Oxygen owners out there!

Henry42
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Re: XM-3500

I started with 73.3 volts, went 8.7 miles at full throttle (upper range) and 1.3 miles at varying speeds (average half throttle) and ended with 65.6 volts. According to the speedometer I was going 55 mph (~44 mph actual) except for hills, which reduced the speed to 50 mph (~42 mph actual). The actual distance is 10.0 miles; the odometer read 19.6 miles traveled. The return trip started with 66.1 volts. The return trip was the same mileage and speed (except in reverse order). With the same 10.0 actual miles and 19.6 miles more according to the odometer. The ending voltage was 65.2. All batteries were 3.27 - 3.28 except #11 which was 3.18. The charge is now 70.0 volts.

I noticed that each time the batteries are charged, the voltage for each battery increases. (i.e. the first charge or two the batteries were 3.34 volts, then ~3.4, ~3.5, and now it is ~3.6) There is a wide range of voltages (3.34 to 4.25), however more batteries are obtaining a higher voltage reading with each charge. With a maximum charge of 4.25 volts.

PJD
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Re: XM-3500

Henry,

The problem is, charging to 4.25 volts per cell is too high for very long cell life.

I suspect that the cells were installed in the scooter with their partial factory charge. In this condition, they are wildly out of balance and unsuitable for putting into service. Before the cells are installed, each cell must be conditioned by charging, discharging at least 50%, then charged using the CC/CV method to 3.7 volts at a uniform terminal current, . Only in this way are you assured that each cell is in reasonable balance. A terminal current of about 0.3 amps is about right. Frankly, if the factory isn't doing this, they are not selling a product that is suitable for sale to an ordinary consumer. It is also not suitable for sale to the consumer if it doesn't have a BMS (charge management plus low-voltage cutoff for each cell) but I already covered this.

If you are unfamiliar with CC/CV battery charging, you may want to look up some references. Basically, charging a battery is analogous to filling a tall water tank in which there are also frictional pressure losses in the pipe to the tank. Remember that voltage is analogous to pressure and current is analogous to flow rate. As fill the tank and the water level rises, the pressure at the pump increases. The pressure also increases if the pump is sped up because the frictional losses are a function of the velocity of the water in the pipe (we are neglecting some complications). So, we can't just run the pump at full speed until we see a pressure at the pump equal to the weight of the water times the height of the tank, because there is still flow-related "back pressure" in the pipe. So, the tank won't actually be full. So, we run the pump at full rate until we see the "full tank pressure" then we throttle the pump back maintaining the full-tank pressure until we see the full tank pressure even at a very low flow rate- now we know the tank is filled to the desired level.

So, by analogy, in CC/CV charging, the charger first runs full current through them battery until the voltage reaches a desired level - in LifePO4 cells, this is should be 3.7 volts, then the charger throttles back the current, so the voltage holds right at 3.7 volts, until the current is down to some low value - a couple tenths of an amp. Then it shuts off. (lead acid chargers switch to a "float" stage but I won't get into that).

Since liFePO4 batteries are so new, finding a 3.7 volt CC-CV charger to charge each cell may be difficult. But an ordinary benchtop power supply for electronics work with adjustable voltage/current will act as a CC-CV charger. Cheap ones are available here:
http://www.mpja.com/products.asp?dept=2&main=1

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Re: XM-3500

Hmm... just wondering if 18 good cells (with 1 less quality cell) could propell the scooter up to 43mph max and if all 19 good cells could go up to 50mph, how fast could the scooter go with 20 good cells? Would the controllor be able to accept 77.2 volts of power from 20 good cells? Would their detuning method be simply taken out the 20th cell? (remember "...tested to 62mph but is detuned to 55mph..." from X-treme's ad?) Maybe we should figure out if the possbility of adding the 20th cell to solve our lack-of-speed problem. I realize someone somewhere on this forum has already added one more cell, but what is the result?

V

Mik
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Re: XM-3500

With this in mind it is best then with this type of ballance algorythm to break in the battery pack. While you would do this at your own risk an acceptable methodology, considering 40 mile real world, would be to make three 4 mile runs charging between each (10%). Then two 10 mile runs (25%) and then two 20 mile runs (50%). Lastly at least two no more than 30 mile runs with charges inbetween (75%). Remember it is important to use the included charger to fully charge the pack between each run.

This breakin period should, at least theoretcially, force an equalibrium between the cells on their capacity charge levels. I would especially not trust the pack is fully charged out of the box with all cells to the same capacity level. And if you take it out and let the voltage cutoff kick in without proper cell levels you are certainly placing some cells at dire risk.

Most interesting...

Would this sort of approach also work for Ni-MH batteries?

Mr. Mik

This information may be used entirely at your own risk.

There is always a way if there is no other way!

antiscab
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Re: XM-3500

that process only works for batteries that have significantly greater shunt current loss at higher states of charge.
It most certainly wont work for thundersky lfp40s.
If you added a 3.6v zenner and a 1 ohm resistor to each cell it would.
though it wont hurt to do it either, just wouldnt help.

im actually quite interested in how xtreme got their chager to detect when a single cell hits 4.25v and shuts down.
this means the BMS required is really really simple. (ie a zener and resistor for each cell, no monitoring since the charger stops charging when one cell reaches 4.25v).
its the monitoring and automated charger shut down thats difficult (expensive) to implement, especially when dealing with high voltage packs (288v+)

from what i understand, nimh has very significant shunt current loss when charging to 100%.
so it *might* work.
however it also has significant shunt current loss when charging at a low state of charge, so dont go far.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
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TANWare
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Re: XM-3500

Since NiMh is memmory free as well the break in procedure, for a non BMS monitored array, would not hurt the cells. This could be a mixed camp though as I've heard others say to fully range a cell for the first five charges to break it in as this then gets the best overall capacity out of them. While the 5 cycle method has never been proven, some swear by it.

The reason you are getting two at 4.25 and the others lower is the cells were wildly out of ballance. as you do each charge the cells with the higher charge will start to level off as the other cells catch up and have their capacities increased. This is a default behaviour of the algorythm they are using when the cells are imballanced. They will only slowly come into ballance where a true active BMS keeps them in ballance from the get go.

This is also the reason I recomend the break in period. It will help bring the cells to ballance without severly risking individual cells. If you suspect a severe imballance I'd even recomend doubling the runs as 5 at 10%, 4 at 25%, 4 at 50% and then 4 or more at 75% capacity and try to never run below 90% after breakin with oppertunity charging at 50-75% whenever possible.

Again I state: this mathamatically gives a matched Lithium serial cell pack with short charge passive cell ballancing the best bet at a long healthy life while retaining maxium designed capacity and maintianing safety.

Again stay well away from the voltage cut off. For it to work single cells will quickly drop off into the danger level to get the pack voltage low enough. Some cells, as Henry saw, can drop well below the 2.5 volt design limit while other cells are still at a healthy charge. You must also rememeber a cell will look good right to that last 5% then there is a SHARP drop off. If you do get to the voltage cut off I'd fully charge it and break the pack in again as you may have placed one or more cells in jeopardy and this could help rebuild a ballance. The other option is to check the cells and fix any imballances.

EDIT:

One last thing too. If while under load you see a sudden drop off of capacity it means you are rapidly loosing the charge from one or more cells. Time to shut down as soon as possible. under this rapid loss by shutting down and removing the load the cells terminal voltage will revive somewhat hopefully saving the cells that are in danger.

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