DOn't mean to hijack the "3500" thread but it had gotten very long...
THe last from PJD
Here is a bit of an update based on some communication with a representative of elite power solutions - an authorized seller of Thundersky products. She is of the opinion that one can get suitable performance without a BMS if charging voltage for the pack is kept no more than (3.6* no. cells) + 0.6 volts, and a proprietary Thundersky charger is used. This charger has 3 stages - the usual CC and CV stages, plus a pulse stage. The idea is to stay away from the highly non-linear part of the charging curve, where some cells can shoot up to damaging voltages. The penalty is about 5% less capacity. Not sure what the pulse stage does, but it may be an equalization protocol that Thundersky has come up with.
This is effectively the point that TANWare was making.
I remain skeptical, but this may not be as big an issue as I thought it was.
but, at any rate, the Gary Goodrum BMS is relatively inexpensive and it assures cell balance and low voltage protection. I'd be willing to assemble the BMS's and wiring harnesses for small fee. But the problem with this BMS is the voltage on the Thundersky chargers would be too low for it to work properly.
BTW, you may be interested to know that Elite Power Solutions has a clone of the XM-3500 - but with 21, 60AH cells, delivering a claimed 62 to 95 mile range and 53 mph real-world top speed. Pricey though - $4,780.
http://elitepowersolutions.com/products/product_info.php?cPath=2_19&products_id=22
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>>>>>Now my question,
I am more than a little confused, I understand that we may not need the BMS but if we do use one we void the Xtreme warranty but the THundersky manufacturers recommend we use one?
Very interesting post from PJD.
Dave - to answer your question: if you use a BMS and wish to retain the Thundersky warranty then it must be a Thundersky approved BMS. If you fit any other type of BMS you will void the warranty. (Think of it this way - if you had to provide the replacement cells would you want someone connecting some unknown device to those cells?)
But, so far it seems that not having a BMS is acceptable - as long as you use the Thundersky charger. The charger supplied with the XM-3500 is the Thundersky charger. So, we should be good without having a BMS (not that it excuses the lack of BMS!)
That's my read of the situation.
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.
I found some information on another battery manufacturer's site about LiFePO4 battery characteristics. gebattery.com.cn/product/lf.asp While they do suggest using a BMS, they seem to suggest that it is not as necessary for LiFePO4 as for other lithium types. They say that LiFePO4 can be overcharged as much as 10% and can effectively self-balance for the most part when charging a series-connected pack. They also indicate a charging voltage plateau of 3.4V per cell which would say that the 69.5V cutoff on the charger might be OK as well for a 20-cell pack. It was good to see that after a short ride the individual cell voltages were all right at 3.2V too.
Haven't done any measurements myself .. but .. in the middle of that long thread on ES about the GG BMS design someone reports on their LiFePO cell voltages through several rides showing the voltages pegged at each other very close. On the other hand somewhere else I saw someone state equally assertively that LiFePO batteries don't have strong voltage correlations - that wasn't the phrasing and I don't remember the phrasing but the idea was you can't trust them to not wander their voltages a bit.
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki