Paul, In your case, *you* were the BMS. But you were lucky that you noticed the lower end voltage and knew something was off. You were lucky that the charger finished charging without cooking the other cells to death You were lucky that the bad cell was a short and not a partial failure or you might not have detected it in time or faced a much more severe situation than a shorted cell. Anyone less knowledgeable than you about your EV (and that would be 99+ percent of all drivers) would not have caught this failure and continued until disaster, so your vehicle can be used by you alone, because you are an essential part of that vehicle! (namely, the BMS) It looks like you just confirmed the thing that you tried to disprove...
Cor van de Water Chief Scientist Proxim Wireless office +1 408 383 7626 Skype: cor_van_de_water XoIP +31 87 784 1130 private: cvandewater.info www.proxim.com This email message (including any attachments) contains confidential and proprietary information of Proxim Wireless Corporation. If you received this message in error, please delete it and notify the sender. Any unauthorized use, disclosure, distribution, or copying of any part of this message is prohibited. -----Original Message----- From: Paul Dove [mailto:[email protected]] Sent: Thursday, June 18, 2015 11:13 AM To: Cor van de Water; Electric Vehicle Discussion List Subject: Re: [EVDL] Bicycle battery I had this issue early on in my vehicle and at the end of charge my pack voltage reflected this and I removed the bad cell. The cell was shorted but the rest of the pack was fine. I have no cell monitoring. Pretty sure it was caused by a loose connection. That said, apart from expense I have no problem with a BAttery Monitor that looks at every cell but prismatic cells have multiple cells in them in parallel so you can't monitor them individually. Still of the opinion it is an unnecessary expense. You haven't convinced me that cell discharge or drift between SOC is a real issue. I cat see it ever being an issue. Sent from my iPhone > On Jun 18, 2015, at 12:44 PM, Cor van de Water via EV <[email protected]> > wrote: > > Paul, > It is true that in series string the same *load* current goes through > all cells, but the principle of self-discharge is that it occurs > internal to the cell, so it is invisible to the outside world except > when you measure each cell (or blow them up due to overcharge or > under-discharge). > I have actually measured cells and seen the self-discharge. > As you say, it is a small effect in good cells. > But still, there is about 1:2 difference in self-discharge between the > cells I monitor and the differences add up over time. > It might not be a problem in the first year or even in the second. > Then in the 3rd year you try to squeeze an 85 or 90% discharge from > the cells and boom - one reverses (or more) and it is destroyed and > you might only find out from the fireworks when you try to charge it the next > cycle or when you drive it. > Oh BTW - one reason to monitor *all* and *every* cell is exactly the > issue with the infamous bottom-balancing without BMS approach: > Some of the cells that I have were abused and too deeply discharged. > Guess what happens? They become a resistor. > Some are "low" resistance which is OK when they resemble a wire, > others are still "in doubt" whether they want to become a piece of wire or > rather a heating element. > This has at least 2 disastrous effects if you do not detect this immediately: > > 1. The pack voltage has dropped and each cell now gets a much higher > finish charging so you might have been charging conservatively with > all cells in the string, but with some cells "removed", all the rest > is dividing up the difference and may easily be charged to destruction now! > > 2. If charging does not harm the pack, what about discharging hundreds > of Amps through a resistor that is nicely embedded in the pack, > insulated from the outside by the cells around it? If it even drops > 10V across it at hundreds of Amps, you now have a multi-kiloWatt heater > inside your pack without much cooling. What do you think will happen? > > Just some easy illustration of the *need* for a cell-level BMS, from > practice by measuring what happened to a used set of Lithiums (that > indeed was run without BMS) and I have been monitoring since taking it out of > service. Learn from it or get your own experience, your choice. > > Cor van de Water > Chief Scientist > Proxim Wireless > > office +1 408 383 7626 Skype: cor_van_de_water > XoIP +31 87 784 1130 private: cvandewater.info > www.proxim.com > > > This email message (including any attachments) contains confidential and > proprietary information of Proxim Wireless Corporation. If you received this > message in error, please delete it and notify the sender. Any unauthorized > use, disclosure, distribution, or copying of any part of this message is > prohibited. > > > -----Original Message----- > From: EV [mailto:[email protected]] On Behalf Of paul dove via > EV > Sent: Thursday, June 18, 2015 9:57 AM > To: Lee Hart; Electric Vehicle Discussion List > Subject: Re: [EVDL] Bicycle battery > > Excerpt:In theory, all batteries are identical. If wired in series, you would > therefore expect them all to charge and discharge equally. But in practice, > there are differences. New batteries that are all the same brand, same model, > same date code (and without "lemons" or quality control defects) will still > have small differences. Each cell 's self-discharge rate, amphour capacity, > internal resistance, and charge/discharge efficiency will be slightly > different. This makes them drift to different states of charge. For example, > cells with a higher self-discharge rate run down faster just from sitting. > Cells with a lower amphour capacity get more deeply discharged on each cycle, > which lowers their efficiency (so they need a bit more current to fully > recharge). Cells with a higher internal resistance run a little hotter, which > affects their efficiency and self-discharge rate. These differences tend to > get larger over time. If not corrected, you can have a pack with some cells > almost full, and some almost empty! > Ok, here's the deal this is fiction. Self discharge is so low (milivolts over > years) as to be non-existant.Secondly, if they are in series then all current > passing through cells is equal. It is impossible to draw more current from > one cell than another in a series pack. It can short and pass all current or > it can open and pass no current but it cannot discharge faster or slower. > If it has higher resistance then it will cause cells to heat faster that is > all. > Please show evidence that your theory is possible. > The only person I spoke with having issues with new cells is when they were > placed in parallel with old cells. In that case the wires melted because the > new cell was handling more of the current. You may damage cells if there is a > big difference in resistance only if you exceed the max current draw from the > cell. If one never approaches the max current draw this will not be an issue. > > > From: Lee Hart via EV <[email protected]> > To: Electric Vehicle Discussion List <[email protected]> > Sent: Thursday, June 18, 2015 10:53 AM > Subject: Re: [EVDL] Bicycle battery > > Paul Dove via EV wrote: >> That was a very good summary. Cell balancing is something that was >> done with other chemistry a and many have tried to apply it to Li Ion >> chemistries but it doesn't work with Lithium since cells do self >> discharge or drift. >> >> If one wanted to balance them it has to be done below 3.38 volts >> otherwise you are still charging the cell. Thanks for your input. > > Lithiums need balancing even more than other chemistries! You can only > get by without a BMS if the cells are so well matched that they > accidentally stay in balance. (Do you feel lucky?) > > Look at my own balancer at http://www.sunrise-ev.com/balancer.htm > I built it to *use*, not to sell. It doesn't load the cells, or clamp the > voltage at some arbitrary level, and doesn't have failure modes that murder > cells. > > It basically does what you would do yourself, if you had the time and > inclination. It measures the voltage of every cell, and charges the ones that > are low. > -- > The greatest pleasure in life is to create something that wasn't there > before. -- Roy Spence > -- > Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > > > > > -------------- next part -------------- An HTML attachment was > scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20150618/dc > 23c230/attachment.htm> _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
