This sub-subject gets done to death pretty regularly but I like gathering what I hear from both camps. Definitely going to use a BMS but I really like the non-BMS workaround of getting *two* expensive battery packs and using a really simple gauge that shows their comparative voltage. If one is sagging more than the other, pull over and find out what is wrong. If you have a smart BMS... it takes care of it for you until you can get home ;)
sean On Thu, Jun 18, 2015 at 3:58 PM, Cor van de Water via EV <[email protected]> wrote: > 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) > > -- Sean Korb [email protected] http://www.spkorb.org '65,'68 Mustangs,'68 Cougar,'78 R100/7,'60 Metro,'59 A35,'71 Pantera #1382 "The more you drive, the less intelligent you get" --Miller "Computers are useless. They can only give you answers." -P. Picasso -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20150618/5175d1ba/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)
