We are not speaking of commercial products. At least I wasn't we are talking about our own conversion and the pros and cons of a BMS.
I wasn't lucky. My pack is the same voltage after charger to two decimal places. I simply look at the voltage when I unplug the charger. Only overcharging causes fires. And then not as likely with LiFePO4. We took a full cell and put 60 amps in it for three hours and all it did was vent and melt the plastic around the terminal. $125 cell was much cheaper than a BMS. I have had no issues since and it's been 18 months now. The pack is the same voltage after every charge. If I was using one of the other two types of cells maybe I would consider it but I doubt it. I would have to play with those cells some to see how they react. I design electric circuits for a living and I trust the battery much more than the circuit. These batteries are very stable. All the cars I have seen that burned had a BMS. I have heard of no one without a BMS burning down a car. Sent from my iPad > On Jun 18, 2015, at 2:58 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 a lmost 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) > _______________________________________________ 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)
