I think it’s been said before but basically you are saying that you can be your own BMS and not spend the money on a physical system. That may be true and you may be able to get away with it for a long time but if you get complacent or someone else is driving your vehicle who does not understand things like ‘its feeling sluggish I need to STOP now’ you will eventually run into the reason for having a BMS and may have a damaged battery pack as well.
The effect of this is very dramatic in my NiMH packs on the R/C car, the time from full performance to sluggish decreases with each full cycle. You need to stop, let the batteries cool and then trickle charge or fully discharge and recharge them to regain full performance. The chargers for my R/C batteries all have the ability to run charge/discharge cycles, voltage limits on top and bottom, over charge timers and for the LiPo cells they monitor each cell during charge and discharge to prevent over or under charging. This cell monitoring on the LiPO’s was added specifically because too many people had fires in their packs when trying to charge the cells in series when they became unbalanced. In summary, the problem is real, the severity may be variable and how well you can manage without probably depends as much on you as anything. The BMS ensures that if you forget or aren’t watching when things start to go wrong it can catch the problem either deal with it or let you know before the pack is damaged. All commercial vehicles have a BMS and cell level monitoring, I suspect that tells us something, if it could be left out they would save the money and leave it out. Your comment on parallel cells is a bit misleading as well. Cells in parallel will self balance, the capacity will be related to the weakest cell but you won’t see one cell at a lower voltage than the others. The stronger cells will dump charge into the weaker one keeping them all at the same state of charge but affecting the overall capacity. So the BMS monitors modules in a series string - each module may be a single cell or group of cells in parallel. Lawrence > On Jun 18, 2015, at 11:13 AM, Paul Dove via EV <ev@lists.evdl.org> wrote: > > 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 <ev@lists.evdl.org> >> 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:ev-boun...@lists.evdl.org] 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 al > most 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 <ev@lists.evdl.org> >> To: Electric Vehicle Discussion List <ev@lists.evdl.org> >> 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/dc23c230/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)
