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 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 <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... 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