Don, I am afraid that you still do not get it, because you are still thinking about NiMH (which has large self-discharge, esp towards the top SoC) while we were discussing Lithium-Ion technology that should have almost no self-discharge. Note that there are two types of resistance, easily mistaken and causing exactly the confusion that we are dealing with here:
- Self-discharge of a cell can be modeled as a (large value) resistor in parallel with the cell, causing a current to flow directly from cell into this resistance, *not* through the other cells, so current can be different in a series string of cells, due to current "escaping" the series path via the parallel resistors of self-discharge. In good cells, the self-discharge current is low (high resistance value). - Internal resistance of a cell can be modeled as a (low value) resistor in series with the cell, so all current flowing through the cell also flows through the internal resistance. The only effect of internal resistance is the voltage drop in the direction of the current, the amount of current is still equal in all cells in a series string, cells might show higher or lower voltage under load due to the voltage drop across the internal resistance. We were discussing Lithium-Ion cells, where the self-discharge is so low that the current escaping the series string is neglible, so all cells see the same current. But the voltage drop in the internal resistance can be significant and vary per cell, due to abuse or defects. Only in extreme cases will the self-discharge also come into play, but then you are talking about really bad or abused packs. I have one and I need to charge some cell independent from others due to current loss in self-discharge. Still, it is good enough to survive several months as long as I do not discharge the pack too deep. I use this mostly as battery pack for my e-Bike. Hope this clarifies, Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: [email protected] Private: http://www.cvandewater.info Skype: cor_van_de_water Tel: +1 408 383 7626 -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Thursday, June 20, 2013 1:33 PM To: [email protected] Subject: [EVDL] Resistance Bill & David and all I am in total agreement and understand about the flow of current in series. Each cell is delivered an equal amount of power. The electrons go through the pack equality so no cell receives more or less energy. The resistance losses I am addressing, do not change the volume of current delivered to each cell in series. It is the differences between cells, which also include resistance, which changes the capacity retained in the cells. Capacity retained in cells can and is different if cells or modules lose a part of that energy at a greater rate then others during charging. This is nothing new To balance a NiMH pack a common method is to over charge the pack with the full cells bleeding energy as heat. The current is exactly the same to each of the cells but the NiMH chemistry is capable of losing a substantial amount of energy as heat. The fully or nearly fully charged cells have higher resistance readings and because of this create more heat then cells still gaining capacity. As I stated before resistance in a cell is not a constant. As a cell reaches a full charge the resistance increases. I have measured this so there is no doubt, cells with a higher state of charge lose a greater amount of energy as heat. While I don't recommend others to do this. I have manually balanced a pack by removing or adding capacity to modules while charging. When removing capacity it basically has the same effect as resistance differences in series, except instead of the energy lost as heat the energy is removed. Either way the energy is no longer retained in the module. When needed I have adjusted the load to zero out the incoming capacity to this module while the rest gain. Each module in the pack was still being charged at the same rate and so of course there was no loss or gain in any other modules. I understand completely in series the amount of energy delivered does not change. In the last 7 years I have been working mostly with used NiMH modules from several different manufacturers with EV pack voltages of 300 to 420 volts. If I had only one method of selection in using modules it would be resistance readings. I have gone though a 1000 used NiMH modules and the closer you can keep resistance readings the better your results will be. If cells have even slight differences in resistance, they don't all heat up exactly equal on charge and discharge. Don Blazer In a message dated 6/19/2013 11:30:57 P.M. Pacific Daylight Time, [email protected] writes: Message: 12 Date: Wed, 19 Jun 2013 19:52:56 -0600 From: Bill Dube <[email protected]> To: Electric Vehicle Discussion List <[email protected]> Subject: Re: [EVDL] Resistance Message-ID: <[email protected]> Content-Type: text/plain; charset=ISO-8859-1; format=flowed The cells heat on both charge, and on discharge. (Ohmic heating has no polarity.) Again, no electrons are lost. They all go around the entire circuit without losing a single one. Each electron flips a chemical ion from one plate to the other plate through the electrolyte. With Li-Ion cells, unless the electron is forced to flip the wrong ion (like when you over charge, or over discharge and damage the battery,) there is a one-to-one ratio to the electron flow and the state of charge. Bill D. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130620/28e5 0f46/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)
