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/20130620/28e50f46/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)
