Hello Lee I agree with everything below However this does not address when resistance varies by cell or module. Don Blazer In a message dated 6/20/2013 1:02:59 P.M. Pacific Daylight Time, [email protected] writes:
Message: 7 Date: Thu, 20 Jun 2013 11:23:50 -0500 From: Lee Hart <[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 On 6/19/2013 7:07 PM, [email protected] wrote: > It still does not answer or address where the energy comes from that was > lost in raising the cell or cells with the higher resistance over ambient > temperature. During charging, the charger supplies more energy, since the pack voltage is higher (due to the resistance). Perfect 100v 100ah pack (with zero resistance): - Charger supplies 100v x 10a x 10h = 10kwh - Batteries get 10a x 10h = 100ah of charge Same 100v 100ah pack, but with resistance: - Charger supplies 110v x 10a x 10h = 11kwh - 1kwh of heat is produced in the resistance - Batteries get 10a x 10h = 100ah of charge During discharge, the batteries supply the same amount of energy, but some of it is burned up as heat (in the cell resistance), rather than powering the load. Perfect 100v 100ah pack (with zero resistance): - Controller loads batteries 100v x 10a x 10h = 10kwh - Batteries deliver 10a x 10h = 100ah of charge Same 100v 100ah pack, but with resistance: - Batteries sag to 90v under 10a load - Load gets 90v x 10a x 10h = 9kwh - 1kwh of heat is produced in the resistance - Batteries still deliver 10a x 10h = 100ah of charge > As an example NiMH cells in the past have a very high self discharge > rate. You can charge up a pack and they lose energy while reaching a full > charge. Cells that have higher resistance or are in a higher state of charge > lose more energy as heat then the others. If you let the pack sit in a fully > charged state they self discharge and the energy loss produces heat. Enough > that it actually helps keep the pack warmer in the winter. Both lead-acid and nimh have "side" chemical reactions (gassing and heating) that can occur. For nickel-based cells (NiCad, Nimh, NiFe) these side reaction starts above 1.45 v/cell, and by 1.54v/cell this side reaction is consuming essentially 100% of the applied current. For lead-acid, it starts above 2.27v/cell, and by 2.5v/cell the side reactions are consuming essentially 100% of the applied current. The side reactions are strongly related to voltage and temperature. A small increase in voltage or temperature causes a dramatic increase in the current bypassed by them. Lithium cells (mostly) don't have these side reactions, so the self-discharge rate is much lower. On the other hand, there is nothing short of destruction of the cell to limit the voltage rise. -- Don't worry about people stealing your ideas. If your ideas are any good, you'll have to ram them down people's throats. -- Howard Aiken -- Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130620/a05ccb59/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)
