Indeed. Current in a series string is equal (by definition). Only thing that is suffering from the internal resistance is the *Voltage*. When charging, the bad (high resistance) cell will cause a voltage drop across its resistance, causing extra heating and higher voltage (this is the loss that you were expecting, turning energy into heat) but the cell is still charged with the same current, so it just runs hotter (depending on charge current, cooling and other factors). When discharging (driving the EV) the internal resistance causes a voltage drop that *reduces* the apparent cell voltage (Bill sketched the model: a resistor in series with an ideal cell, we call the value of that resistor the internal resistance). This voltage drop again causes heating of the cell, which can be excessive if the voltage drop is large - if the internal resistance is large enough, the output voltage can even become *negative* which means that the voltage drop across the resistance is larger than the output of the cell. In those cases it is better to remove the cell from the string, not only due to the bad efficiency but more due to the risk to set fire to the battery pack. One example to illustrate: Say we have Lithium cells (any chemistry, but say the cell is at 3.5V rest voltage). Due to construction or abuse, the internal resistance of the cell has increased to 10 mOhm and you try to pull 500A from the string of cells. The resistor drops 0.01 (Ohm) * 500A = 5V while the cell tries to deliver 3.5V so if you measure the terminals of the cell under this load, you will see the cell at 3.5 -5 = -1.5V. The ideal cell is delivering a power of 3.5V * 500A = 1750 Watts. The internal resistance is sucking up and producing heat to the tune of 5V * 500A = 2500 Watts. Total power delivered by the damaged cell is -750 Watts (it is consuming 750 Watts of power from the adjacent cell by dropping part of the adjacent cell delivered voltage across its internal resistance)
Still, if the heat does no damage and does not affect efficiency of accepting charge, then the high-resistance cell will stay in balance, it will just be inefficient and possibly disastrous in its operation if the internal resitance causes dangerous heating to occur. It is comparable in electrical effect to a bad (corroded) terminal on a lead-acid battery, which can (and has, on my truck) heat up to the point of burning itself off the battery. However, in case of Lithium, an overheating cell can be quite dangerous while it is rare that a lead-acid battery burns. 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 Bill Dube Sent: Monday, June 17, 2013 8:54 PM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Resistance Intuition would make you think so, but your intuition turns out to be wrong in this case. Reread Lee Hart's post on this subject. He has it correct. All cells get/produce the same current because they are in series. The cells all are charged and discharged at the identical rate. Thus, have the identical state of charge. Any imbalance is caused by unequal self-discharge, which is a strongly influenced by temperature. The variations in temperature are indeed caused by variations in internal resistance. You can visualize that resistance as a separate resistor in series with the (ideal) cell. It does not influence the state of charge because the current is the same in all cells. It is the fact that the current is identical that is the key. All electrons that enter one end of the string emerge on the other end. None are lost. Each electron flips an ion in each cell. Whatever voltage is needed is what there _will_ be, or electron flow will stop. True fact. Bill Dube' On 6/17/2013 12:56 AM, [email protected] wrote: > Bingo > > You got it. > > The energy stored in cell has been reduced by losing some of the energy as > heat. > Mostly upon discharge because the loads are higher but also when charging. > > Since resistance changes by the state of charge so does the amount of > energy lost. > > Its why I recommend if possible to keep the SOC of the pack 40% to 80%. > > If your charging above 90% there is higher resistance and more heat. The > cells that have the highest resistance lose more energy as heat which in turn > causes them to have a shorter service life. > > If you deeply discharge a pack to where it is in a lower state of charge. > Resistance again increases as the state of charge drops. The energy lost as > heat is greater in cells with the highest resistance. > > So both by charging or discharging. Your loses are greater when you push > cells to their limits as the resistance increases. > > Since resistance varies cell to cell under best case conditions. When > pushed to their limits these differences between cells also increase and have a > greater impact. > > Don Blazer > > > > In a message dated 6/14/2013 9:59:43 P.M. Pacific Daylight Time, > [email protected] writes: > > Date: Fri, 14 Jun 2013 22:57:43 -0400 > From: "Al" <[email protected]> > To: "Electric Vehicle Discussion List" <[email protected]> > Subject: Re: [EVDL] Resistance Jack Rickard of EVTV.me > Message-ID: <00a701ce6974$1bcff760$4101a8c0@alkb2ayu> > Content-Type: text/plain; format=flowed; charset=iso-8859-1; > reply-type=original > > That doesn't sound right. > Wouldn't the cell with the higer resistance lose some of the Ah as heat? > > Al > ----- Original Message ----- > From: "Peter Gabrielsson" <[email protected]> > To: "Electric Vehicle Discussion List" <[email protected]> > Sent: Friday, June 14, 2013 10:22 PM > Subject: Re: [EVDL] Resistance Jack Rickard of EVTV.me > > >> While I agree that resistance is important you are unfortunately not >> really >> correct that it causes imbalance due to energy loss in series strings. >> >> Batteries are primarily electron storage devices, that's why their >> capacity >> is measured in Ah (1Ah= 5767*10^19 electrons). In a series string the >> amount of electrons you shove through each battery is always the same >> regardless of resistance. If you put 5Ah into a string of two empty 10Ah >> cells they will both end up at exactly 50% SOC even if one has 1000 times >> the resistance of the other. > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130617/42e2 9a64/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)
