On Thu, Jun 20, 2013 at 2:34 PM, <[email protected]> wrote: > Hello David > > Sounds good in theory but what I find is cells with higher resistance never > have as much capacity as modules with lower resistance when charged in > series. >
Don, you are confusing a couple of things. First, power does not equal current (as you claimed in another post). Second, we are not talking about a difference in capacity between cells correlating to the cell's IR. We said that the IR in a LiFePO4 cell (and as Cor pointed out in his response also Li-Ion cells) doesn't cause the cells to get out of balance. While it is true that the higher IR cell receives more energy on charge and gives off less energy on discharge than a lower IR cell but each one gains the same amount of charge and gives off the same amount of charge. Take two LiFePO4 cells of any capacity, say a 10Ah cell and a 40Ah cell. Discharge them so that they rest for several hours at 2.50V. This assures that they both are at the same state of charge. For this example call this 0%SOC. Put the cells in series and charge until 10Ah have been put through the circuit. Both cells will have changed their state of charge by 10Ah but each will be at vastly different states of charge. Now put a load on the string and discharge 10Ah from the cell and both cells will eventually rest at 2.50V each. This is what happens and is independent of the amount of energy each absorbs or gives off. We want the states of charge to match up at empty in this example. The state of energy of one cell essentially goes from 0%SOE to 100%SOC and back to 0%SOE and the larger cell goes essentially from 0%SOE to roughly 25%SOE and back to 0%SOE. The state of charge, on the other hand, goes from exactly 0%SOC to exactly 100%SOC to exactly 0%SOC in the small cell where the larger cell goes from exactly 0%SOC to exactly 25%SOC and back to exactly 0%SOC. Again, power does not equal current. An amp-hour is not energy. Here are some definitions and equations: Power: Watt defined as the rate at which energy is used in Joules/second Energy: Watt hour or Wh. This is a strange unit actually since it includes two time units which are different so don't cancel out. Ampere: measure of current or coulomb/second Coulomb: a specific number of electrons approximately equivalent to 6.2415093×10^18 electrons. Note that it doesn't matter what potential these electrons are at. Any voltage will do. Volts: a unit of electrical potential or electrical "pressure". These relate as follows: For power: W = V * A or power = volts * amps For energy: Wh = V * Ah or Watt-hours = volts * amp-hours When Ah are counted no voltage value is necessary. This is why an Ah counter like a CycleAnalyst, e-meter, JLD404, or any other similar meter does not need to know the voltage to count Ah. The meter does, however, need the matching voltage at each one of its samplings to calculate the power (W) and energy (Wh). When you look at the above you will see why these statements you made in another email are not true: ========================== 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 measure of the number of electrons is independent of voltage. See the definition of an ampere. Each cell is not delivered an equal amount of power because each cell is at a different voltage (W=V*A) which results in each cell getting a different amount of energy (Wh=V*Ah). > Even brand new LiFePo4 cells have different resistance readings. Which > slightly affects their useable capacity with each cycle. > Again, useable capacity is not what the original question was. My example above demonstrates the principle. > Don Blazer > > > In a message dated 6/19/2013 11:30:57 P.M. Pacific Daylight Time, > [email protected] writes: > > This is > why, as Cor pointed out, two cells with vastly different IR but the > same charge capacity, when starting at the same State of Charge, can > run out of charge, and thus energy, at the same time even if the > energy each delivers is vastly different. > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130620/74ea288b/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) > -- David D. Nelson http://evalbum.com/1328 http://www.levforum.com Nokia Lumia 920 Windows Phone 8 _______________________________________________ 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)
