On Sat, Mar 8, 2014 at 3:18 PM, Michael Ross <michael.e.r...@gmail.com> wrote:
> Why is 2.75V right for BB'ing a LiFePO4 pack? What is that based on? > Take one of your cells and charge it with a bench power supply where you can control the current and watch the voltage over time. Attach a load, like a light bulb to keep a somewhat constant current, to the cell and watch the voltage over time and you will quickly find out why 2.75V or 2.5V is a good BB value. For a more technical reason check out the chemistry of the batteries in detail and it will make sense then too, if you want to be a chemist. I top balanced my pack because of the fact that my charger won't shut off based on both a voltage and current value. It only shuts off based on a voltage and time at that voltage. I charge to 3.455V and after doing a no load rest after charge and finding the cells resting at no higher than 3.38V (theoretical 100%SOC) I have a safe cutoff voltage. IIRC, the highest cell was at 3.36V so I'm cutting off a little below 100%SOC. I also top balanced because it would give me an easy comparison of cell voltages right at the moment that the charger finished charging. I started checking voltages weekly and quickly went to monthly because of the boredom with getting the same results within a narrow margin. After a year or so I went to every other month and now I just looked and see that the last time I checked was October 2013. The high and low cells are not always the same ones but this makes sense since the SEI layer in the cells is constantly changing as they charge/discharge and that can have an effect on the absorption rate or effective IR. I haven't balanced the pack since July 2011 and it hasn't drifted like several on this list told me that they would. They are still working just fine. I typically don't use more than 70% of the capacity and I don't draw more than 500A for a brief moment out of this 200Ah pack. These are TS-LFP100AHA cells in buddy pairs. The only extra instrumentation I have on the pack is a variation on Lee Harts Batt-Bridge. I have a digital volt meter giving me pack balance. Read about it in my blog if you are interested in what I did. I figure that within two more years I will be at the break even point on cost compared to if I had gone with lead acid packs instead of LiFePO4. Even if they died tomorrow I'm still ahead on the convenience factor alone. Balance them once, don't over charge and don't over discharge. Properly make the terminal connections like Bill mentioned earlier, and you have a set it and forget it pack for a long time. I would not run a lithium pack of any sort without an Ah counter of some sort, at least in my road going EV. In my electric push mower, I put a pack of eight 20Ah LiMnFE cells and bottom balanced them. I can see quickly when the voltage drops on the volt meter I installed and the mower power will drop rather quickly, as you will understand if you play around with a LiFePO4 cell on the bench. You can bottom balance any number of cells in parallel you want. It is just that if they are half or nearly full it will take a really long time or a really high current to do so. It is usually easier to put them in series and discharge the higher voltage pack until the lowest cell hits a cutoff voltage of 2.5-3V. Then take that cell out and continue the process until you are ready to put them in parallel. You can pick up several lengths of stainless steel welding rod to use as a load on each cell in your parallel pack. Just put loop on each end of the wire to hook to the terminal. To bottom balance my lawn mower pack I just used a short length of nichrome wire attached to alligator clips on a wire. I could move the alligator clips on the nichrome heater wire to get the current draw I wanted. Just be careful to not burn your self. I discharged to 2.5V and then let the cells sit individually until they all rested at the same voltage. For a larger pack I would put them in parallel for a couple of days after bottom balancing then disconnect them and let them sit at least another day then take voltage readings on them to make sure you don't have any duds. They will show them selves buy continuing to discharge. What ever you do, DO NOT have an imbalanced load on the pack. That, no matter how small, will knock the pack out of balance over time. If you are interested in my spreadsheet on my cells just PM me. It is in OpenOffice.org format. -- David D. Nelson http://evalbum.com/1328 http://www.levforum.com _______________________________________________ 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)
