/How do you know if some cells go down way to low? And how to prevent that any slightly weaker cells to go ..a reverse situation at very high loads, or when it is -40` outside or so and the pack might not be perfectly and evenly heated up yet?/
My way of handling this is to never drive the pack down below about 30% or so SoC as indicated by my TBS gauge (charge counter). /I guess the answer to the first line will be the reason of doing bottom balance, all might reach "low" at the same time, and to me THAT argument make at least some sense, (if all of them can deliver the same current in all temperatures and the same SOC(Ah left in cells.) )/ It is true cells will survive and still work well since none will be reversed if well bottom balanced, but according to Whitacre there will be some loss of capacity if a cell is driven under 1 - 1.2V due to lithium coating out on electrodes. /Bottom balancers: How do you guys do fast charge? or regen hard with full pack, or handle a cold pack, regen if it is at freezing degree, or maybe less capacity cells in a pack and so on without damaging the cell, if just one or a few was close to full before the others and you regen hard in a cold pack. I dont like that, It WILL happen to me sometime in the future,, if I dont have the system to protect the cells from that overcurrent-situations./ When my pack was bottom balanced I recorded voltages of cells near end of charge with a dvm - every charge for months. I set "Full charge" on my TBS gauge for when my lowest capacity cell hit 3.45V and adjusted the limit voltage on the Manzanita charger so the charger shut off when that cell was about 3.45V (bit more complex than that since this varied with temp and charge current). With fast charge I would have had to watch the TBS gauge during charging and terminated charging manually to ensure I did not over charge my lowest capacity cell. I don't think there are any diy'ers (including me) using fast charging stations at this time so it isn't an issue for us. Regen is easy. The Curtis controller has a User_Overvoltage parameter that can be set by the user in sw. Just set it to a value that ensures your lowest capacity cell is never overcharged by regen. I've had this cut in a couple times when going down hill after a full charge. The car seems to be suddenly accelerating! Which it is of course due to gravity, with no regen to hold it back as normal. Glancing at the current gauge shows the regen current has decreased to zero. My pack has never been below 48 F (9C), and that was when I left it parked at the airport a couple nights without heating. Normally it is kept at 65 F (18C) when parked in the garage, using battery heater pads/controllers, and remains above 50F if parked for 4 - 5 hrs in temp around 25 F (-4C). I think not heating a pack is penny wise and pound foolish if you live in a cold area. Another advantage is range is about the same winter and summer except for some increased friction losses and of course cabin heater losses. /Also,,when I see the fast rate things do change in the very end of a charge or discharge cycle.. It is just not practical to do anything "manual" there if you have a lot of individual and slightly different cells in series./ Yes, that's why people have been saying for years now to stay away from the ends of the charge/discharge curve. Operate between 25% and 95% SoC or so and there is no problem. With most shunt balancers you have to go above 95% to get balancing, but the HVC function will ensure you don't overcharge a cell. /I get that you MIGHT not need cell-balancers at all, or maybe not big ones capable of many Amps in all normal charging (if the cells are ok, and new, and matched production units, and so on) but without any measuring at cell level, how can you say you know anything about that individual cells situation in driving/charging/regen?/ It's a leap of faith. :^)) By "know" people mean they "believe". You don't know if you don't have data. Only small shunt currents are necessary if a pack is initially well top balanced, as even 0.2A shunt is orders of magnitude greater than any leakage currents - the main source of which is the bms itself. But it corrects that small difference in loss easily each full charge cycle and gives you HVC and LVC. -- View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/self-discharge-of-CALB-180Ah-LiFePO4-cells-tp4669067p4669154.html Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.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)
