Tom, I am afraid that your calculation has 3 orders of magnitude error: If nominal drain is 4mA +/-20% then max difference between the drain from different balancers is in the order of 2mA (milli, not micro), unless the nominal drain is 4uA (microAmp) then the difference can be less than 2uA but that is an unbelievable low power that usually is only possible with a pure analog solution or a very careful designed digital system with extensive power-saving modes and short wake-up times, not a run of the mill digital design. I do not know the miniBMS, so I am assuming that the nominal drain is 4mA which means 0.1Ah per day.
I browsed through the specs of the cell units and I did see a mention of max 7mA drain but it was unclear to me if that was the header (central) unit or the per-cell unit. If I want a BMS then I will probably create my own version of MiniBMS per-cell unit, because I do like their approach of a simple daisy chain to indicate a problem somewhere, though I may concentrate several boards together so it is not spread out across the pack, but sitting in a safer location right next to the pack. I am quite sure that I can get the per-cell drain low enough to not cause much imbalance and make sure that shunting is active on every charge cycle, or I might go for Lee Hart's balancer which makes sure that all cells are at equal voltage, so there is no need for shunting the excess away. Cor van de Water Chief Scientist Proxim Wireless office +1 408 383 7626 Skype: cor_van_de_water XoIP +31 87 784 1130 private: cvandewater.info www.proxim.com This email message (including any attachments) contains confidential and proprietary information of Proxim Wireless Corporation. If you received this message in error, please delete it and notify the sender. Any unauthorized use, disclosure, distribution, or copying of any part of this message is prohibited. -----Original Message----- From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of tomw via EV Sent: Friday, June 19, 2015 8:05 AM To: ev@lists.evdl.org Subject: Re: [EVDL] Bicycle battery /"That is a reason I don't use a cell level BMS. With a cell level BMS like the miniBMS there is a constant drain on the cells running the BMS boards and it is nearly impossible to make sure that each board uses the _exact_ same current regardless of voltage in the cell."/ The constant drain is less 4mA with about +/-20% variability, so the difference between cells is at most < 2 microamp. /"Furthermore, boards like the miniBMS expect that you will balance at the end of every charge and last I saw, that balance voltage was at 3.6V or so which is over the theoretical 100% SOC level for LiFePO4 cells which leads to potential overcharging of the cells on every charge cycle."/ Balance voltage on the minibms for LiFePO4 cells has always been 3.5V, from the very first ones made to the present. Check the original minibms thread on diyelectriccar if you don't believe it. I purchased some of the first boards and am still using them. Also, 3.6V while charging is not necessarily over the theoretical max SoC. According to Whitacre cell rest voltage of 3.4V is 100% SoC. The voltage will be higher while charging while near full charge due to the voltage drop across the cell effective internal resistance. The higher the charge current, the higher the cell voltage will be at a given SoC. Of course if you go too high in cell voltage (4.2V according to Whitacre) you start to break down the electrolyte. When I charge my pack to 3.53V average cell voltage, they are at 3.344V average after > 2 hours rest. CALB and others used to spec charge to 4.1 or 4.2V at 0.05C rate, which would result in a full charge and rest voltage of about 3.4V per cell. They haven't spec'ed that for years though. They relaxed it to 3.6V. I would guess for longer cell life and the fact that there is little difference in useable charge. /"If you stop charging without the balancing taking place and/or let the pack sit for extended periods of time then the different current draw of each board working 24/7 introduces an imbalance in the SOC of the cells in the pack."/ I do partial charges without any shunting > 70% of the time. When I do charge to 3.53V per cell to get shunting, the first cells start shunting about 10 minutes before charging terminates, and by end of charge all shunt LEDs are on. The highest cells reach about 3.55V. I've let the car sit for over 2 weeks while away for work or on vacation and the pack voltage reads the same voltage when I return as when I left. The next full charge there may be several cells that don't shunt, but they all do after a couple of full charges. The pack has 5 1/2 years and about 45,000 miles on it. I think you have to keep the scale of things in mind. These effects are small. I've found them inconsequential in daily operation of the vehicle. I think the main effect on cell life is temperature. I expressed concern about that several years ago when the Whitacre video first came out and he said don't exceed 50C to 60C. I questioned then which was the limit, pointing out that 50C was a big problem for a place like Phoenix. But I am not going to let the car sit in hot weather. I use it as my daily driver. If hot weather shortens the pack life, then so be it. I want to see how it operates as a car, not a hobby. Yesterday the cells were at about 105F (41C) after about 75 miles driving in about 96F ambient. That's pretty typical for the last 5 years during June - August, but I have measured as high as 116F (47C). -- View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/Bicycle-battery-tp4676242p4676353.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) _______________________________________________ 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)
