The BMS does not need to know the SOC. The User Interface (dash) often presents an estimation of the SoC and this means that either the car (Motor Controller) or the BMS needs to measure the current and integrate over time to estimate the Ah in/out of the pack so that an estimation of SoC is available, occasionally re-calibrated automatically when the pack approaches full or empty and the BMS starts to become active to protect the allowed voltage range of every cell in the pack, which gives a calibration point near full/empty that the SoC estimator can use to reset itself from accumulating errors in the measurement of current and integrating over time these small errors can start to grow into big errors...
As said, the BMS is simply there to protect cells, so it is fine to only measure voltage and possibly temperature if the voltage measurement needs to be temp compensated or if the protection of the cells include a protection of working temp range. The BMS simply maintains the cells with operational parameters and stopping overcharging means monitoring cell voltage to detect a cell approaching max voltage and throttling the charger (or brake regen) back. Avoiding under-dischargesimply means detecting cell voltage dropping to the lower limit and cutting back on the allowed current draw for the motor controller. At some point that current may approach zero and the car should shut down completely. You can also think about temperature protection, both too high and too low, there may be heaters or a coolant loop that can be controlled by the BMS, you can think about isolation (detection of leakage to the car frame) but the essence is that a BMS only needs to protect the cells and it is useful if a BMS not only monitors but also actively balances cells, which typically happens at the top (end) of the charge cycle, unless the chemistry in use allows estimating SoC directly from cell voltages, such as Li-Ion Polymer or Manganese chemistries. Keeping the driver informed of available charge (SoC) is a function of the car and this function may be allocated to the BMS for convenience of design, but for the operation of the BMS it is not necessary. Hope this clarifies, Cor. -----Original Message----- From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of Bill Dube via EV Sent: Tuesday, August 08, 2017 11:26 AM To: Lee Hart; Electric Vehicle Discussion List Subject: [EVDL] Voltage SOC on LFP (was Curtis 1238-6501 State of Charge) It is difficult or perhaps even impossible to determine the SOC on LFP from voltage measurements alone. At least to do it with any accuracy. You can measure the temperature on each cell in addition to voltage to _perhaps_ get a better handle on the SOC, but still you can't get a very accurate SOC. From a practical point of view, it is easier and cheaper to count the amp-hours in a single place, than to make very precise voltage and temperature measurements on every cell in a high-voltage pack. That is why it is done that way universally in OEM cars. Bill D. At 10:15 AM 8/8/2017, you wrote: >paul dove via EV wrote: >>Sure you can. Voltage is the only way to discern SOC. >>You just can't measure accurately under load. You have to measure open >>circuit voltage. >>www.mdpi.com/1996-1073/9/11/900/pdf > >This paper mainly discusses the higher-voltage Li-Ion cells, which have >a larger voltage change between full and empty. > >For the LiFePO3 cells being discussed here, the voltage change is much >smaller. As Bill says, the change is so small that it can't be reliably >measured outside of laboratory conditions and precisions instruments. >Other factors, such as age, temperature, manufacturing variations, and >the accuracy of the equipment play a larger factor. > >-- >Where ignorance is our master, there is no possibility of real peace. > -- Dalai Lama >-- >Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com >_______________________________________________ >UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub >http://lists.evdl.org/listinfo.cgi/ev-evdl.org >Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag >racing at 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 Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at 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 Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
