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
and integrate over time to estimate the Ah in/out of the pack so that an
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
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
or if the protection of the cells include a protection of working temp
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
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
there may be heaters or a coolant loop that can be controlled by the
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
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
SoC directly from cell voltages, such as Li-Ion Polymer or Manganese
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
operation of the BMS it is not necessary.
Hope this clarifies,

-----Original Message-----
From: EV [] On Behalf Of Bill Dube via
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

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.
>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, 
>Read EVAngel's EV News at Please discuss EV drag 
>racing at NEDRA (

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