[Ovmsdev] MS_V_BAT_SOH

[Robin O'Leary]

On Wed, May 23, 2018 at 10:42:32PM +0800, Mark Webb-Johnson wrote:
> What is MS_V_BAT_SOH used for?
...
> Which is it?
> An overall indication of battery health? 100% being perfect, and 0% a brick?
> Or battery capacity (100% being a new car, and 50% being one who’s range is only half that of a new car)?

If the car provides its own SOH value, then we should report it, even if
we don't know exactly what it means.  In some cases, it may be important
for warranty claims.

For the Leaf, the BMS-reported SOH seems more complicated than just a
percentage of new capacity; e.g. increased internal resistance and cell
imbalance are also factors.

If the car does not provides its own SOH value directly, then maybe
it's better not to fake it; just show whatever values are available.
Or perhaps it could be user preference which value best serves their
particular purpose: total capacity depletion, cell imbalance ...
or some combination.

...
> Nissan Leaf uses:
> StandardMetrics.ms_v_bat_soh->SetValue(ah / newCarAh * 100);

You may recall, we had a recent discussion in favour of changing the
Leaf's ms_v_bat_soc and ms_v_bat_soh to report the values directly from
the BMS.

We do now report SOC as both direct-from-car and scaled-to-new values
in Leaf-specific metrics ("m_soc_instrument" and "m_soc_new_car")
and we have a config preference for which one to copy to ms_v_bat_soc
(defaulting to the instrument value).

We have not yet done that for SOH, but hopefully will do so soon, adding
"m_soh_instrument" for the BMS value and moving the above calculation to
"m_soh_new_car" .
...
> Find out the difference between SOC LIM MIN and LIM MAX, and use that as an indication of imbalance in the pack.
> 
> Or should we just add SOC_MIN and SOC_MAX if these are used in many cars? Or an imbalance percentage?

Having a way to report cell imbalance is a good idea, but trying to
express it as two percentage values seems rather crude.  The Leaf can
report voltage for every cell, but (as far as I know) does not indicate
the nominal voltage value (and I wouldn't be surprised if these differ
between the three(?) generations of chemistry already in circulation), so
converting each one to a % would be arbitrary.  Also, the distribution of
voltage values for the modules is often tightly grouped, with just a few
outliers, so just picking the lowest and highest is a bit too simplistic.
A better way would be to report all the cell voltage measurements
directly, or at least some basic statistics (n, average, stddev).

How widely are such detailed values available for other vehicles?
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