Re: Battery (SLA) modeling

On 12/16/2024 1:16 PM, ehsjr wrote:

On 12/13/2024 7:06 PM, Don Y wrote:

On 12/13/2024 4:20 PM, ehsjr wrote:

On 12/13/2024 1:57 AM, Don Y wrote:

At a fixed time and temperature, is it safe to model a battery as a
fixed voltage source behind a series resistance?  Possibly, additionally,
a parallel RC to cover transients?
>
In particular, does the model need to change based on whether the
battery is being charged or discharged?  (i.e., to estimate that
series resistance)

>
For what value of "safe"?
>
Probably better, in general, to use manufacturer's
discharge curve, at least as a starting point. If
your application differs appreciably from the test
conditions in the manufacturers specs, you need to
test it yourself, again depending on what you have
in mind by "safe".

>
Discharge curve depends on knowing the starting point SoC.

 No. The state of charge indicates the location on the curve.

Isn't that what I said?  Knowing the shape of the curve and points
on it doesn't help me unless I can map my battery's current
state (SoC and age) to that.

That location tells you how much time remains to reach the
terminal charge or discharge points, under the conditions
that were used to produce the discharge curve. The curve is
a design tool, not something you produce dynamically with
each charge/discharge cycle in actual use.
 

For a battery in continuous, cyclic use, you need to
*determine* the SoC at any given time.  I.e., when to
STOP charging, when to stop DIScharging, how much charge
you can expect to have available, how long until you
are likely going to reach "full" charge, etc.

 Exactly.
 

It's better to design the system to accommodate the
battery at its best and worst SOC/capacity levels.
Also the age of the battery and how it has been
treated over that age is a factor, so take that
into account too.

>
The hope is that by continuously updating the (albeit
simple) model, you can reflect the effects of age IN
the model and, potentially, indicate when replacement
is required (which largely depends on the service it
has seen)

 From your last paragraph, it seems that you do want to
model dynamically, using the dynamically updated model
as (put simply) the yes/no decider for replacement.
Certainly you could gather data from charge/discharge
voltage and current during same and idle time and write
code to do the analysis.

There are two (three?) issues here, each addressable (?)
with an appropriate model.
The first is to be able to ascertain the SoC of the battery
to determine how much charge it needs to accept to be returned
to "full" charge and the best way of delivering that charge
to meet a given timeframe, etc.
The second is to be able to ascertain the SoC of the battery
to determine how much longer it can support the *current*
load and how that load may want to NOW be shaped to meet a
given support time.
The third is to be able to notice how the model has changed,
over time, to reflect aging in the battery.  Determining
when/if replacement is necessary will depend on the SoH
of the battery as well as its load and charging history;
a battery *might* appear to be "shit" yet can still be
suitable for certain smaller loads or shorter support
intervals.

When would the analysis be "safe"?  I dunno. After enough

I'm not concerned with the "safety" of the analysis but,
rather, the safety of the *model* -- the two are distinctly
different.  I.e., one can have a perfect model and use it
poorly.

total cycles of replacing batteries you could arrive at a
better figure than something like "replace after 500
charge/discharge cycles whether it needs it or not", or
"replace every 5 years whether it needs it or not", etc.

It will never be that cut and dry.  Rather, it will be
"YOUR usage patterns suggest the battery should be replaced
soon.  Failure to do so can result in loss".
Or,
"Based on your usage patterns, you might consider changing the
battery to something larger (or smaller)"
[No reason to force people to buy more than THEY need]

But that's using much more than fixed time and temperature
and voltage data to arrive at whatever you have in mind by
"safe".
 Ed