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From: ehsjr <ehsjr@verizon.net>
Newsgroups: sci.electronics.design
Subject: Re: Battery (SLA) modeling
Date: Mon, 16 Dec 2024 15:16:22 -0500
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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.
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.

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

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