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From: Martin Brown <'''newspam'''@nonad.co.uk>
Newsgroups: sci.electronics.design
Subject: Re: Battery (SLA) modeling
Date: Fri, 13 Dec 2024 16:43:15 +0000
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On 13/12/2024 15:54, Liz Tuddenham wrote:
> Don Y <blockedofcourse@foo.invalid> 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)
> 
> Design for the worst possible series resistance and then some.  I have
> been caught out by that a couple of times.  Screaming and apparently
> untraceable audio instability in the middle of doing the P.A. for a
> public event is highly embarassing. ("Yes, we put in a new battery - oh
> no, someone must have put the old one back in the box with the others -
> terribly sorry!")

I've seen kit intended to be powered by lead acid accumulators that has 
three diodes in series with the supply for when the battery is fresh off 
the charger and nearer to 14v than 12 with a switch to short them out 
when the battery starts to get low. It avoid the annoyance you had.
> 
> Also, whatever the manufacturers clain for the battery capacity, design
> for half that amount.  Batteries don't last forever but your customers
> will assume they do and won't want to change them until they are well
> below half capacity.
  I think most regular users of SLA's are aware of their weaknesses.

I love the way they swell up spectacularly inside certain brands of UPS 
making them almost impossible to remove. Cruel environment for any SLA.

-- 
Martin Brown