Re: Battery (SLA) modeling

On 12/13/2024 8:54 AM, 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'm primarily looking at ways to determine SoC while charging and/or
powering loads.  In the first case, to guard against overcharging.
In the second, to guard against too deep of discharge.
In the second case, I obviously can't "disconnect" the battery to take
a peek at the open circuit voltage (which could be an option in the
first case).

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.

The batteries are just acting like "super caps".  I am more concerned
that they can manage a particular load (for a predictable time -- even
if that time is determined on-the-fly) than how LONG they can supply power.
SLAs are preferable because of cost, reputation (lack of association
with spectacular fires), availability of replacements (want more
capacity?  buy larger replacement batteries!) and ease of disposal
(you can drop them off at many businesses to be recycled -- hazardous
waste -- and even select "recycling facilities" if you want to
recover some of their monetary value)