Re: Battery (SLA) modeling

On 12/13/2024 3:59 AM, Martin Brown wrote:

On 13/12/2024 06:57, 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?

 Depends what you want to do to the battery for any non-trivial charging or discharging rate you ought to take its temperature into consideration too. I^2R heating with a fudge factor probably a good enough proxy.

Yes.  I'm just interested in taking a peek at it at a point in time.
E.g., look at it while under charge (knowing voltage at terminals
and charge current) and then briefly under DIScharge (same observations).
If series resistance is the same, I can infer the potential of the voltage
source from a set of linear equations UNDER THESE CONDITIONS.
I.e., I suspect having to "wait" a bit (hence the RC in my proposed model)
as I alter observation conditions -- but don't want to have to let the battery
come to some particular state before adjusting my assessment of it's SoC.
I want to use them to bridge brief "brownouts" for a ~3KW load (supplemented
with ~500W from the mains)

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)

 It might at least near fully charged states or about to be damaged by deep discharge. Manufacturers datasheets are usually helpful.
 Not all SLA's are as good as each other for every usage. The ones used in UPS's and invalid buggies seem to be particularly bad & overpriced.

The value they have (for a large energy store) is they are not as likely
to be seen as "dangerous" (e.g., lithium) by customers.  And, are
available cheaply and locally -- as well as legally disposable without
special measures.