Re: Battery (SLA) modeling

On 13/12/2024 13:31, Don Y wrote:

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.

Except for a few special cases, electrode-electrolyte interfaces are
very non-linear and you may find that the potential of the inferred
voltage source is different for charge and discharge conditions.
John

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.