rick wrote:
On 09/04/2021 11:05, Dave Plowman (News) wrote:
In article ,
rick wrote:
On 07/04/2021 10:32, GB wrote:




The voltage on a lead acid isn't dependant on the capacity. But rather
obviously different sizes are going to take different times to charge
from
low.


I realise its Volts per cell that makes up the 12V and number of cells
is the same.... but with massively bigger cells thought there may be
other factor, differnet back emf, resitance, etc.

The voltage is "electrochemistry". There's a professor in the
chemistry building at your local university, that's all he does :-)

It's an electrochemical potential. If you sit still long enough,
the electrochemistry prof will teach you the Nernst equation.

See the third page - the battery acid is roughly 12 molar when
the battery is fully charged. That is giving slightly over 2 volts
per cell. There's also a temperature term in the equation, and battery
chargers should really know how warm the battery is.

http://ecee.colorado.edu/~ecen2060/m...tery_paper.pdf

It is modified by the boundary layer (the concentration of materials
there can be different than the bulk material). There is a layer of material
next to the plate, and after the battery is charged, that's a bit
"disturbed". After a few hours, the cell settles to its final (real) voltage.

A cell has resistance, but that only affects the voltage under load.
If the cell has a resistance of one ohm, one ampere of current is
drawn, then the cell will appear to be delivering one volt less
voltage than before the load was applied. A car battery will have
resistance values, well under an ohm. For example, by measuring my
own car, one operating point is "9V @ 150A". When the starter
starts cranking, the battery voltage drops due to its internal
resistance, and those two values are what I measured with two meters
set to "peak hold" mode.

The behaviors of the battery are also measured by various pieces of
equipment. At the Battery University website, they talk of
"impedance spectrum", which is related to the transient behavior
during charging. It is possible to collect health information about
a battery, by studying the impedance. And doing it with pulses of current.

There is no back-EMF as such. Back-EMF is a property of motors
(and by analogy, likely generators or alternators as well).

If you change the chemical composition of the lead paste used
in the plates, that's more likely to alter the potential. Just as
lithium iron and lithium cobalt have radically different voltages.

Paul