Re: Sealed Lead Acid Battery Tester Strategy?



Mike wrote:
> >I'll check out the link. The low point is one of the contentious values
> >that varies. Some manufacturers quote "watts available to 10v" and
> >other voltages up to 11.5v as a minimum point. That's a big range.
>
> Having looked at the Yuasa manual link, it's raised some interesting points
> that I wasn't aware of :-
>
> Yuasa quote their capacities at 20 hour rate to 1.75v per cell (10.5v) so
> I'm using the right stopping voltage for my tester as "stop discharge" point.
> 11.0v and higher are "over cautious", in my opinion, and are not achieving
> the full capacity of the battery (highly relevant in a battery tester). It's
> probably better to use 10.5v and higher as a stop point for a piece of
> gear being powered by the batteries. But not for a tester.
>
> My capacities come out slightly under the nominal because I'm using 10 hour
> and 1 hour rate timings, and higher currents mean lower effective capacities.
> At least now I have a graph of how the current draw derates the capacity
> away from nominal.
>
> Yuasa quote discharge levels that go down as low as 1.6v per cell in their
> calculation tables (9.6v!) which reinforces a figure I'd seen in CSB Batteries
> and Camden data sheets. I think that's a bit low. However, context is
> important here ... it would seem that the acceptable "stop" voltage varies
> with the discharge rate, e.g. :-
>
> 0.1C or below/intermittent load=1.75v (10.5v)
> ...
> 0.6C=1.60V (9.6v)
> 3C=1.30v (7.8v)
>
> That caught me out.
>
> So really when discharging at "C" I've not been truly flattening the
> battery: I've been stopping at 10.5v or thereabouts. I should be stopping
> somewhere around 9.2v. That's on load, actual voltage. It looks like these
> figures are already trying to compensate for the internal resistance of
> the battery, among other effects.
>
> However, estimated battery capacity (off load) doesn't use the same "end
> point". This is one of the things I got wrong. I thought the discharge
> termination voltage was the same as the off-load "empty" voltage of 10.5v
> or less.
>
> The offload voltage should range from 11.5v-about 13v for 0-100%, in a
> fairly linear way.
>
> This partly explains why my discharge measurement and "remaining estimate"
> were at odds: I wasn't totally flattening at higher currents, and my
> estimator was using a range of 10.5v-12.96v, which would bias the estimate
> readings as too high. It also partly explains the "voltage bobs back up"
> effect. It seems I'm not the only one to make this mistake, as my Belkin
> UPS shares my over-enthusiastic estimates, seeing how it relates the
> terminal voltage to battery capacity.
>
> I will go and code wrangle and see if it improves now. Thanks for the
> pointers!

Happy to see you're on target now. ;-)) See my 4 Jul post--although I
haven't reviewed the Yuasa manual, little has changed in 30yrs with
lead acid batteries. Keep in mind that in addition to discharge rate,
plate design will affect to some degree the final numbers for total
power-on discharge as will temperatures--manufacturers data will help
here.

"Storage Batteries" by Ralph W. Ritter/552B International Textbook
Company, Scranton, Pennsylvania--Copyright in Great Britain, 1939--will
provide additional data on old time solutions/equipment for your
problem.

Again, you've done some good work--best wishes and let us know how it
all turns out.

Tut
> --
> --------------------------------------+------------------------------------
> Mike Brown: mjb[at]pootle.demon.co.uk | http://www.pootle.demon.co.uk/

.