Re: Sealed Lead Acid Battery Tester Strategy?

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