T i m wrote:
On Sat, 16 Sep 2017 00:50:48 +0100, (Roger Hayter)
wrote:
snip
And why I'm trying to apply *some* science to whatever I end up with?
If I set the low voltage threshold to a voltage that would represent
50% DOD when discharged at 5A the alarm *will* sound prematurely when
running at a higher current. Now I could take that on the grounds that
at least it wouldn't over-discharge the batteries but I wouldn't get
the full capacity either. ;-(
This is where I think you are slipping into a fallacy. Although the
capacity has been less at the higher current, you have *lost* that
capacity by using a higher current and you can't get it back.
Yep, I understand that.
Even
though 50% DoD has been reached proportionaliy sooner, it *has* been
reached
No, it won't have been reached if the meter is using an arbitrary low
voltage threshold though?
and you can't reclaim the loss of capacity, except by
over-discharging the battery.
Of course (if that was the case).
This is why for battery management, as
opposed to journey management, the reduction of capacity with higher
current is irrelevant to you. You still want to stop at 50% DoD however
soon you reach it.
*Agreed*. Now, what low voltage is *correct* for what current, as one
voltage can't be correct for all currents can it (or they wouldn't
publish a graph showing different voltage cutoff points for different
currents presumably)?
Please don't get embroiled in the thoughts that I want to actually
monitor the actual battery capacity because I don't ... I just want to
do a bit more that use a single 'catch-all' values for what is already
known to be a conditional range. ;-)
Cheers, T i m
p.s. I have various electronic measurement tools including a watt hour
meter but it's no use as a form of protection tool and it doesn't
compensate for the varying capacities at varying loads. It could give
me a 'worse case' value though (30A x the voltage x the period
measured as a percentage (50% say) of the potential at that rate).
The model of the battery as a voltage source in series with a resistance
is quite a good one I think, and the on-load voltage plus predicted
voltage drop is quite a good measure of what the off-load voltage has
fallen to. Which just leaves the temporary loss of capacity at high
load, which would recover if the battery was left off load. The latter
might be a relatively small effect at the moderate current of about 10A
per battery that you are using. But as you might guess I don't know
any better than you.
An interesting experiment would be to run the battery down to the
indicated 50% discharge depth, then, without charging, use it again the
next day and see if you get a significant amount of use before it falls
to the same voltage. If not, then your measurement system is doing
quite an accurate job of protecting the battery without wasting
capacity.
--
Roger Hayter