T i m wrote:
On Fri, 15 Sep 2017 23:08:41 +0100, (Roger Hayter)
wrote:
snip
Ok, but if I understand the above info correctly the difference in
capacity changes significantly within the range of currents I intend
to use therefore (in my mind anyway) making it worth of inclusion in
any gauge trying to display the cutoff voltage for a given depth of
discharge? As you say, we need the 'how' and I was hoping we might be
able to extrapolate that from the information we *have* been given?
It depends whether you are trying to predict capacity at a given
current, which is where your Peurkert equation comes in.
No, I wouldn't say I was trying to predict anything (at this point
anyway g), but calculate it from the current characteristics?
Or just
finding out when you have reached 50% capacity.
Yeah, that's the badger. ;-)
The latter will be
indicated by the voltage they mention,
(at the discharge rates they mention, so far ...)
and the fact that it was reached
proportionately sooner with a lower effective capacity at high current
is irrelevant to the problem of detecting when it has been reached.
Ok ...
The battery has *been* inefficient at high current and it is too late to
do anything about it.
Ok, so, let's say that the battery would reach 50% depth of discharge
if we were going along on speed 1 (of 5) in 3 hours. The alarm beeps
and I know I might damage the battery any further so stop (the device
has done it's job).
Then I fit the second battery but it's starting to get dark so we want
to get home quicker so I put it on speed 3 that will have the alarm
going off in 50 minutes (again, the device has done it's job).
The point is that in each case the only information that will be used
to determine the battery voltage but with a rider that is a function
of the current current being drawn.
And this isn't me imagining anything or making things up, this is
scientifically proven fact that is measured and well published.
This phenomenon is separate from the *temporary* loss of capacity after
a period of high current.
Of course.
The only way to deal with this is to stop
discharging the battery well before 100% use of capacity, which is what
you are being advised to do when you use 50% as an end point.
Quite.
This
makes sense to me though it is a bit speculative.
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. Even
though 50% DoD has been reached proportionaliy sooner, it *has* been
reached and you can't reclaim the loss of capacity, except by
over-discharging the battery. 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.
I'm not asking or expecting any solution to be an accurate measure of
the DOD, just that I think I should be able to better than using one
set voltage over a range of currents when we *know* the voltage will
vary as a function of the current drawn?
Cheers, T i m
--
Roger Hayter