PV System Worksheet --Charge acceptance factor [RE-wrenches]

[John Berdner]

Windy:

For the type of charging algorithms we have in PV systems the charge
acceptance of the battery will be defined by the Amp hour law.
According to the Amp hour law the maximum instantaneous charge current
will be proportional to the number of Amp hours out of the battery.
Essentially this says the closer you get to full charge the slower you
have to charge the battery.
Hard to model when you don't kow how many Ahrs have been discharged.

In classes I try to explain that it is kind of like pouring a cold one
and trying to fill the glass right to the top. 
At first you can go glug, glug, glug and you won't spill a drop.
Once the glass starts to fill up you have to slow down or you end up
with foam spilling out (gassing in battery speak).
At the end you have to put in one drop at a time or it will spill.
After you get the glass full you have to keep adding a drip now and
then to compensate for evaporation (self discharge in battery speak)




>>> windy at dankoffsolar.com 6/6/2003 10:30:49 AM >>>
>Windy & Wrenches (& windy Wrenches...and sunny boys, too, I guess),
>I should have been more clear. I have unprotected the sheet and gone
into
>the formula bars to tweak here and there. Sometimes I succeeded at
getting
>the info I wanted, sometimes not. What I suggest in a companion sheet
is
>descriptions of the logic of the equations from which the results are
>derived (huh?). For example, "watt-hours per day equals quantity times
watts
>each times hours per day times days per week divided by 7". That's a
very
>simple example. Some of the formulae are quite complex and yet
insufficient
>to cover all situations, such as calculating generator run time. The
>inverter-generators have a very different efficiency than the
contractor
>specials.
>This is a request, Windy, not a criticism.
>Allan


Dearest Allan,

I wrote the guts of this 6 years ago. You think I remember the logic? 
As for the generator run time, that's the only formula that's 
severely twisted. It came to me in a rare flash of brilliance, to wit:

=IF((G27-(G38*G32*(K20+(K20*B32))))/(J17*B30)<0,0,7*(G27-(G38*G32*(K20+(K20*B32))))/(J17*B30))

What's the logic? I couldn't wrap my head around it 6 SECONDS after I 
wrote it! I just tested it up and down with examples to see that it 
worked, sighed relief, and nobody's reported any problems.

I CAN tell you that it's simplified. It assumes the battery is 
accepting the full "Battery Charge Amps" for the entire charge cycle. 
(I will add notation of this to the instruction sheet.)

There are way too many factors to handle here, like how full is the 
battery when the generator starts up, and how far does the current 
taper down before the generator is stopped? And, of course, the 
C-rate (ratio of battery AH to charge rate). This would take another 
whole sheet, and too many assumptions in my opinion.

However --

I can easily add a "Charge Acceptance Factor", with a default of say 
75%, to show the run time /.75. This would reflect a realistic 
average for a period of bulk charge at full current, then some 
tapering. Sure it's a guestimate, but better than assuming 100%.

Oh, I guess we could enter a C-rate and it would adjust the factor 
further automatically?

Tell me what you think.

Thanks!

Windy

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