Design efficiencies (was: cost per kWh) [RE-wrenches]

[Ray Walters]

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I talked to Ralph the other day about that very issue. I had been using 94% 
for battery efficiency too based on the Trimet info. But Ralph explained to 
me that that was amphours only and did NOT include the fact that charging 
voltage will always be higher than discharge voltage. As Don noted, we need 
to use KWH efficiency in our designs. Ralph said that 94 to 97 number was 
for programming his meter only and that we shouldn't be using it for other 
calculations.
Boy you and I have certainly been thinking in stereo over the past few 
years without even knowing it.

Ray


>96% for battery efficiency, however. This is based on something I learned
>years ago from Ralph Heise of Bogart Eng., in the context of setting up
>battery efficiency on the TriMet. He distinguishes between per-cycle
>efficiency and system efficiency. Per cycle efficiency refers to the direct
>relationship of how many Ahrs it takes to return a battery to the same state
>of charge each cycle. I use 95-97% for this, and find that 97% in my own
>home system (12 T105s in good shape) it is quite accurate (that is, the
>accumulated error in the "% full" display will be very little even if I
>haven't completely filled the batteries/reset the monitor for a week or
>more). This is distinct from system battery efficiency, which factors in the
>energy used to maintain float on a full battery and to routinely equalize. I
>think that system efficiency, which is much lower, is where we have gotten
>the 80-85% battery efficiency figure that most of us use. But I figure that
>per-cycle efficiency is more accurate when doing a load analysis.
>
>Any other Wrench opinions on this?
>
>Allan at Positive Energy
>
>----- Original Message -----
>From: "Ray Walters" <remotech at taosnm.com>
> > Windy and I have been using a spreadsheet that you can punch in efficiency
> > numbers for  battery storage, inverter, wiring, and PV to battery voltage
> > mismatch. I have been using a battery efficiency of 85% (consider day
>loads
> > are higher, night a little lower), inverter at 85 to 92%, wiring at 95%
> > (shouldn't be worse than 90%) and with MPPT, I set the mismatch factor at
> > 98%. This gives a combined factor of  64% to 73%.
>
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