Voltage drop musings [RE-wrenches]

[Windy Dankoff, Dankoff Solar]

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>Hi Windy,
>
>>We sell LOTS of Solar Boost. I have one in my system and have 
>>measured 20-25% in winter cold with batteries at about 24.0 V.
>
>Tell me how you are testing to get this figure.
>
>Christopher Freitas was calling into question the way the 
>manufacturers calculate it.


OK, yes, it's easy to miscalculate based on the meter in the Solar 
Boost, though I'm sure the mfgr is hip to that. The meter can read 
battery volts, amps in and amps out. The amps in is at the max power 
point, and is thus a bit lower than it would be if directly connected 
to the battery (see any IV curve). So, if you calculate the % gain by 
dividing amps out by amps in, you reach a falsely high conclusion. 
The meter is useful and honest device, just don't misinterpret it.

Instead, here is how I got my gain measurements. I put a SPDT 
(transfer) switch on the PV+. In one position it throws PV+ to the 
Solar Boost input. In the other position, PV goes straight to the 
battery. I compare the amp readings at the battery (shunt to a 
Tri-Metric) with Boost in and with Boost out. Simple. As long as 
battery voltage is low enough to not cause any regulation of the 
charge, you have a comparison of the Solar Boost to the best possible 
(zero loss) conventional controller.

Windy

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