Peri Hartman via EV wrote:
Well, if it's current that varies, it makes sense. Maybe I
misunderstood, but your prior post says "any voltage from 0 up to...".
Bob is right; PV panels do behave somewhat like a current source. If you
look at a graph of voltage vs. current at some fixed light level, it
looks roughly like a quarter of a circle, centered on 0v, 0a. Here's a
crude ASCII graph (view with a fixed-width font).
voltage
max X X
|| . . X <--peak power point
|| . . . X
0v. . . . X
amp 0 . . max
The light level changes both the max voltage and max current it can
deliver. Peak power is delivered somewhere in the middle (MPPT
algorithms vary the load on the PV panels to find this point).
Near its max voltage, the PV panel act somewhat like a voltage source.
Thus, a 15v max panel can be connected directly to a 12v battery and
won't overcharge it. Above this, you'd need something to limit the peak
voltage. The PV voltage varies with light, so connecting it directly to
a battery also means it won't get maximum power, and won't charge at all
when the light level is lower.
Near its max current, the PV panel acts more like a current source. That
means the current barely changes despite the voltage it is delivering. A
450v PV panel would charge a 6v, 12v, 24v, or 48v pack all at about the
same current. The current depends on light level. If the PV panel's
short-circuit current is 4a, then it would charge any of these batteries
at about 4a.
Again, you won't get anywhere near the peak power that the PV panels can
deliver. And, you can't leave it connected or it will overcharge the
batteries to death.
DC requires DC-rated switches, contactors, circuit breakers, and
connectors. Above 30v or so, arcing becomes serious! Make *sure* the
parts you are using are rated for the peak voltage and current!
They're not common, but it's not too hard to get parts that can handle
450v DC at 4 amps. They typically have arc ladders, magnetic blowouts,
or exceptionally large contact spacings. Kilovac makes some contactors,
and Anderson Power Products has some Power-Pole connectors that can do
it. Schrack relays usually have DC ratings on their data sheets, though
you have to connected multiple poles in series to reach 450v DC (for
example, two 250vdc, or four 150vdc contacts).
Be careful!
--
I have no special talents. I am only passionately curious.
-- Albert Einstein
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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