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PV panel impedance matching?

 
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Vortex



Joined: 16 May 2006
Posts: 6097

PostPosted: Sun Jul 13, 2008 2:38 pm    Post subject: PV panel impedance matching? Reply with quote

I have a small 5W 12V regulated panel (Maplins shite) to charge an electric fencer.

Just for fun I connected the fencer WITHOUT its battery to the panel ... and the fencer couldn't quite run properly, even with the panel in full sunlight.

(The fencer needs around 100mA at 12V)

Now, is this an impedance thing? Is the current demand dropping the PV voltage too low?

In other words, will I get a ton more benefit from the PV if it's 'looking at' a 12 volt battery plus fencer?

Or is the PV simply too puny?
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Steve Houseman



Joined: 06 Feb 2008
Posts: 17
Location: Eastleigh

PostPosted: Sun Jul 13, 2008 7:48 pm    Post subject: Reply with quote

Hello vortex ,

I currently know nothing about solar panels etc altho I hope to change
that in the near future . Ditto for electric fencers (!).

Looking at the sanyo panel (only pv that I have a pdf data sheet on)
it appears to be a current source dependant only on the irradiance ,
and as such would have a high impedance which may be the problem
if the 'electric fencer' (is that a real name?) is expecting a
(low impedance) battery or similar source.

The battery itself probably would act as a low impedance source
when connected in parallel .

If you have a meter , then I would suggest , connecting the battery ,
as well as the fencer , and measuring the dc current from the pv ,
and ditto the electric fencer , and that should reassure you (or not) Smile .

I would expect the battery to fix the problem (famous last words) .

Cheers,

Steve
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mikepepler
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Joined: 24 Nov 2005
Posts: 2904
Location: Rye, UK

PostPosted: Mon Jul 14, 2008 2:01 pm    Post subject: Reply with quote

Solar panels have a "maximum power point" (MPP), where the product of their voltage and current are the maximum. If you try and draw more current, the voltage drops, if you draw less it goes up, but either way you get less power. Some of the more advanced inverters take account of this and maintain the panel at the optimum voltage - I think you can buy DC-DC products to do this too, but they're expensive. The name for them is an MPPT (maximum power point tracker).

Ultimately though, if your panel can't supply enough for the fence, then it won't help - you just need more power. I guess a crude way of making the post of the panel you have would be to set up a fixed DC-DC converter to allow the panel to run at 17-18V and the fence at 12-13V? I don't know how well this would work in practice though.
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adam2
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Joined: 02 Jul 2007
Posts: 6550
Location: North Somerset

PostPosted: Mon Jul 14, 2008 2:23 pm    Post subject: Reply with quote

The electric fence probably consumes an average of 100ma, not a steady 100ma.
A 12 volt 5 watt module should in practice be able to supply about 250ma in good sun, but contains no energy storage whatsoever, therefore if the electric fence requires brief pulses of 500ma it wont work.

Also in between the pulses the fence may draw no/almost no current, under these conditions the voltage will rise to the open circuit voltage of the module which may be about 25 volts, high enough probably to damage equipment designed for 12 volts.

You will therefore need a battery and charge controller, together with the PV module. A 5 watt PV module wont be sufficient to power the fence 24/7 in winter, and maybe not in summer (though it would greatly extend the intervals at which the battery would have to be charged)
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Vortex



Joined: 16 May 2006
Posts: 6097

PostPosted: Mon Jul 14, 2008 7:11 pm    Post subject: Reply with quote

That was my theory ... on the basis that by winter we should have an ISO container on-site plus a very small wind turbine to charge batteries.

(We have no shortage of wind sweeping across the land!!!)

Even if we have to swap out batteries, it will only be a short walk/stoop lugging the things to/from the container.
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Vortex



Joined: 16 May 2006
Posts: 6097

PostPosted: Sat Jul 19, 2008 5:24 pm    Post subject: Reply with quote

So far so good: this sunny weather is keeping the tiny 4AH lead acid battery I am using for the fencer nicely charged.

Five rip roaring watts from the panel are doing their business!
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