Q? small solar panels 20w / 40w amp output.

forgotmyname

Looking for a small 40 - 80w (ish) panel(s) for the shed to keep some batteries topped up.

Checking the figures puzzled me, why does a 40 watt panel only produce 2amps at 12v instead of the expected 3amps?

Checked a few sites explaining the different types but did not spot an explanation for the watts/amps not being what i would expect them to be.

Anyone explain?

Thanks.

zeupater

forgotmyname wrote: »

Looking for a small 40 - 80w (ish) panel(s) for the shed to keep some batteries topped up.

Checking the figures puzzled me, why does a 40 watt panel only produce 2amps at 12v instead of the expected 3amps?

Checked a few sites explaining the different types but did not spot an explanation for the watts/amps not being what i would expect them to be.

Anyone explain?

Thanks.

Hi

I'll have a go ....

I take it that you've put a multi-meter into the circuit when generating to establish the 2A at 12V ?? ... if that's the case, did you assume 12V or measure the voltage ? .... are you using a charge controller ?, was the panel hot?, was the panel in full direct sunlight ?? .... else, how about a link to a technical spec to the panels you're looking at ? - (In my experience "12V" panels are rarely 12V)

I've got some old nominal 12V panels which are used for various things and, as far as I can remember, when I've had a multi-meter connected they've never produced 12V, always more, probably somewhere around 18W IIRC - after all, how else would power flow in the right direction ?? .... to me a nominal 12V panel rated at a nominal 40W which claims a 2A typical optimal current would suggest that it's also designed to operate at a similar voltage .... 18Vx2A=36W.

Anyway, if you're looking to set-up a system without dedicated charge control you'll need to include a circuit diode to prevent battery discharge overnight, some (/most) small panels aren't designed with direct charge connection in mind so don't have this feature. You'd be better looking for a panel which is designed specifically for direct connection trickle charging a caravan battery (etc) to make sure (or design/build a suitable circuit, or buy a charge controller) ....

HTH
Z

forgotmyname

I have not bought the panels yet, Currently i have several batteries with small panels 1.8w to 4w on each to keep them topped up so they dont run flat. I still charged them every now and then with a CTEK charger to ensure they stay in peak condition.
Just a preventative just in case i forget to charge them and dont want them to run flat.

I thought about setting it up properly with a larger panel(s) and charge controller.
This is just 20w panel but gives the figures.. www.ebay.co.uk/itm/400949623035
Current at max power 1.16A where i would have expected closer to 2A.

Figures here if this works..
Open-circuit voltage (Voc) Av 21.6V, sd 0.3
Voltage at maximum power (Vmp) Av 17.5V, sd 0.2
Short-circuit current (Isc) Av 1.31A, sd 0.01

Current at maximum power (Imp) Av 1.16A @ 12v

Maximum power at STC (Wp) 20W ±3% Operating temperature -40oC to +85oC Maximum system voltage 1000Vdc Power tolerance ±3%

Which figures do they use to calculate the wattage?

Another Q? about charge controllers if i may, If the LEDs stay powered 24/7 wont they put some drain on the battery every night?

zeupater

Hi

Power is only produced if current exists, therefore the figures you're looking for are under maximum generation and load conditions, so you're looking at a 17.5V(Vmp) panels which can provide 1.16A(Imp) ... 17.5x1.16=20.3W ... the maximum system voltage is stated for anyone looking to connect panels in series (do not do this to charge a 12V battery!) ...

Regarding the charge controller's LEDs ... a single LED shouldn't draw much power at 12V, probably around 100mW, but it really depends on the design quality of the entire circuit and the capacity of the batteries you have .... a 100Ah 12V deep cycle battery would theoretically keep a single LED running for close to a year (natural discharge excluded) ....

HTH
Z

forgotmyname

I has a small 4w panel attached to the charge controller to just have a play, it was connected to a 10Ah SLA battery that is well past its best.

Before bed last night it was at 12.2v, i checked it again about midday and it was at 11.9v. I have disconnected the charge controller and put the panel directly to the battery.

Its has 5 LEDs, 3 for the battery voltage and 1 for load which is always on and the charge light.

Just conencted the charge controller to a watt meter and its .02 - .05amps/ 0.3 - 0.5 watts. So a tiny amount.

Maybe the charge controller doesnt like the tiny 4w panel?

forgotmyname

OK a few days of testing with 2 12v 10ah SLA batteries.

1x 4w folding panel and a BSV20A controller = the battery voltage is lower after 4 days than it started out at. 0.7 volts less.

1x 4w folding panel + 1x 1.5w panel also, CMTP0210A controller and again the voltage is lower than it started out at. This time its just 0.1v less but this battery is on better condition that the other one.

Panels not in an optimized position for the best sun, But receives at least 4 hours of direct sun on the panel each day.

I think i need to switch them off at night.

I have one 4w panel connected directly to the rubbish battery and will check the voltage in a few days, see if it has increased.