Solar Charging Batteries

Submitted: Thursday, Jun 22, 2006 at 10:35
ThreadID: 35172 Views:1801 Replies:7 FollowUps:9
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Hi All. Wondering if anyone out there can tell me if 20W Solar Panel for 12vdc application would be enough to keep batteries charged to Keep 110ltr 3way fridge running for any length of time . Thanks Matthew W
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Reply By: Gerry - Thursday, Jun 22, 2006 at 10:40

Thursday, Jun 22, 2006 at 10:40
Unfortunately, NO. I believe the 12V element is somewhere in the region of 180 watts although this may vary with fridge size, make model etc. The 12 Volt setting is really only designed for use while travelling so the car alternator is charging. While stationary, I would only ever use the gas or 240V if available.
Cheers
Gerry
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Follow Up By: Matthew w - Thursday, Jun 22, 2006 at 13:51

Thursday, Jun 22, 2006 at 13:51
I thought this may be the case but was not sure many thanks. Matthew
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Reply By: brian - Thursday, Jun 22, 2006 at 10:40

Thursday, Jun 22, 2006 at 10:40
if 20w is not a typo it would be totally inadequate
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Reply By: disco1942 - Thursday, Jun 22, 2006 at 10:53

Thursday, Jun 22, 2006 at 10:53
A 20W pannel would only be large enough to supply one high efficiency light (11W PL type tube) for 2-4 hours per night. On the other hand it would be too large to maintain a battery between trips without using a regulator with it.

PeterD
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Follow Up By: Matthew w - Thursday, Jun 22, 2006 at 13:54

Thursday, Jun 22, 2006 at 13:54
Thanks Peter Have regulator but was wondering if it was possible .
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Reply By: Rokkitt - Thursday, Jun 22, 2006 at 12:19

Thursday, Jun 22, 2006 at 12:19
Hi,

Try this site for calculating power requirements:

Site Link

Ohms Law

A 20 watt panel connected on a 12v system will give 1.66 amps - I think this would be per hour? dependant upon resistance but I think this would be the actual output......no doubt someone will correct me if I have got it wrong....been a long time since I touched this stuff.

Rod
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Follow Up By: Matthew w - Thursday, Jun 22, 2006 at 13:56

Thursday, Jun 22, 2006 at 13:56
Thanks Rod
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Follow Up By: Member - Mike DID - Sunday, Jun 25, 2006 at 17:08

Sunday, Jun 25, 2006 at 17:08
Actually a 20 watt panel will never put out more than 1.1 amps at any instant in time - and that's only when it's in direct sunshine.

It puts out 1.1 amps - time doesn't come into it. If its in full sunlight for 8 hours, it'll put 8.8 amphours into your battery - not much, is it !

Forget converting the 20 watts to amps - it's a smokescreen used to make panles look good.

Mike
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Reply By: Wizard1 - Thursday, Jun 22, 2006 at 12:23

Thursday, Jun 22, 2006 at 12:23
If the fridge is 3 way why not run it on gas?
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Follow Up By: Matthew w - Thursday, Jun 22, 2006 at 13:58

Thursday, Jun 22, 2006 at 13:58
Thought I may have benn able to run on the cheep guess it will have to be gas. Matthew
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Reply By: Darian (SA) - Thursday, Jun 22, 2006 at 13:30

Thursday, Jun 22, 2006 at 13:30
Yep - I reckon Rokkit's figures re output for the 20W seem correct - anyway, Matt - seems all 3 way users who have stumbled in here regard 3 way optimal running as gas when camped (or static for whatever reason), 12 V purely for mobile and 240V when you can get it (to save gas). The 12 volt system is ultra hungry (8-9A on a 40 L model / maybe more on yours)..... needs the car battery feeding from an alternator to make it a proposition. Keep in mind that even under ideal power supply, the 12V system cannot compare to the performance of the gas system. Gas is the winner mode.
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Follow Up By: Matthew w - Thursday, Jun 22, 2006 at 14:05

Thursday, Jun 22, 2006 at 14:05
Thanks Darian . I will remember the above advice. Matthew
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Follow Up By: disco1942 - Thursday, Jun 22, 2006 at 15:21

Thursday, Jun 22, 2006 at 15:21
There is no way you will get 1.66A from a 20W pannel. The peak power OP of a pannel is not calculated at the 12V point on the current Vs voltage curve. For starters if you wanted to calculte power OP for batery charging then you would have to use a figure of around 14.5V – that is the voltage needed to charge a battery.

The peak output of a 36 cell pannel occurres at around 17-18V. This is useable in water pumping applications as some pumps are disigned to accept this. If your pannel peaks at 17.5V then the current at peak power is around 1.15A. If you load its OP to 14V you will get a little more and you will get more again if you short circuit the pannel.

An example is a Model NE-80E1U pannel listed on the 12V Shop web site. It has an open circuit voltage of 21.3V, a peak power voltage of 17.1V & current of 4.67A and a short circuit current of 5.31A.

To add to that we generally mount our pannels flat on the roof of our van- don't we? I do but I have compensated for the losses with a bigger pannel. If you can get more than half the short circuit current out of your pannel into your battery for a few hours a day in the far north of Australia in Summer then you are doing well. You can improve this a little if you are prepared to sit around camp all day and adjust the angle of the pannel every now and again to face it squarely at the sun.

PeterD
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Follow Up By: atoyot - Thursday, Jun 22, 2006 at 21:41

Thursday, Jun 22, 2006 at 21:41
I've never thought about running a 3 way from a battery, and when you do some rough sums, it's pretty scary. Assuming 8 amps (constant, as these things don't cycle) in a 24h period, it will use 192amps. If you had say 8 hours of good sunshine, you'd need your solar panels to provide 24amps per hour just to keep up with the fridge. You'd then be looking at 3 100watt panels, at the very least.

Andrew
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Follow Up By: Darian (SA) - Friday, Jun 23, 2006 at 14:33

Friday, Jun 23, 2006 at 14:33
Re Disco's comment on outputs - I've got a 10W BP solar (given to me) - configs are about 16V x .7A - surely a 20W could peak around 1.6A......
Anyway, far as I'm told, all Solar users have to be mindful that all claims made by manufacturers are for ideal circumstances - air temparture, clear sky, sun directly overhead.... as soon as you depart from any of those ideal factors, the actual output starts dropping - markedly, in many cases.
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Reply By: Member - Jeff H (QLD) - Thursday, Jun 22, 2006 at 22:30

Thursday, Jun 22, 2006 at 22:30
. Disco 1942,
. Maybe you got just a little technical there: Michael asked a question in ignorance (as I have done, and will continue to do, so shoot me.)
. We run a 123w Sharp over the Rodeo cab. Doesn't bloomin work though, 'cause the kayak shades it out. So we arrive somewhere good, slide the panel out, (pardon, did you say yours are fixed on your roof ? I am so sorry.), and if there happens to be water , we'll fish.
. To each their own, and I feel that blokes like Mike (onya buddy;great but optimistic question),could do with basic common help - not quoted figures.
. Disco, not looking for a blue: I've only been in a (financial) position to buy into solar very recently (90000 km. kirra kirra),and love. Just bought Collyn Rivers book 'Motorhome Electrics' (google collynrivers).

. Let's not forget that we're not all paid up members of the SAS Regiment/UQ Physics Dept.
. Hope that Michaels interest in Solar blossoms.
. Jeff H.
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