Tuesday, Jan 12, 2010 at 23:04
Colin,
Your question sounds pretty straight forward so there should be a straight forward answer. But there isn't.
The power you need will depend on sunshine, how often you are willing to move the panels so they face directly at the sun, and especially, how cold you run the fridge. And of course, are we talking summer in
Alice Springs or winter in Tasmania.
If you've read our blog (refered to above) you'll have some idea of all the factors involved. I'll try to be a bit more specific:
First, what is a 900 CC AGM battery? Do you mean 900 CCA battery? If so, it is a starting battery, not a deep cycle battery, which would be much better for the job. A starting battery will run the fridge and lights, but not for as long as a deep cycle battery and it will die earlier than a deep cycle battery if it is run down too far.
It may be a dual purpose battery which will handle both starting and deep cycle usage, though not as
well as the correct batteries for these jobs.
For my rig, I use 145W of solar panels. You are right in saying that you don't get all this out as usable power, but that's how the panels are rated and that's how we talk about sizing. In practice you must simply bear in mind that it isn't all there when you start making calculations. I also charge from the vehicle, but the panels handle the fridge and lights ok when stationary for a week or more.
Your ideal battery would be a 100 Amphour AGM, that is about the same physical size as a big (900CCA) starting battery. This should run your fridge and lights for 2 or 3 days without any charging unless you are running the fridge very cold. (Running it as a freezer you'll maybe get 1 day.)
You need to replace the Amphours you use each day using the solar panels. For that (assuming the fridge is not working very hard) you will need about 140 to 160 watts of solar capacity (that's manufacturer's watts, not usable watts). With that much solar capacity you will charge at up to about 10 amps in strong sunlight, so you MUST have a controller to save the battery from being overcharged.
So, the simple answer to a pretty complicated question is - panels rated at about 140-160 watts should do the job for you if the sun is shining and so long as you're not in the tropics in summer. If the sun doesn't shine, start the car and then use jumper leads to connect the trailer battery to the car battery and run the engine at fast idle for half an hour a couple of times a day. Disconnect the jumper leads when the engine is stopped.
Hope that helps
John
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Follow Up By: On Patrol & TONI - Wednesday, Jan 13, 2010 at 07:27
Wednesday, Jan 13, 2010 at 07:27
Thanks John
That helps, and I am still trying to digest the blog info.
As for the battery type John, thats not going to change for some time as it is still very new and expensive (2weeks old)
Thanks, Colin.
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Follow Up By: Member - John and Val - Wednesday, Jan 13, 2010 at 07:50
Wednesday, Jan 13, 2010 at 07:50
Understand about the battery Colin. It should work fine, but suggest aim to never leave it seriously discharged for long. Running any battery
well down, but especially a starting battery, will shorten its life. And as you know so
well, they aren't cheap!
Good luck with it.
Cheers
John
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