On Thu, 05 Jan 2006 21:24:35 GMT, "Bryan Martin"
wrote:

The unit I will be using is 12V 0.5A

12V X 0.5A = 6 Watts
6W x 24hours = 144 Watts per day
144W / 12V = 12 amp hours

Now given a battery with 110 amp hours
(http://www.defender.com/product.jsp?path=-1|328|51495|306219&id=51496)

We should only draw the battery down 50% so we will actually only get

110 / 2 = 55 amp hours

12 amp hours needed / 55 amp hours available = 4.58 days
so with this battery we have enough juice to potentially run our unit for
4.58 days w/out charging.

Now if the above is not correct then please explain. And the next question
is how would I now figure out approx how large of a panel I would need to
keep the battery charged. I am really looking for the formula and a
understanding of how you got from A to B as oppose to "this panel should
work".

I would approach it a bit differently. And I think your error in your
calculations is an assumption that the battery charge/discharge is 100%
efficient. It's closer to 80%.

1. Daily Load is 12AH (@ 12V) as you calculated. But with DC loads you
can do your calculations directly in Amps. So 0.5A * 24 hrs = 12AH (@12V).

2. Add 20% for system losses and a safety factor -- 14.40 AH. You don't
get 100% efficiency in charging/discharging batteries.

3. Calculate your PV requirement based on the number of effective sun
hours at your site during the worst month. Not knowing where you live it's
tough to say, but, for example, the San Francisco area that might be 4 hrs.

That gives you a required PV array current of 3.60 amperes (14.40/4). Now
you can go look for an appropriate PV panel (or two, if need be). A BP365
panel is rated at 3.69A at PMax, as an example.

4. Now decide on your DOA (days of autonomy) to obtain required battery
size. 5 DOA with a 50% maximum discharge -- 144AH battery.

Or, working backwards, your 110AH battery would give you 3.8 DOA.


-- ron (off the grid in Downeast Maine)