Re: [VAC] Solar Power

[Don Horn <donhorn@xxxxxxxxxxxxxx>]

Jim,

Good description.  I agree with all you've said.  If I may add a little
regarding system capacities...

My sources suggest that you discharge the battery to no less than 50% of its
capacity, and that you expect to only recharge it to 75-80% of capacity
using your solar panels.  This means that the capacity of your battery
should be approximately 4 times your usage between charges.  Your situation
fits into this exactly on a daily basis, i.e., about 20 AH usage per day,
which would take an 80 AH battery from 60 AH to 40 AH and solar panel output
of about 20 AH that would  bring it back up to 60 AH every day, just about
indefinitely.  

However, if you only expect to be out 2 or 3 days maximum on any trip and
always start with a fully charged battery, then you can get away with higher
usage and a smaller panel/battery combination, as long as you don't drop
under 50% charge capacity during those few days.  The same goes if you
expect to be out only a week.  The panel doesn't have to bring the battery
back up to 75% every day, as long as you get to a 120V AC source to recharge
before it falls below 50%.   You need to run the numbers of your actual
usage vs. daily solar panel output vs. battery size to figure the minimum
size panel and battery combination you need.

As an example,  if you have a solar panel that puts out 15 AH per day, and
your usage is 25 AH per day, then you have a deficit of 10A H per day.   If
you start out with a fully charged battery at 80 AH (actual), this takes the
battery to half charge in 4 days, after which you'll need to find another
source or cut your usage.  If you're not planning any trips over 2-3 days
away from 120v AC, this setup would be more than adequate.  If you travel
every few days, your tow vehicle would also add a small amount to this, but
would likely not bring the battery up to full charge, because the voltage
regulator is not matched to the acceptance rate of deep cycle batteries.

I don't suggest that you'd necessarily want to cut it that close, but if
you're on a budget you could get by with your existing battery and a small
panel to start, planning to add panels plus more and/or larger batteries in
the future, and still have adequate power for short trips in the meantime.

Once again, I recommend  the book "Managing 12 Volts" (even if you're not
considering solar), and "The RVers' Guide to Solar Battery Charging" from RV
Solar Electric.  

Don