Hi,
You have 3 choices to charge the battery if not hooked up to AC power.
A generator, solar panels or wind power.  All three are expensive, wind
power not very practical in most cases.  Solar works best in the
South/Southwest and better in summer then winter and is expensive. In
general solar panels of 15 watts or less are only good for keeping a
battery topped off/charged up while not being used.  You would need one
or two 50 watt or larger solar panels plus a charge controller to
recharge your batteries with any efficiency.  A lot depends upon your
location and your usage when sizing solar panels.  They need direct
sunlight, no shade.

 A marine type battery should be your second choice for batteries.  A
deep discharge battery like used in golf carts would work better.  Two 6
volt golf cart batteries wired in series would be ideal.  They are about
$50 each at Sam's Club.  They would give you approximately 220 Amps of
power and about 80-90 amps of usable power before they should be
recharged.  You should try and not run them down below 50% max.  A group
31 12V deep discharge battery would work well to.  You would have
approximately 130 Amps for one battery.  But they are more expensive
then two 6V golf cart batteries.  A typical Group 27 marine battery will
give you approximately 100 Amps of power and about 20-40 amps of usable
power before they should be recharged.  Marine batteries are a
combination of starting and deep cycle and cannot take as many or as
deep a discharge as true deep cycle batteries.

To calculate the power you use you need to know the wattage of the
items you intend to use.  If you have a 12V light that uses 12 watts of
power you need 1 amp of electricity per hour plus losses to light it.  
Amps = Watts / Volts.  So if you have that light lit for 4 hours you
will have used 4 Amps plus any losses from your battery.

Add up all the wattage for the 12V items you intend to use in one day
divided that number by 12 to get amps and multiply that number by the
hours you will use them.  That will give you the number of amps you will
use for a day.  How many days you can go without charging depends upon
the amps used per day and the size of the batteries.

Hope this helps.

 

Solar and wind is not cheap,

Your cheapest most reliable solution would be a set of 2 or 4 golf cart
batteries($50 each), a 1093a vector charger($50((referb))-100) and a small
portable generator. ( a coleman pulse 1850 ($400) but loud or a much more
reliable honda 100W inverter equiped ($750 and much more reliable, quiet and
lighter) and maybe a 400W inverter ($30 sams club). total cost ($730-$1050
depending on gen choice).

Solar: Same battery setup ($100(2)) $200 (4), I paid $1000 for (4) 45W
panels (delivered) another $100 for a trace c40 controller and $30-50 for
cables to wire panels. another $40 for holed angle iron for frame to mount
panels. Also, keeep in mind that your geographic location will effect how
much power can be harvested from the sun as well as the capacity of your
battery bank.

Hope this gives you some info.

Ed

If "money is very tight":

Forget solar.  Too expensive.  Invest in a set of booster cables,
and work on your charm.  At most buy an extra battery and battery
switch.  Maybe a cheap charger from Walmart.

You need more options?  Concentrate on making more money.

www.arcatapet.net/bobgiddings

The usual strategy is to add a deep cycle battery for cabin use and
reserve the original battery for starting the engine. This is to insure
being able to start the engine when necessary.  Walmart carries a good
deep cycle battery (105 amp-hours) for around $60.

Your alternator/voltage regulator might be adjusted to charge your
cabin battery reasonably fast by running your engine. This approach
requires some learning about battery charging and voltage regulators.
Costs are something like $20-120 depending on your alternator (the
alternator/regulator system must be able to put out about 14.5 volts
and must have an external "sense" terminal.)

A non-obvious strategy is to buy an inverter to make 120v AC so you can
use inexpensive compact fluorescent lamps that use a lot less energy
than incandescents (and also allows you to use many other AC gadgets.)
The inverter can also be used to power an efficient battery charger so
that when you run your engine the battery gets charged quickly. The
costs involved are around $60 for a Walmart Deep Cycle Battery (105
Amp-hour), $60 for a 20 amp Vector charger, and $60 for a suitable
inverter - about $180-200 total.

The inverter/charger/battery strategy has the advantage of shutting
itself down when the battery voltage falls below around 11.5 volts.
This is usually a high enough voltage to start the engine.

There are 12V fluorescent lights you can use that'll save a lot of
power. A fluorescent light takes less than 1/3 the power of an
incandescent.

12V rechargeable fluorescent camping lanterns are pretty good too.

Dave