PV Gap

Charlie Said:

When using a marine battery, what solar panel size wattage to use to charge the battery?

We Answered:

at least a 5 watt

Dwight Said:

How many solar panels?

We Answered:

Go to Real Goods and find their worksheet. Need to figure the refrigerator will run almost constantly, the motor run time, and how many watts for the lights. Now figure for inefficiencies.
OK, all that said, I have 1750 watts of PV at 24 volts, 20 6-volt golf cart batteries, and a 2400 watt inverter. I have a propane refrigerator, and an electric chest freezer. I have enough battery for about 3 days, and enough PV to run dishwasher, washer, gas dryer, but not all at once.

Ruby Said:

How long will it take for a 12-volt, 60-watt solar panel to recharge a 12-volt marine battery?

We Answered:

Batteries are rated in units of Amp-hours.

A 12 Volt, 60 Watt panel makes 5 Amps of electricity, because Volts times Amps equals Watts.

So it depends on how many Amp-Hours capacity your battery has.

The Sears DieHard Marine Deep Cycle/RV Battery, Group Size 24M has a capacity of 80 Amp Hours. 80 divided by 5 is 16. That means that at 100% efficiently a 5 Amp 12 volt solar panel could charge a 12 Volt, 80 Amp-Hour battery in 16 hours. But it is never 100% efficient. You can usually count on using maybe 25% more electricity to charge a battery than you can get back out of it. So count on 20 hours. And that 60 Watt solar panel only makes 60 Watts maximum, when pointed directly at the sun on a clear day. If you have the panel mounted on a roof or fixed mounting that does not constantly turn to face the Sun like a sunflower does, it will make less than 60 Watts most of the time. So I expect it would take 2 or 3 days to charge up that battery from zero to full in a typical case. Use 2 or 3 panels in parallel to get 12 Volts and 10 or 15 amps to speed that up to one day. And as another answer said, use a charge controller between the solar panel and battery.

Lucy Said:

Solar Panel (70W), Deep Cycle Marine Battery, Inverter for my Conversion Van; Most Efficient Set Up?

We Answered:

Now, someone is coming right down my alley!

It all depends upon what all you intend to run with this
system as to how much of what you will need. You
have an excellent start with the solar panel, and two
batteries. However, do not think that marine batteries
are really a true deep cycle battery. They are not!

What the difference is with a marine battery, or even any
regular car battery is the amount of discharge it should
have allowed by use before recharge. To drop the
battery state of charge below the allowable percentage
will damage the battery so that it will not soon be able
to take the recharges. Most marine batteries are not
designed to drop below a 10% discharge before recharging.
Actually, car batteries are about the same. The only
difference between them is the difference between the
cranking amps versus the reserve capacity.

Reasonable batteries can be found where you can drop
about 25% charge before recharge, but most of those
you can get at places such as Farm and Fleet, and
similar companies, will not be listed with cold crank amps
at all, and a very low crank amps. They are designed
merely with a much higher reserve capacity.

For a true deep cycle battery you would have to get
an actual battery designed for solar recharge. These
batteries can be had in all sizes, and different capacities.
Their drawback is the price. It can run as high as $600 to
$800 each! (Not economically feasible for the casual user
of solar energy.) However, the life span of these batteries
being used for solar charge, and running an inverter system
for power is many years. I have seen some in use that
have been in operation for over 30 years now.

Another thing about batteries. You should get them all
about the same time, and all of the exact same type.
To get one, or two now, and another one or two to add
later is not good. Get what you need to take care of
your maximum use all withing a two to four week time
period.

Next it about your wire sizes.
The cables between the batteries, and the inverter must be
able to carry whatever load is demanded. The best way to
think on this is to use regular battery cables, and keep the
distance short. If there is a great distance (over just a very
few feet) you would have to get cables much larger. You can
have a remote switch to turn on the inverter, and that does
not require any special wire just to work that switch. It can
be regular lamp cord wire if you like. All it is doing is to
extend the switch on the inverter to another location away
from the inverter.

As for the wire from the solar panels you don't need any
wire larger than what is on them to begin with. It is almost
always a small wire about 14 ga or less. Many come
from the factory with 18 ga. The reason for this is that
you are only transferring a maximum wattage of the
solar panel. You use a whole lot more than that with a
toaster, and it doesn't use a wire larger than 14 ga at most.
Even a speaker wire is large enough to run a 100 watt light.

For the length of wire from the inverter to the loads you
demand from it you can use any normal wire that you
would use on those appliances. If you have four outlets
on the inverter, but need six circuits you would use the
size wire to the converter needed for the amount of load
of both the circuits you have split into from the one outlet
on the inverter. The size wire for each of the circuits
need not be any larger than the load on that circuit. In
other words, you can use a large wire to an outlet box
where you would plug in the other two circuits, but
regular wire that can handle the load to each separate
circuit.

As to your batteries for the volts, and current you should
use as series, parallel system wit more batteries. You can
stay at the 12 volts, but increase the available amps.
Many large systems use a 24 volt inverter to run the 110 AC
since it is easier to gain more amps with this system.

Now, it would seem that the one 70 watt panel would be
more than enough to handle what you might need in a
mobile unit such as a van. Unless you are going to live
outside a home where you would need a lot going each
day, two batteries can run a TV, and a bunch of lights
for several hours without running out of juice, and if you
are running during the day your system is being charged
with the sunlight while you are using the power from the
batteries to run your inverter system.

I don't know what model of panel you have, but the
efficiency of it could be one of the low ones, or high.
As for the charge controller, I don't know what you have
so I can't tell just how dependable it is for your need.
Of course, I don't even know your use needs to calculate
what you might need anyway.

The whole idea of anyone using solar is excellent! In
reality solar is the best resource.

Lynn Said:

Want to power this with solar panels?

We Answered:

You will find a much larger selection of system components to make a 12V solar system rather than a 6V solar system. So you might consider finding a switching regulator to drop 12VDC down to 5VDC, then use a diode to further drop it to about 4.4V. A switching regulator can be had with efficiencies in the 85% range. Avoid a linear regulator; its efficiency will be less than 5/12 = 42%.

Assume the following design criteria (adjust to your liking):
1. battery to be a deep-cycle (a.k.a marine battery)
2. battery discharging stops at 20% capacity
3. battery nominal voltage 12Vdc
4. run-time on battery 3 days between charging (due to cloudy days)
5. effective charge time 6hours/day at full output (this depends on your lattitude and season).
6. 12Vdc/5Vdc switching power supply with 85% efficiency
7. Load is a continuous 0.66A at 4.5V
8. Battery charging is 78% efficient
9. Solar panel operates at 17.1V output (maximum power point)

Calculations:
The needed battery capacity (to 20% charge) in ampere-hours is:
0.66A x 24h/day x 3 day x 5V/12V / .85 / .80 = 29.1AH

The number of kWh needed per charge cycle (6 hours) is:
29.1AH x 17.1V / .78 = 638 VAh ==> 0.638kWh

The average current required during the 6-hour charge cycle is:
29.1AH / 6H / .78 = 6.22A

The nominal solar panel size required is:
6.22A x 17.1V = 106W

Most solar panels today are constructed of strings of 36 individual cells. These panels require that a solar charger be used to regulate the charge current (to preclude overcharging the battery). A simpler system is to obtain a solar panel with 33 cells per string. This is considered to be a self-regulating panel that can be connected via a diode to the battery, without a charge controller. It is a less expensive system, but it won't charge as well as a 36 cell panel will on an overcast day.


As cheap and simple as possible...
Consider relaxing your design criteria. E.g., allow outages after 1 day. Power down the access point at night or during periods that it isn't used.

Good luck! Sounds fun.