Hi,
I have not been able to find an answer to this basic question: Is it possible to charge a 12V battery using a 12V solar panel? I see some MPPT controllers[1] "stepping up" the input voltage. Not sure how. However, my question is, how is it done?
One possible way is to use an OpAmp as a Summing Amplifier. But, unless I'm mistaken, it would require another energy source, perhaps a smaller panel?
Basically, is it possible to charge as 12V battery using only a 12V solar panel?
Thanks in Advance.
[1] http://www.solarfuture.com.au/solar-panel-info-3/controller-info/
What are the actual specs for the Solar panel?
You need the V/I curve , as its common for 12V panels to actually be 18V panels, with no load.
In which case, you can charge a 12V battery.
A lot of the solar panels on the market sold as "12V" type are 36 Silicon pv cells in series, and can do Voc ("Voltage open_circuit") of up to 21V in full sun, about 18V in weak sun, about 13V in 1% sun (rather cloudy), and these can charge a 12V battery with nothing more than a plain silicon diode to prevent current leakage going backwards overnight. Like that you do need to watch for the battery going over 13.2V when you should disconnect it because it is full.
A 4.5Ah 12V burgler alarm battery charges from 1/4 to full in an afternoon from a "30W 12V" solar panel on a windowsill facing west.
A 60Ah scrap automotive battery can charge from that "12V" solar panel, but beware of battery internal leakage current, as I have some old ones which lose most of their charge overnight. In the best case it can charge over a fortnight storing a few Ah per day.
The first thing that you should do when you get your panel is to prod it with a multimeter to measure Voc in the best sun which you can find. Next look at the spec sheet for Isc ("Current short_circuit") in 1000W/m^2 and choose an ammeter setting which can take that much. Then prod the terminals again just to see.
For example a 60 cell panel Voc 37V Isc 7 Amps was ok to short by going straight through a multimeter on its 10Amp setting, and on that day in weak sun showed about 1.5Amps.
If you are there to disconnect it when full, then any "12V" panel with Voc > 18Volts in good sun can certainly charge it, and has some volts to spare to go over the 0.7V loss in a "solar charge controller" which automates the disconnection for you. If you can afford a "pwm mppt" solar controller, those are desirable as they can convert, by high-kHz pwm, about 18V 0.22 Amps to about 12V 0.3Amps. Nice to have, though not strictly necessary. Only the battery and solar photovoltaic panel are minimally necessary.
Note that arduino pwm at 0.9kHz could only buck down from about 18V to 12V with an exceedingly large inductance such as a car ignition coil or some other scrap coil.
As for stepping up a lower voltage to 12V, go and read up on "buck" and "boost" circuits containing inductors. As it is always possible to series connect more than one solar panel, you should not need to boost.