Unfortunately, it will be impossible for a 6V solar panel to charge a 12V battery. So, don't bother trying this thing. After all, a 12V battery needs a solar panel with a wattage of at least 5 watts.
To charge a 12V, 100Ah battery, you actually only need one 175-watt solar panel. Keep in mind the battery's depth of discharge when calculating the solar panel size. Related article: How Many Solar Panels To Run A Fridge?
There is no danger in trying to charge a 12v battery with a 6v charger. There is not enough electricity involved to fill the 12v battery. The first lesson is that smaller voltage-rated chargers do not provide enough energy to charge larger voltage-rated batteries. So, for example, you cannot use a six-volt charger to charge a twelve-volt battery.
You can charge a six-volt battery directly without a solar regulator, but you do so at significant risk. A solar regulator on the cheaper end is around $50. However, the regulator's cost is minimal if you use the solar panel to charge the battery over many years.
The solar panel will provide a little over 9 volts at its peak. Given that a six-volt battery is 100 percent charged at around seven volts, the pairing of the panel to a battery works when both are six volts. While that sounds good news, it is not always a good fit. Are we talking in circles? Nope, and here's why.
With a simple PWM 6 volt charge controller (or a direct connected panel), charging a 6 volt battery will reduce the panel's "wattage" by about 1/2... For example (just guessing on the numbers). Assume 45 watt "12 volt panel" (actually, Vmp is ~17 volts for the typical panel). So:
1. Assemble your Parts — You will need a 6v solar panel, a 6v battery charger, a solar regulator — PWT or MPPT, a voltage meter with DC setting, tools such as screwdrivers or pliers, and a cap or electrical tape to seal the connections. Sometimes all of these pieces will come with snap clips.
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