The simple answer is no, a 6V solar panel cannot directly charge a 12V battery. There are two main reasons for this, which I have discussed below, followed by some alternative solutions.
This might sound weird, but both are correct and useful: Nominal 12V voltage is designed based on battery classification. With solar panels, we can charge batteries, and batteries usually have 12V, 24V, or 48V input and output voltage. It is the job of the charge controller to produce a 12V DC current that charges the battery.
Both regulators will help the solar panel charge your six-volt battery and do that safely. Another consideration for charging batteries with a solar panel is a battery backup bank. While charging a single battery, you can also charge a battery bank. The energy in the bank will allow you to charge your devices when the solar panel is inactive.
However, when the solar panel is connected to the circuit, the voltage is a consistent 4.485V and current is drawn from the battery since the charger is "off" (circuit will only "turn on" with >=6.5 V power supply). Even when we connect another 6V @ 9W solar panel, we still measure 4.485V.
Namely, we have to come to terms with the fact that there are several different voltages we are using for solar panels (don't worry, all of these make sense, we'll explain it). These solar panel voltages include: Nominal Voltage. This is your typical voltage we put on solar panels; ranging from 12V, 20V, 24V, and 32V solar panels.
36-Cell Solar Panel Output Voltage = 36 × 0.58V = 20.88V What is especially confusing, however, is that this 36-cell solar panel will usually have a nominal voltage rating of 12V. Despite the output voltage being 18.56 volts, we still consider this a 12-volt solar panel.
With solar panels, we can charge batteries, and batteries usually have 12V, 24V, or 48V input and output voltage. It is the job of the charge controller to produce a 12V DC current that charges the battery. Open circuit 20.88V voltage is the voltage that comes directly from the 36-cell solar panel.