In order to charge a cellphone (or any battery), you need to provide a flow of charged particles (current) with enough driving force (voltage). A wall socket or USB port provides a steady current at a standard voltage but any current with enough voltage will charge your phone.

Certain fruits and vegetables can be a source of charged particles, particularly squishy ones with lots of minerals, salts or acids. Citrus fruits like lemons or grapefruit work best, but you can use potatoes too. Watermelons could work but don’t have as many minerals/salts/acids as a lemon, despite being a bigger fruit.

Charged particles on their own are not enough – which is why your phone doesn’t charge if you plug it directly into a lemon. There’s no driving force (voltage) for the charged particles to flow into and out of phone. One way to apply voltage is to use two different metals to connect your phone to the fruit. For example, the difference in energy between copper and zinc will create a voltage of ~1.1V, a little less than a AA battery. When stuck into a fruit, the charged particles move away from the zinc and toward the copper, which causes electrons from the copper to pass through your phone to the zinc. This flow of electrons (current) has enough driving force to charge your phone.

Any YouTube video that claims you can charge your phone using only a cold watermelon is a fake, but fruit-based batteries are real. The secret isn't the choice of fruit – it’s the choice of metals you use to make the connection.

For more information, check out this video:
watch here

If you’d like to try making fruit batteries at home, check out this website:
click here

I have to answer this in two parts: the watermelon battery itself and some practical concerns.

The first part is about the watermelon itself. Yes, it is possible to draw electrical power through a watermelon by turning it into a battery. To make a battery, we need to put two metal electrodes inside an electrolyte (battery acid), then connect the electrodes by a conductive wire. For example, one of the electrodes can be zinc (like on a galvanized nail) and the other can be copper (like a penny). The watermelon provides the electrolyte, watermelon juice.

The electrical power that this battery generates does not actually come from the watermelon. Instead, it comes from a pair of chemical reactions that happen when all of the battery parts are connected, called reduction and oxidation (or together, redox, for short). In our example, the oxidation reaction pulls two electrons off of a zinc atom, causing zinc ions to dissolve in the watermelon juice. The reduction reaction takes the electrons and combines them with two hydrogen ions from the juice to make hydrogen gas. Together, these two reactions move electrons from the zinc electrode through the wire to the copper electrode, making electricity.

Now for the second part, the practical concerns. You might not be surprised to hear that commercial batteries are much more powerful than watermelon batteries. They use better combinations of electrodes and electrolyte to generate much more electricity. To get enough power for your iPhone to even detect, you would probably need a few watermelon batteries linked together in series. Finally, there is the question of what the iPhone itself will do when it detects the watermelon battery. If the power source is not a good match, the phone might be able to block it to protect itself.