How to drop the voltage of a power supply?
Your understanding of the setup is wrong,
A single resistor will not help, all that can achieve is current limiting which would be I = V/R = 21V / 3.9R = 5.4A @ 21V. This would not change the voltage in the slightest.
You would need to create a voltage divider to drop the voltage, to do this you would need something like a 3.9R (R1) &amp; ~6R (R2) to get around ~12.7V (Vout) with Vin @ 21V. This is not recommended as the battery will act as a load and will affect the division based on its internal resistance. See the image below,

To do this correctly you would need a regulator to step down the voltage from 21V to 14V, by the time you get ones of these you would probably be better off buying a Car Battery Charger. In the UK it looks like something dirt cheap costs around £20 and looks like this.