You have not given data to what leds you are using, but on an average for a basic led a resistor of 560 ohm to 1K would do for a 12v supply, it only needs to be 1/4 watt but anything bigger will work to.
The calculation is.............
12 volt - led voltage / led current (amps)
Example........
12 - 2.2 = 9.8v
9.8 / 0.020 = 490 ohm resistor
So a 560 ohm would be safe for this.
But as the charging voltage of the battery may get as high as 14 volt than the calculation should be done for 14 volt
14 - 2.2 = 11.8
11.8 / 0.02 = 590 ohm so as you see 560 ohm is a bit low.