The easiest way I can see to do this is to add a checkbox to the item they have to pay for, something like "Check this box to get the AC charger FREE" or whatever it is. In the description, I'd include a link to the product they will get free. In the description of the product they will get free, I'd put a comment like "get this FREE when you buy the flashlight it charges!" and a link to the product you charge for. Not particularly elegant, but it will work, everyone is happy to check a box to get something free (plus it's a reminder to you to include the free item when you print the invoice or packing list), and no one has to remember a coupon code.
If you really want to go the coupon route, you could set up a coupon for the amount of the free product, but I'm not sure you can require purchase of 2 products for the coupon to be valid (and I'm too foggyheaded right now to go check it out).