I imagine it will, but the amount if power an led takes will make it hardly noticeable, I'm not entirely sure on size but I imagine it will be about the size of the button cell battery maybe a little bigger but thinner? I'll see, it is a pain changing button cells and they are consumable whereas I recharge AA's and use them again! I'm glad LEDs are working out for you, the 9v led pack looks good, how heavy is it? Are you planning on using a pack like that for carriage/train lighting? I bet they last a long time running off a pp3 battery, yeah I like this forum too, it is closer to the type of forums I already post on,