I've had good luck with my zapper... Depending on how many rats you kill, the batteries last a good while. It'll tell you when it's getting weak. I'm pretty sure that I killed 20 or so and it still had plenty of juice left. You just bait the thing with dry cat food and wait, you'll have a rat in there by the morning. Rats are very xenophobic and will check out the trap for a while before they go inside to get their final exit... But, if they are hungry one after another will get zapped. I can remember setting the thing up and I got a rat in 20 or so minutes (I was in my coop watching, VERY still). I would dump out the dead guy and reset it, 30 minutes later another fatty bit the dust... It'll kill mice all day long too... It took a little over a week, but I've had no rats since... Give it a try, I'm telling ya, it's no joke...
M.