Motorhead toys batteries.

I own these two Motorhead toys


http://www.lovehoney.co.uk/product.cfm?p=33076

http://www.lovehoney.co.uk/product.cfm?p=33073

I've used energyizer batteries and they have lasted about a month. They have been used a lot in that time and obviously the Motorhead toys have very powerful vibrations. Just wondered what brand of batteries people use and how long do they last for?