I am using rechargeable Lithium LiFePO4 batteries instead of the old ones rechargeable NiMh. They have 3.2V instead of 1.5V and so when the old ones are used in series (most part of time as I know), I can replace for 2 of them for 1 LiFePO4 + 1 3D printed dummy battery :)The dummy batterie includes a hole to use a metal paper clip so energy flows through it.I designed this 3D models using the OpenSource FreeCAD (http://www.freecadweb.org/) and printed on BEETHEFIRST.