These wattmeters are all over Ebay and Amazon for about £10. They can tell you a lot about how your bike works and everything about your battery if only you could see it while riding. You could thick battery wires up to the hand lebars and back, but not only is that ugly, but you also get some power loss. Here's how to fix all that for almost zero extra cost. All you need is some three-core wire, thinnish, but not too thin - IIRC 22g or thicker is about right; a small piece of vero-board or some