bit of camera tape with the number on, like Peut, but I have green tape one side (charged), red tape the other (depleted), so charged batteries are green side up, but when the batteries are used/need charging, they go back in the bag red tape up. This way I get to cycle through my batteries evenly, and if one fails or plays up I can keep an eye on it.
Speaking of batteries, does anyone know how to change the number? I use Airdata to get the TXT data for my Flight Log (post PfCO) and it gives me the battery number as a single digit, but they are all over the place, like one is #7, and I only have six!