Measuring battery capacity isn't rocket science. We offer this one, which is basically the same as the other two versions for those who recognize what a case that's been CNC-machined of 6061-T6 aircraft aluminum brings to the game.
ProTip: connect a 2-wire JR-style lead for NiCd/NiMH packs (which typically don't have a balance connector for measuring individual cells) at the 2-pin port. With regard to lithium-type packs, e.g. LiPo, LiIon, LiFe, and super duty LiFePO4 (A123) because they have a balance connector, then making the connection to the tester with it gives you more than just total pack voltage. This is because you can measure the voltage of each cell in the pack at the individual pins of the connector. Arming themselves with this information (voltage of the individual cells) is the 'how' behind pros staying ahead of weak cells (for determining when to put a pack out to pasture).