What Battery Guardian is not
We would rather you know this before you buy than after.
It is not a four-wire lab measurement. Cell voltages come through your pack's own balance connector, which has its own contact resistance. A dedicated milliohm meter with Kelvin connections to each cell terminal is the more accurate instrument, and if absolute accuracy is what you need, that is what you should buy.
It measures at one load current, not across a range. The test draws roughly 870 mA. Internal resistance is not a single fixed property; it depends on the current you measure it at. This reading describes your pack at roughly 870 mA. It tracks your pack's condition over its life. It does not predict how the pack will behave at a flight current of tens of amps.
It is a bench and pre-flight tester, not a monitor. It does not watch your pack while it charges, it does not live in your plane, and it has no temperature sensor. You bring a pack to it and it runs a test on demand.
It cannot measure 5S or larger. That is a hardware limit, not a setting. The board has four measurement channels, and the largest balance socket takes a 4S plug.
It does not compute a true state of charge. The percentage is a straight line drawn between two voltage points, not a real discharge curve. It runs optimistic on every chemistry, and the gap is widest through the middle of the range, so do not use it to decide whether a pack has enough left in it. It is least meaningful on LiFe, whose voltage barely moves across most of its charge. The cell voltages themselves are measured; the percentage is an estimate drawn from them.
It has no reverse polarity protection, no fuse and no input clamp. Connect the main lead backwards and you will damage the board. Check your polarity.
Readings compare to each other, not to somebody else's meter. Test the pack when it is new, test it again every few months, and watch the change. That is the measurement that matters. A single number in isolation says less than you would like it to.