How to detect low battery under load?
You decide what the "low" threshold voltage level when the battery has its load on it. This way you will know when the battery gets to critical level of capacity corresponding to what load it is expected to power. Trying to do this when the battery is unloaded will not give you a realistic in system usage scenario.

Think about your car. Its battery stays connected to the alternator / voltage regulator all the time -- unless you remove the cable to do service work.