Oftentimes you can increase the Low Voltage Cutoff value on your controller. Its better for the controller to shut down than to let your BMS do it. If the pack's voltage sags below LVC, and the controller shuts you off, all you have to do is power cycle the display and you are back in business... just use low assist on the rest of the way (straight) home to try and get home under power. But if your BMS does the cutting off, the battery will stay off until it senses that its been connected to a charger. Which usually means you have to get home unpowered to fix the problem.
A cheap Chinese watt meter of any stripe on AliExpress is typically going to be off all by itself, in addition to your display being off. If you want a more accurate reading, buy a quality multimeter. No need to blow big money on, say, a Fluke. $35 will get you a decent Klein. $75 will get you one with True RMS for a slightly better answer. But really, once you check it one time and know how far the voltage is off, you never need to concern yourself with it again. Its not something you need to buy is what I'm getting at. A half-volt of error is typical and you can more than soak that up with an LVC change on your controller.
The fact that there are factors that can vary the battery voltage across dozens of cells and a half dozen cell groups in your pack is not something you can do anything about, and not really something you need to concern yourself with as an ordinary rider. The most likely thing to cause your pack to shut down on a routine basis is voltage sag. You hit the battery hard and the voltage sags low. Then you stop for a minute and you can see on your display the voltage level goes back up again. Sag it down far enough and it hits LVC. When you are nursing a low battery back home, you want to minimize sag by using less power, which may be uncomfortable, but not as uncomfortable as riding home with the power off.
