I have the Cable Techniques Hirose sled and get the same battery gauge indication on my MixPre. I checked the output voltage on the CT sled and it’s internal DC-DC converter outputs a constant 7.2 volts until the power source drops below ~9 volts, then it loses regulation and drops out. The MixPre battery gauge range in L-mount mode is full at 8.4volts to empty at 6.3volts, so it follows that 7.2 volts will stick the gauge in the middle. As we know, a 2S lithium-ion pack used in L-mount batteries has a nominal 8.2-8.4 max charge voltage, so this all makes sense. Cable Techniques was clever to choose the 7.2 volts, because that allows the battery supplying power to discharge a bit more if it’s able, giving additional run time. As jbell said, one must monitor the gauge on the battery pack itself to determine remaining charge. The MixPre has no way to know that information, as it only sees the steady voltage.
BTW, a USB power bank works with the USB-C input on the Mixpre 3/6 exactly the same way, except with a steady 5.0 volts.
Hope this is useful.
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