I would use a pre-made buck-boost converter, like this one I got from amazon:
or just search for buck boost converter -? there are multitudes of models, some with meters, etc.
You set its output for 12.0V (as measured by your DVM), and if the input voltage is higher, it regulates it down, and if the input voltage drops lower, it boosts it up to 12.0V.? Works very well to give a constant regulated voltage to the QMX.? And it is very small, so you can attach it to the housing of your battery.? Like any converter, it is not 100% efficient (varies between 85% and 95% depending on voltages), but that only reduces the runtime from your battery a small amount, while it removes any concern about the voltage level into your QMX.
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Stan KC7XE
