I'm very new to electronics, and I have a sensor which is being powered by 2 batteries in series, each 6V/3.5Ah. I believe this means the total Watt Hours is 6x3.5x2 = 42Wh. When the sensor started, it detected the batteries had 12V initially, but after an hour had decreased to 8V. It no longer functions properly once it hits 8V. However, I need it to run, uninterrupted, for 6 hours, with enough voltage.
What kind of external battery should I look for to facilitate that?