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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?

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  • \$\begingroup\$ First, define together. It can be seen that you wanted to mean in series, however, you should explicitly say it. Other than it, we need to know the power consumption data of the sensor to answer your question. \$\endgroup\$
    – muyustan
    Jun 13 '20 at 6:03
  • \$\begingroup\$ unfortunately the manual doesn't say anything about its power consumption, but I did notice it dropped about 4volts in an hour. Does that help? \$\endgroup\$ Jun 13 '20 at 6:06
  • \$\begingroup\$ the voltage drop characteristics of a battery due to the discharging changes from battery to battery, so, I don't think we can establish a direct relationship \$\endgroup\$
    – muyustan
    Jun 13 '20 at 6:08
  • \$\begingroup\$ Gotcha. Yeah, unfortunately the spec sheet only has "12V rechargeable gel cell" in the power section... \$\endgroup\$ Jun 13 '20 at 6:11
  • \$\begingroup\$ well, then, my recommendation would be testing the sensor with an adjustable DC power supply adjusted to 12 V and see what current flows through your sensor. Maybe you can make some recordings on fixed time intervals to average later. Then calculate a power consumption using that data. Then update this question. \$\endgroup\$
    – muyustan
    Jun 13 '20 at 6:15
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Two 6V 3.5Ah batteries connected in series is equivalent to a 12V 3.5Ah.

The available capacity of the gel battery and the voltage after discharge, at different discharge rates, would be as shown.

enter image description here

It follows that the load could have been in the region of 2.5A (assuming the batteries carried 100% charge to start with). This may be confirmed by measuring the sensor current using a 12V DC power supply or using the same batteries after a recharge.

Were that to be confirmed, a 12V 18Ah gel battery would be just right for this application.

In my opinion, a 12V DC power supply would be the way to go.

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