I wired the heater circuitry into the node's solar charging harness and mounted the thermal sensor on the long narrow side of the battery.
Both the sensor and heater pcb are held in place tight with a small tie wrap around the battery. Not the one shown.
Then I wrapped this assembly in two layers of thin foam for insulation.
The battery assembly sits atop a 1/4" foam pad to further insulate it from the case.
I also updated the node's firmware to the latest stable release while I had it in the shack.
The white connectors shown attach to the RAK 4631 mesh node circuit pcb.
Bottom to solar jack and battery's to the battery jack.
I'll get pictures next time it's in the shack.
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Then I put the node back on the tower to wait for a sunny, freezing day.
Today (12/20) was a perfect test with temps in the single digits and bright sun part of the day.
The battery was down to about 60% from a stretch of cloudy days.
The simple circuit warms the battery as long as the temperature sensor is open (<32F), else all the current goes to charge the battery.
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Meshtastic has some nice built in metrics for visualizing it's power status over time.
In it you can plainly see when the battery is being warmed (low half of the "square wave") and charging when the voltage is higher.
It also gives a good idea of the duty cycle which tells me the heating is gentle.
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While this is only day one of testing, the results are very promising as a solution to the LiPo battery cold weather problem in solar powered remote nodes.
The first two hours were bright sun then filtered through clouds the rest of the day.?
Interestingly, it was still charging, tho not as fast.
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73
Mike N0QBH
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N0QBH Mike Berg