A few weeks ago the box on the hill stopped transmitting. A walk up the hill revealed a slight amount of water ingress to the box (but not touching electronics), a healthy solar panel output, but a dead microcontroller.

Readings on the solar charge controller were:

• set pin (mppt knee voltage) = 2.67V
• (NOT)charge = 3.52V
• (NOT)fault = 3.52V
• VBat = 3.52V
• Load = 3.52V
• Vin(microcontroller) = 3.52V
• Votlage at solar panel (open circuit) 19.2V

The battery bank is a 2x18650 LiIon cell, so shouldn't be down this low.

The (not)Charge pin is pulled up to VBat by 10k resistor, and pulled low by the LT3652. Quote datasheet: "After C/10 charge termination or, if the internal timer is used for termination and charge current is less than C/10, the CHRG pin remains high-impedance.

So the charge was deemed to be terminated by current or internal timer.

(not)Fault is also pulled high, and was reading high.

" If no fault conditions exist, the FAULT pin remains high-impedance"

i.e. no fault.

After disconnecting the solar panel and battery, (not)Charge was pulled low:

" During a battery charging cycle, if required charge current is greater than 1/10 of the programmed maximum current (C/10), CHRG is pulled low."

We have disabled the timer based charge cessation.

What appears to have happened is that C/10 current rate has been met which put the chip into charge complete mode. It then failed to wake again when the battery float voltage drops by 2.5%:

"VFB (Pin 7): Battery Float Voltage Feedback Reference. The charge function operates to achieve a fi nal fl oat voltage of 3.3V on this pin. Output battery fl oat voltage (VBAT(FLT)) is programmed using a resistor divider. VBAT(FLT) can be programmed up to 14.4V. The auto-restart feature initiates a new charging cycle when the voltage at the VFB pin falls 2.5% below the fl oat voltage reference"

I will check this pin next time.

Anyway, after unplugging and reconnecting everything, the battery appeared to be accepting charge, but the microcontroller was (I assume) putting itself into sleep mode as the voltage was too low. I vowed to return with a replacement battery as charging would take a long time with winter light levels.

I forgot about it for several weeks, and then whilst exploring the spectrum with an RTL-SDR, happened across the LoRa transmissions - the unit had come back alive all by itself!