Hi there

I have built a LoRaWAN node with a Whisper Node Lora Rev 1.0. Everything is working perfectly when it runs on USB power, but I'm having problems switching to the single AA battery. Every time my node wants to sent data to TTN, it obviously reboots itself and I see a activation in the TTN console. I am only sending two bytes of payload, and the AA battery is almost new and never used. It is connected to the VBAT and GND pins on the header.

Is a single AA battery not enough for a LoRaWAN transmission?

best regards,

Hi milhouse83,

It should handle the TX current with a single AA cell, specially if the TX is just a single short burst. But that would be just in the "edge" and you wouldn't be able to use much of your battery. After some usage, the battery voltage would drop, for example, to 1.3V and it might not work anymore...

The step-up IC MCP16251 delivers up to 130mA when VIN is 1.5V and VOUT is 3.3V, while the maximum TX current for the RFM95 is 120mA.

Saying that other things might affect the step-up performance. Make sure things are tested at ambient temperature. Additionally, the connection between the battery and the Whisper Node must be done with a very short cable capable of delivering the required input current without voltage drop.

LoRa transmission rate would also affect the power consumption. The slower the transmission speed, the longer the RFM95 will spend draining the battery. If you have an oscilloscope I suggest monitoring the battery voltage (at the Whisper Node VBAT, not at the battery) during the TX and see if there's any significant drop.

In any case, such configuration is just about on the "edge" and any voltage drop will indeed make the step-up shutdown. Every time you drain current from a battery, the voltage will drop. The size of the drop will depend on the battery state and capability.

There are a few things you might wish to try so you could step back from the "edge" a bit:

- Increase the TX speed or reduce the TX power. This should reduce the demand for high-power to a short burst only, which the capacitors would assist.
- Reduce the MCU frequency (this might give you only a few mAs)
- Add a larger Input capacitor, directly to the Whisper Node VBAT
- Add a larger output capacitor in the Whisper Node 3V3 Rail
- If you have size constraints and the options above do not help much, you can still try a different battery like Lithium AA or CR123A (non-recheargeable)

Regards,

Hello

thank you very much for the very extensive answer! After reading all of it, I came to the conclusion that even though using a single AA battery could be possible, for my usecase it's the easiest to just use two alkaline cells. I connected two in serial and now everything is working fine. I will still implement some of the hard- and software measures you mentioned.

Thanks again and best regards from Switzerland.