BeWi - Ultra-low power wireless Beacon sensors

I removed LDO from my NodeUSB (ESP8266) board, and use LiFePO4 battery to power it directly.

700 mAh able to run about 6 hours (345 minutes):

2016-07-08 13:04:30.324560

2016-07-08 18:50:05.973346

Then, I charged LiFePO4 battery (3.3V LDO directly) and ran another test:

2016-07-08 23:40:50

2016-07-09 01:15:34

That is 95 minutes, so about 27% of capacity, 190 mAh.

Second test is much batter (I charged longer this time, for 3 hours):

That is about 47% of the capacity.

If use 3.5V LDO to charge LiFePO4 battery, it can reach 95% of capacity. I will test it next week.

Today I started testing LiFePO4 battery. I had the idea of using LiFePO4 battery for few months. Now I just received the shipment early this week.

Few benefits use LiFePO4 battery:

1. Can use LiFePO4 battery directly power MCU, without LDO or buck converter. LiFePO4 battery discharge at 3.2V. This is huge for ultra-low power applications that average 3-15 uA. Even the really good LDO still consume 2-5 uA when MCU is sleep.
2. Better high temperature performance.
3. Long life, 2,000 charge/discharge cycles
4. Large overcharge tolerance and safer performance
   
5. Simplified battery management system and battery charger

Here is the schematic of BeWi based on Atmega168 and NRF24l01+:

1. Atmega168 read sensor data
   
2. Atmega168 use SPI to talk to NRF24l01+ and send 2.4G BLE beacon packets
3. Atmega168 control AO6604 to enable/disable battery charging.
4. Atmega168 read battery voltage and decide when to charge, keep battery voltage low can greatly extend battery life
5. HT7333 provide power to Atmega168 and NRF24l01+
6. C2 is 0.22F super capacitor, depends on the setup, it can power BeWi alone with small solar module

After testing different chips/modules for WiFi, BLE beacons.

I am leaning toward to use NRF24l01+ send and listen BLE beacon frames.

The reasons:

1. TI does not provide API to use CC2540/CC2541 in sniffer mode
2. To write software on CC2540/CC2541, you need IAR, which cost $4,000 USD!
3. ESP8266 chip use too much power, it is take too long to boot and transmit the packet, the best I can do is 200ms.

Use another MCU (Arduino) with NRF24l01+ , anything can develop the firmware fairly easy. Cost will be low.