Friday, February 12, 2016

Battery node

New node boards have arrived. I've ordered first time from seeed, and they look quite good. Nice sharp traces, I use usually bigger ones, as I tend to hand solder sometimes. Nicely aligned solder mask, and silkscreen without smudges. I will use them in future for sure.
This board was also for me first board to be made in KiCad, instead of Eagle, as I needed bigger PCB area for new gateway(main board). I can just say all as expected, if you check your Gerbers, there should not be any surprise. I feel a bit more confident to spend $50 for the big boards.

So, speaking of hardware, I soldered the battery option node. There is place on PCB for either battery or DC or even both if you want dual power (3.3V and 5V). And it works! Happy man :). To put it in numbers:

Battery voltage is more then 3.4V:
ATMega328P, and HW69W both sleeping:  0.09mA
ATMega328P working , and HW69W sleeping:  7.2mA
ATMega328P working , and HW69W receiving: 8.5mA 
ATMega328P working , and HW69W sending: 61mA

Battery voltage is less then 3.4V:
ATMega328P, and HW69W both sleeping:  0.12mA
ATMega328P working , and HW69W sleeping:  14mA
ATMega328P working , and HW69W receiving: 17mA 
ATMega328P working , and HW69W sending: 123mA

I tend to use it with li-po batteries, in theory they have self discharge ranges about 5% in month, then 1–2% per month (plus 3% for safety circuit). By raff numbers, and using the scenario of a remote sensor sleeping for 5-10 minutes, reading the sensor and doing the math for 500ms. Finally transmitting for at full power for 10ms, it gives me battery life for about 2 years with 2000mAh battery.

Now the question here is where it is a practical and economical solution to buy a li-po and let it basically drain by self discharge. Eneloops NiMH have seems to be more suitable, but li-po's are getting cheaper on eBay or such. Another solution is to go for a small li-po, around 200mAh and tiny solar panel. This or another, I'm happy with its wide input voltage and consumption. With this setup I can go from AA, to li-po or NiMH.

Going back to design, it uses MCP1252, which is inductorless, buck/boost, DC/DC converter with low quiescent power: 80 μA (Typical), and giving 120 mA (150mA max, high power RFM69HW can be used) output current. Add couple of ceramic capacitors, and you are set. On picture capacitors a a bit bigger then pads, since I have put what I found in drawer, and not the proper 1206.