Using Rechargeable Coin Cell Battery. Wifi operation only when battery is charging

Hardware
1

Hi,
I am working on an IoT product and want to keep the form factor of the product as small as possible. We are thinking of using a coin cell rechargeable battery like the one shown below.

http://www.mouser.com/ProductDetail/Panasonic-Battery/VL-3032-GUFN/?qs=sGAEpiMZZMuXcNZ31nzYhaSsneKiWkB%2FFdJFShkFwL0%3D

Since this battery will not be able to supply the current needed for a Wifi operation, we were thinking of building the device such that all data is saved on the chip and the upload operation occurs when the device is plugged in for charge. I wanted to bounce this idea off the community here.

The device will be in sleep mode most of the time and will wake up to do a weight measurement once every hour. Any thoughts on this approach would be welcome.

Thanks

2

The problem with this is that if a device is unplugged from charge during a connection then the connection will fail; it’d likely be better if you just picked a slightly bigger battery (a pouch Li-Ion cell which can deal with 250mA discharge doesn’t have to be very big).

Alternatively you can get pretty small primary cells which will run an imp fine, eg a CR2 lithium.

3

Thanks. I do not know a lot about rechargeable batteries and have started looking into them. The app notes on battery powered applications provided in the dev center are a great help.

I am looking into pouch Li-ion and have found some on hobby sites like the one below. However, I am not able to find any batteries from a source like digikey etc. Any pointers for online battery sources.

4

Generally, I’d recommend that you ALWAYS get a cell with a protection circuit attached, as shorting out a plain pouch can be… interesting.

eg: https://www.sparkfun.com/products/731

Batteries are interesting to get from suppliers. Generally you need to get them from a specialist vs eg digikey. You can try Aliexpress for example, depending on how many you need, but always get one with a protection circuit attached.

5

If Li-ion cells are a bit of a scary prospect perhaps a supercapacitor in parallel with a coin cell could reduce the effective source impedance for small bursts?

6

The problem with that is that it doesn’t help much when there’s a longer burst (eg an OS update). Supercaps - or even just big caps - are very good to improve transient handling for most batteries though, generally at least 220uF across the battery is a good idea. We’ve seen this make 20%+ difference on LiMnO2 cells like the CR123.