Projects: Smart Watch – Power

So, I spent a LONG time looking around the internet for suitable batteries. They had to be small and have decent capacity.

I decided 150mAh was the minimum. And in the end I settled on these little 3.7v Lithium-Polymer cells with 250mAh.

250mAh LiPo Battery

Since these are only 3.7v each I either have to put two in series OR use a voltage booster to achieve the 5.5-12V range.

A nice little voltage booster like this should do.

The batteries haven’t actually arrived yet – I ordered them from China so it could be a week or two still before they turn up. There’s no point buying a voltage booster until then.

But I actually had very little idea how much my circuit used. So I tested it using my ammeter and as it turns out, the Bluetooth module uses 5mA when connected and idle, ~20mA when connected and transceiving, and ~35mA when unconnected.

5mA is acceptable and so I will do my best to keep data transmission to a minimum in order to minimize power usage.

I was pretty shocked to find that the arduino uses around 30mA when running even the simplest of loops. And so I spent a couple of days sorting this out.

I found that if I put the arduino into “sleep mode” the power consumption dropped to about 0.5mA which is much more up my street. Luckily, the serial pin on the arduino could be used as an interrupt to wake the module up from sleep, so all I have to do is send some junk data to the arduino so it wakes up in time to catch the real data.

However, in the periods when the arduino was awake, ie when flashing LEDs, transmitting data etc it still used about 30mA which is pretty unacceptable.

By reducing the clock frequency from 16MHz to 1MHz, this can be reduced to around 16mA which is just about acceptable as hopefully it will be asleep most of the time. I daren’t reduce it any more.

So, as it turns out there are approximately 3 general states the watch can be in:

Completely idol using approximately 5mA

Idol but with the bluetooth module awake using approximately 16mA

and fully going for it at 32mA.

Overall I estimate that the average usage will be around 8mA which should give a battery life of well over 20 hours. Yay.

On top of this will be the LEDs, but by using largish resistors I hope to keep the current down to 2-4mA each and they shouldn’t be on too long anyway.


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