This is applicable to any project that's looking at doing truly low idle power consumption.

Note that excessive optimisation is pointless on some battery technologies - the internal self discharge may be orders of magnitude more important on lifetime.

For example, a standard 2600 mAH 18650 lithium ion cell at 20 degrees may lose e.g. 17% over a year.

That's 442 mAh per year, or an equivalent current of 50.5 microamps.

Optimising a circuit to draw 0.1 uA instead of 0.5 uA is not going to be very productive if used in such a system.

However, there are certainly applications for low current draw, and it is interesting to see how low power one can go.

For power sources with a higher than needed voltage:

e.g. Lead acid batteries, 9V batteries.

Some form of regulation is needed.

The simplest way is a 3-pin voltage regulator, traditionally the 7805, though a huge range exist.

My Seeeduino has an ams1117, for instance.

The important bits to look out for in this application are:

Quiescent current, and minimum load current.

In the case of the AMS1117, the quiescent current is typically 5 mA. This is unacceptable - we're targeting <1 uA. The minimum load current is the same - 5 mA typically.

In general, the lower total power from a regulator, the lower quiescent current.

Since the scales are mostly asleep, but woken up occasionally, the load when active should also be handled well. Unfortunately, this is 3-4 orders of magnitude more current than idle, and most regulators struggle to have good specs at the low end.

I've not got one suitable - yet!

There's also a very simple regulation - a zener diode.

The reverse voltage through a zener diode is quite predictable, and can be e.g 4 V (to drop froma 9V battery).

Again, there are problems when going from 5 mA to 1 micoamp - there will be a sizable change in voltage.

A voltage that changes (a lot) under load will cause problems in a sensitive system, e.g. an ADC.

I've not got one suitable lying around.

The main benefit is that there's no quiescent current - though there is leakage (the voltage increases at light loads)

There's not much to do with cells at 1.5V or below.

e.g. AA/AAA alkaline, NiMH

They can be boosted by a boost converter - but as an active system the quiescent current is large.

One can if clever use an AVR to operate it's own boost converter - sprite_tm has done this with style: http://spritesmods.com/?art=ucboost. However it needs a button press to start.

This leaves cells or batteries with a voltage the MCU can run off directly

E.g. 2/3 AA/AAA cells, li-ion, li-poly, CR2032 cell...

For these, it is mostly easy.

Simply check that at all voltages they will operate at, all the peripherals work (e.g. my display module), and plug in directly.

As a plus you can indirectly measure the voltage using your AVR chip itself.

e.g. https://wp.josh.com/2014/11/06/battery-fuel-guage-with-zero-parts-and-zero-pins-on-avr/

There is one concern here though - and that is rechargeable lithium ion batteries.

As a rule, you do not want to discharge below 2.5V. You may want to limit at 3.2-3.5 V typically.

*Note the battery voltage will drop a lot at below freezing temperatures.

To this end there are protection circuits that sit on the battery. A common one is the DW01A chip, which switches a separate mosfet to protect from under or overvoltage.

(see http://www.ebay.co.uk/itm/5pcs-TP4056-with-Battery-protection-LIPO-Charger-Module-Board-Micro-USB-TE420-/301905672102?hash=item464afaf7a6:g:YecAAOSwneRXQqwy for an example of a charger with a protection circuit built in)

This has a typical quiescent current of 3 uA. In the example the TP4056 charger chip is also connected, with another 3 uA of quiescent current.

A lot of the lithium cells will come without surch protection circuits, and this is good for quiescent current. However, because I do want to be able to recharge my battery without damaging it, I'd tie in li-ion usage with the AVR measuring it's own voltage. Since the AVR is measuring it - it can warn of a low battery with the display, well in advance of a charge being needed.

Overall for the scales I've decided on an unprotected Li-Ion cell I had lying around. Seems to be around 800 mAh, so lifetime in the years, depending on usage. An old phone battery would be suitable.