While Grocery shopping I found a battery for my PICO $7

1800 mAH
Has two USB ports, the micro is for charging the regular powers my PICO.
Here's a link on Amazon

https://www.amazon.com/Xtreme-Universal-1800mAh-Power-Keychain/dp/B00JJ9PE2C

Unremarkable specs for price. I got one like that for five at microcenter.

One really does wonder how these mfgs stay in business!

I've got one exactly like that here (it cost around $3, delivered): http://www.ebay.co.uk/itm/262524282820

It even featured in some videos :) https://www.youtube.com/watch?v=HKEHXOSLzCQ

However, having said that, both of mine have now died because I let them go flat. Yours might be better made, but mine had no protection circuitry on the battery so when it went flat (because the power supply seems to run all the time) it discharged the battery too much and basically self-destructed :(

It's easy enough to get inside them and check though - mine are a few years old now so I'd imagine something would have changed.

The package claims it is pre-charged, but I had to charge it.
A Red LED lights when charging and a Blue LED lights when it is charged.
So it sounds like the internal circuit does discharge the battery on the shelf.
Will investigate further.
Nice to know it can be found at a lower price if one is willing to wait, but also nice to have a local fast source.

Sounds just like mine (with the red/blue lights). You can ping the top off pretty easily and look inside - but if you find there is no battery protection circuit I'd be sure to keep it always charged - when mine discharged itself the battery actually leaked liquid everywhere!

Otherwise they're great little devices. The power supply doesn't shut itself off when very little power is drawn - so it can keep an Espruino Pico running just fine.

Some other battery packs I've seen don't detect the Pico as there isn't enough power draw and shut off :)

I have similar one ;) My impression is that because of 5v stabilisation circuit (or kind of DC/DC converter) it's draining energy faster. I'll try to disassemble and bypass stabilisation circuit

Those devices have a LiPo battery (nominal voltage 3.7v, 4.2 at full charge), a charging circuit, and a dc-dc boost converter to get 5v out.

You can probably (with some care) remove the DC-DC converter for the output. The catch is - you don't want to bypass the UVLO, otherwise you could overdischarge the batteries.

Amount of 'Loss/Inefficiency' - measured in 'Efficiency' - depends on how many LiPo cells such power bank includes, and also how much current is drawn.

If it is a two cell, then a buck / step-down-converter, such as ti's TPS562210 is an example...

If it is a single cell, and you need only 3.3 volts, you may get away with a low-drop out voltage (LDO) voltage regulator that you connect AFTER the cell protection circuitry. Do NOT remove the cell protection circuitry, because it prevents silent dead by over-discharge (towards and below 3V), and violent dead with possible collateral damage by over-load and over-charge: fire and possible explosion. Your 'heavy' load you connect directly to the protection circuitry output.

If it has one cell and you need 5Volts, ti's TPS61256 is an example for a boost converter. And same story here: efficiency depends on on load as well.

Also with a multi-cell you can connect right after the protection circuitry and before the converter... Most protection circuitries include protection from both and facilitate in-circuit charging for easy use.

Efficiency is comparable, and quite current consumption is quite low... for buck step-down and boost step-up I attached (in same sequence) example efficiency graphs... For best efficiency, the circuitry is tailored to max and min/nominal input voltage, output voltage and output current drawn.

You can even buy just the power bank case with built-in circuitry and supply yourself the readily available and price worthy standard 18650 cells.

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