While USB Power Banks based on Lithium-Ion and Lithium-Polymer batteries offer a fairly large capacity for the weight and size, and are rechargeable, sometimes they take a long time to charge and you just can’t live with that. They might even be overkill for your application. Another cheap eBay “special” – a USB Emergency Charger that operates off two AA batteries for about $4.
The charger itself is a little larger than two AA batteries, and is finished in glossy black plastic. There is a red LED power indicator, a USB A Female connector, and an LED for the “torch” functionality, which, being frankly honest, is a bit pointless.
It’s a charger, how hard can it be to operate?
You can easily fit this with two Sanyo Eneloop batteries for some emergency juice on the go – and it does work. The problem is that the two AA’s hardly match a decent single 16850 – say if you have two Eneloops, that’s a total of 2000mAh at 2.4v, whereas a decent 18650 can provide 2400mAh at 3.6v. Energy wise, that’s only 55%.
So you can’t expect too much out of it – mainly just for powering small Arduino projects and the like. A Raspberry Pi? Probably not.
Inside, it’s a fairly small PCB with very few components:
There are two caps, nothing special, and the main switching inductor and diode on the top. The device actually outputs the battery voltage when the switch is off – and when it is on, the circuit “boosts” the voltage to 5v (roughly). This can be an issue if you plug something in before switching the switch to on, the device may malfunction. Also, the switch itself doesn’t seem like a very durable one – it’s got a bit of play in it and seems like the contact ratings might be a bit low for this application.
As the battery runs down, the voltage output on the USB port dips. This is visible through the dimming of the power LED – but many devices will continue to display “charging” while stagnating or sometimes, actually consuming charge from the battery. At that point, it’s futile to leave the device connected to the port …
Unfortunately, the ICs are not identifiable. Interestingly R1-R4 appear to be used to configure the resistance over the D- and D+ lines to be able to “trick” the device to believe it’s been attached to a charger – this is likely configured for Apple devices.
The device does work in general – and it’s inexpensive, that’s for sure. But the practicality of charging with AA batteries is really not there compared to the power banks – unless you really have no other option and need that AA-buy-it-anywhere convenience. But it does mean that there’s really little good reason to build your own from a kit, given the price on these things …