18650 x 2 Battery Bank

The non-3D printed parts you need are:

1 x 5V Micro USB 1A Lithium Battery Charging Board [see note below]
1 x DC-DC Converter Step Up Boost Module 2-5V to 5V 1.2A [see note below]
1 x Rectangle On/Off Long Rocker Switch SPST
2 x Snap-In Single 'A'-'AA' Battery Contacts 209 [KEYSTONE ELECTRONICS CORP.]
2 x Snap-In Single 'A'-'AA' Battery Contacts 228 [KEYSTONE ELECTRONICS CORP.]
2 x 18650 Lithium ion Batteries [2600mAH x 2]
[Note: I got the Charging board, DC-DC Converter and two 18650 Battery for under $14.]
Application examples:
http://www.thingiverse.com/thing:1232619
http://www.thingiverse.com/thing:1235865

Lithium Battery Charging Board:

Inputs with MICRO USB female, can be directly input to do with the phone charger rechargeable lithium battery,
And still retains the input voltage wiring pads, can be very convenient DIY
Input voltage: 5V
Charging cut-off voltage: 4.2V ± 1%
Maximum charge current: 1000mA
Battery over-discharge protection voltage: 2.5V
Battery overcurrent protection current: 3A
Board size: 2.6 * 1.7CM

The first time you access the battery, it may no voltage between the output OUT + and OUT -, then access 5V voltage charge your energy can activate the protection circuit, and the battery from the B + B -, then draw a line on the short open also need recharged to activate the protection circuit. Note that the charger must be able to output 1A or more to do when using a mobile phone charger input, or may not charge + MICRO USB female and next - pads for the power input, access 5V. B + positive then the lithium battery, B- then negative lithium batteries. OUT + and OUT- connected to the load, such as positive and negative electrodes connected mobile booster plate or other load.
Connect the battery to the B + B-, inserted into the USB female mobile phone charger, the red light is being charged, the green light to full.

DC-DC Converter:

1. Input 2V~5V DC voltage arbitrarily, can stable output 5V DC voltage, the input with two AA batteries can output current of 1000mA, single lithium battery power supply can output current of 1200mA, for mobile phone, camera, single chip microcomputer, digital power supply products.
2. high conversion efficiency, up to 90%, on average about 85%, 3.7V up to 5V/1A typical application efficiency reached 87%, below measurement chart for your reference
3. with indicator light

Electrical specification:
The input voltage: 2.0V ~ 5.0V
No load output voltage: 5.1V +/-0.1V
Conversion efficiency up to 90%, average 85%

Maximum output current: (can work for a long time):
(Vin>=2.0V):600ma
(Vin>=2.5V):800ma
(Vin>=3.0V):1000ma
(Vin>=3.5V):1200ma

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https://youtu.be/J38f1Cxj9Sw
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Zalophus's DesignHouse: http://zalophus.tistory.com/​​ and https://zddh.blogspot.com/​​