DIY Power Bank without auto shutoff

I own a small data logger for recording accelerations. The data logger is a Slam Stick X with integrated 250mAh rechargeable battery. My problem is that the capacity of this integrated battery is far too small for my projects. I intend to use a battery pack with 40,000mAh.

Since the Slam Stick X has an USB port I could use a big power bank as one can use for mobile phones, tablets and so on. Unfortunately all of the power banks I found have an auto shutoff function when drawing too little electric current. When my Slam Stick X draws too little power (for example in standby mode waiting for a trigger event) the power banks automatically shut off.

MIDÉ, the manufacturer of the Slam Stick X, recommends the usage of the power banks from Voltaic Systems. They have an ‘Always On’-Mode implement that means that they don’t have a low current shutoff. See the following link for further information: Always-On mode by Voltaic Systems
But the biggest power bank they have just has a capacity of 19,200mAh. Not even half of the capacity I want to use.

My idea now is to build a power bank by myself. I think it is most practicable to use the 18650 batteries from Panasonic since they are not expensive and available everywhere. To reach the 40,000mAh I need to connect twelve 18650’s in parallel. A step up converter can increase the voltage to 5V so that I can power the Slam Stick X via its USB port.

I need help to put my little electrical project into practice.

• Is it safe to operate a Li-Io battery in ‘Always-On’-Mode or do I need to observe safety precautions?
• I have found out that a lot of Li-Io charger ICs exist. They have mostly overcharge and over-discharge protection implemented. Which one should I use that does not shut off when drawing low current?
• Maybe I would like to increase the capacity with more battery cells. Is this easily possible?

I would be very thankful if you could support me and give me some advice. Thank you very much.