18650 battery pack question

hello everybody, I need to install (build) a recharging battery pack for my plant shelf.  it will be running an arduino nano, just 3 leds (3mm 20ma each) 1xNTC and 1x water sensor probes (nails).--  no LDR no yellow LED.
I was thinking about getting a 2x 18650 battery holder - I need more battery hours, should we wire the batteries in parallel to get more mAh and boost them 3.7V to 5V. so basically this is what I am trying to custom build to get more hours; DIY Mobile Power Bank Case Kit
I would just buy the mentioned product BUT under specifications>battery it says the capacity range: 1000mAh and below. but the batteries I have are 3.7V @ 5000mAh. does this mean that the remaining 4000 will be lost because the system can't handle it? 
basically what I am asking for is how can I make a long lasting arduino nano powering 18650 battery pack BUT also with a recharging module. these are the items I found that may work together, please remember that I have no electrical experience from before. I have learned everything that I know from instructables, this is a too advanced build for me to figure out without asking a few questions. thank you for your patience, let me know if I have to clear anything out.
batt holder: 
am I even in the right track with this thing? can these be connected together and a switch. again, the goal is max battery life.. maybe add 4x 18650 - 2in parallel + 2in series, 7.4V to arduino Vin, eliminate the booster, even more battery life ?
thank you in advance,

love & peace

There's slightly more to it than that, unfortunately since you are using off the shelf
arduinos, and the USB bus is +5V. You need to change a regulator and a couple of other wires,
but then it will work straight off the battery. A simple USB charging
module should then be fine.

5000mAh will give you roughly 5000/30mA, or about a week of CONSTANT operation. The trick is to NOT be on constantly. Blink the leds very quickly, say 1/4 second every two, and your battery will last 8 X longer - or two months.

3.7V is enough to run an Arduino chip on, and the LEDs, no need to boost it, and lose energy there.

Akin Yildiz (author)  steveastrouk2 years ago

so you are saying I can input a single 18650 3.7V 5000mAh battery to my system as is through the Vin, it will come on and run everything, even the LEDs? I am going to try this, I would also need to purchase less batteries if this is true. I need just the green LED to blink every 5 seconds, the red or blue led comes on only if either sensor is triggered. then the green turns off, and the corresponding (blue/red) led turns on and stays on constantly. this will not happen so often, only if the plant gets thirsty or gets too cold/hot. so the battery usage should already be low. what about the recharging module tho? I need to be able to leave the batteries in while charging, instead of removing them to charge.. just a simple USB charging module. would those work together?


I have a modded Nano you can have if you need it. PM me,