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hey!! You can use 3.7v to 5v converter. If you have many phone batteries you can parallel connect them and add a tp4056 lithium battery charger for charging of the batteries . that idea would make a powerful rechargeable power bank (depends of the capacity the battery carries)
well it does not affect the batteries because you add it parallely I made one that was to you but more batteries, i have added 6 cells and hack my power bank to draw more ampere without affecting the circuitry it was good love the capacity it was almost 10k mAh
are you a Filipino??? Can I know where did you buy 18650 batts from?? Brand new would be okay but used 18650 from laptop batteries is myfirst priority
You can buy at eBay, it so cheap $0.99 per piece and you can add tp4056 for charging and protection of the batteries