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I have lots of small (500mAh) LiPo cells taken from Vaping devices (eCigarettes). I want to reuse them in some consumer electronics devices that run off 2 AAA batteries. There is generally space inside the devices to add extra cells/circuit boards so I'd like to add as many cells in parallel as will fit and charging circuitry preferably with a USB port.

Should I stick to parallel cells only for simplicity? I understand that before connecting them it's best to get them all to the same voltage and bottom balancing is better than top. What's the best way to charge/discharge them to exactly 2.5 volts?

Is a TP4056 (with protection) the best board for integrated charging?

I understand the LiPo cells go from 4.2 to 2.5 volts and I need 3V so maybe I could use a buck converter or low-dropout regulator to power the device.

I'm not concerned about charging and powering the device at the same time (power sharing).

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    \$\begingroup\$ "What's the best way to charge/discharge them to exactly 2.5 volts?" That's not how they work - you can't choose the voltage you charge them to. Charge them in the right way to 100% and use regulators as needed if your device can't cope with the full voltage. \$\endgroup\$
    – Finbarr
    Aug 22, 2022 at 21:39
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    \$\begingroup\$ If you have a lot of these cells, you might be able to balance them by connecting each of them to a 100 ohm resistor to a common 3-4 VDC supply. Once they all read the same, you can safely connect them in parallel and then charge the pack with the proper charger. Then use a 3V or 3.3V buck regulator. \$\endgroup\$
    – PStechPaul
    Aug 22, 2022 at 22:04
  • \$\begingroup\$ You want a buck converter to convert the battery voltage down to 3V. If you only charge the batteries to 3V you're wasting most of their capacity! since space is limited and current is not that high, look for a really small one. \$\endgroup\$
    – user253751
    Aug 22, 2022 at 22:10
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    \$\begingroup\$ Discharging them fully will shorten their life and is not necessary to get them balanced. You don't need to get them all to a specific voltage, you need to get them to the SAME voltage. Connect them together with suitable resistors and leave them for a day and they will equalise. \$\endgroup\$
    – Finbarr
    Aug 23, 2022 at 9:55
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    \$\begingroup\$ Yes, one resistor in series with each cell and you can connect as many together in parallel as you like. Anything from about 22 Ohms upwards will do. \$\endgroup\$
    – Finbarr
    Aug 23, 2022 at 12:14

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What's the best way to charge/discharge them to exactly 2.5 volts?

Use an adjustable voltage stabilized supply with an adjustable current limit preferably with a voltmeter and ammeter incorporated. There are some useful boards on the "market".

Is a TP4056 (with protection) the best board for integrated charging?

Use one with integrated switch for battery protection.

You could use something like this for powering your device (Vin > 4.5V).
Just choose a model with the right input voltage.
But see if a lowest "current limit" protection is needed.

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