I have several high capacity 3.7 volt Li-ion batteries. How might I convert them into a USB charger for my phone? I'm not interested in the most elegant solution, or if this would be less financially viable than simply buying one on eBay, I'm just curious what the easiest way to do it would be.



You could use a boost converter such as the TI TPS61090. I think Adafruit sells a board that uses this chip for your particular application. (Ok. here , which also includes a charger).

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If you were building many of them and wanted to shave the cost, there are other less expensive chips available on the market, but the TI one is a good one and easy to get, with all the required information easily available.

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  • \$\begingroup\$ The input voltage is connected to both pins of the C1 capacitor? \$\endgroup\$ – Bence Kaulics May 22 '15 at 19:49
  • \$\begingroup\$ @BattleHamster Yes, that's right. \$\endgroup\$ – Spehro Pefhany May 22 '15 at 19:59
  • \$\begingroup\$ So the input voltage is connected to the power ground? What is purpose of that? \$\endgroup\$ – Bence Kaulics May 22 '15 at 20:49
  • \$\begingroup\$ @BattleHamster It's a regulator- input voltage is power from a 3.7V battery (maybe fused). Input and output power have a common ground (power ground) which is connected to signal ground. Not sure what you're getting at here?? \$\endgroup\$ – Spehro Pefhany May 22 '15 at 22:28
  • \$\begingroup\$ I am confused about this. BTW sorry for bothering maybe I miss something obvious. \$\endgroup\$ – Bence Kaulics May 23 '15 at 9:15

Generally you must not charge with over 4.7V, do not charge past 4.2V on the battery, do not discharge past 20% capacity, and you must watch for overcurrent, undercurrent. These things can all be done with passives, but the amount of component and space for keepout space of each one can get quite large compared with IC battery chargers. Look up PMICs on a part suppliers website and you will get loaded with plenty of LiPo chargers. They can range from under a dollar for a TI part, to over five for an LTC piece. You just have to choose them according to your charging specs and choose their passives according to the charge current you want.

Look into the LTC4053. Very simple and you change the passives from PROG to change the charge current. It also utilizes trickle charging once your phone is full.

As for getting the required voltage out of your battery, you could certainly use a buck convertor. I would just recommend getting a higher voltage battery myself.

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    \$\begingroup\$ Unfortunately, this is an incorrect and even dangerous answer. I will not down-vote it immediately to negative, but consider changing it please. Just connecting two LiIon batteries in parallel can be dangerous because one can be 10% the other 90%, high currents will flow, possibly causing (explosive) cell destruction. Cells need to be balanced to each other before they go parallel (or in fact in series). Added to that, two cells in parallel are still only 3.7V. The second danger is applying a peak voltage (if you do put them in series) of 8.5V to your phone. It'll overheat at the least. \$\endgroup\$ – Asmyldof May 22 '15 at 19:17
  • \$\begingroup\$ Please read my whole suggestion and note I said how dangerous it is. It is literally the cheapest and least elegant solution, which is what was asked for. \$\endgroup\$ – mcmiln May 22 '15 at 22:02
  • \$\begingroup\$ You mean, the answer you edited it into between my comment and now? \$\endgroup\$ – Asmyldof May 23 '15 at 0:12
  • \$\begingroup\$ Yup. Changed it so all parties are happy. Friendship is key on the exchange. \$\endgroup\$ – mcmiln Jun 1 '15 at 0:18

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