I've bought a cheap phone (5$) to be a WiFi hotspot and nothing else. Now, the battery is dead. It's a Li-Ion unit, 3.7V nominal (Sony BA900).

Since I do not want to spend any more money I was thinking to crack the phone open, solder some wires on and use a 18650 cell instead of the phone battery.

What worries me is the charging circuit. Does the circuit have current sensing and current limiting to charge a 18650 nicely and keep the magic smoke inside the charging IC? Does battery capacity affect charging?

The 18650's are Samsung SDI branded cells.

Would such a contraption work? Anyone tried such a thing?

  • \$\begingroup\$ Usually that would just work. In the phone there will be a charging circuit to safely charge the battery. It will stop charging when a certain voltage has been reached. Battery capacity has little to do with that, a larger capacity will just take longer to charge. It could be that the phone detects that you're fooling it as the BA900 has 3 contacts and a 18650 has only two. That 3rd contact is usually for a thermistor to keep an eye on the temperature of the battery. If your 18650 is an unprotected one I would at least add a 2A fuse in series with the connection. Or get a protection circuit. \$\endgroup\$ Commented Nov 1, 2016 at 22:01
  • \$\begingroup\$ Nice to hear. If I added batteries in parallel as I have quiet a lot of 18650's, that would increase the capacity right? And won't affect the charging, just would take light years to charge, right? \$\endgroup\$ Commented Nov 1, 2016 at 22:04
  • \$\begingroup\$ replacement batteries online are about $9usd \$\endgroup\$ Commented Nov 1, 2016 at 22:20
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    \$\begingroup\$ That ain't fun my man, and this was bought with the intention to mess with it. :) \$\endgroup\$ Commented Nov 1, 2016 at 22:30
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    \$\begingroup\$ Don't just place 186050s in parallel as large currents can flow when they do not have the same voltage ! You must first balance them, do this by connecting them in parallel but not directly but with a resistor of 10 ohms or so in between. Leave that to balance for some hours. Then connect them in parallel without the resistor. Yes of course that will take longer to charge. \$\endgroup\$ Commented Nov 2, 2016 at 13:14

1 Answer 1


I have 3 18650 in my galaxy s4 mini with cyanogen 14 android version 6.0.1, use mine as a normal phone as wlell as an ap at home as its faster internet at up to 65 meg, works a treat and charges fine but had to add a charging pcb that also doubles as a USB power sorce, was from a pound shop portable mobile charger.had to add a switch between battery's and phone to turn it off fully to reset the phones battery meter as does not recognise when its fully charge. Lasts a couple of days with normal use. All in all very pleased with results so far. About 4 months.

  • \$\begingroup\$ Thanks for sharing, could your share more about the charging PCB you've mentioned? I'm going to try this HM1S. Any success story with HM1S? Thanks. \$\endgroup\$
    – alphaGeek
    Commented Aug 5, 2020 at 10:24

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