I've decided to take apart my old android phone, and since I've been in a great shortage of batteries for my high energy requiring protable projects, I was wondering If I colud use my phone batteries, that are quite capable in terms of Ahs.

I know the hazards of li ion batteries, and the difficulty in charging them, and I've attempted a solution by using the phone very charging circuit, or actually the pcb containing the usb port and the battery terminals.

My question is, is this safe? I've connected the load in parallel with the battery and charging circuit, and whereas the battery should be 3.7V it remains a costant 4V, even without the charger; When I connect a motor, the voltage drops down as expected, but when the running speed is reached it's still 4 V.

I somehow managed to get the batteries to 0V, but i suspect it was some protection, but connecting the charger brings the battery to 4V immediately, whereas obviously he phone woult take hours to charge.

Also, do you think there is some cheap IC that could replace the phone pcb? I would also need to use these batteries in series or parallel, but I don't know hot to treat them tho.

  • \$\begingroup\$ There certainly are small ICs and breakout boards that will charge for you, but you have to match the charge rate to the battery. \$\endgroup\$
    – pjc50
    Commented Nov 14, 2016 at 10:02
  • \$\begingroup\$ Discharching a li-ion battery is almost as unsafe as charging it. You need protection circuits in both cases. \$\endgroup\$
    – Agent_L
    Commented Nov 14, 2016 at 13:15

2 Answers 2


Batteries contain energy and chemicals and those two things generally tends to be unsafe.

A good battery should contain functionality to stop bad things to happen, such as thermal shutdown and short circuit detection. But do not count on it, even big companies (mentioning no name) have problems with batteries getting too hot. Li-ion batteries does not like getting hot. Some batteries (old models) will even explode directly in your face if you short circuit them and I am speaking from personal experience here.

You should not go beyond the maximum rating for the battery. The maximum ratings for the battery can be stated by the vendor together with the model of the battery in question. If you do not have this information I strongly advice against you using it.

There are a myriad of different battery management chips that you can use to charge Li-ion based batteries. The hard thing is to get a chip is suitable for the battery you would like to charge. Normally the specification or battery manual will point you in the right direction.


The full charge for a Li-ion cell is 4.2V. Getting 4V is normal. 3.7V is the nominal voltage, and 3,2 is often considered as the 'discharged' voltage. To make your setup safer, you could use a Li-ion charger module WITH PROTECTION ; it has distinct battery terminals and load terminals, and the load terminals have over-current, reverse flow and short circuit protection. just solder your battery and load and you're good to go !


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