I have tens of old cell phone batteries lying around and I want to try to make use of them. I've googled around for the answer, but haven't able to come across anything consistent.

LiPo batteries that you can buy for DIY purposes from places like SparkFun contain a PCB protection circuit attached the cell. enter image description here

Is something like the circuit on the green PCB included in these types of phone batteries, or is the circuit inside the cellphone?

enter image description here

If so, is the best way to charge these by just using a charge management IC like MCP73831? If not, how can you make practical use of these cells? Cellphone batteries also have three or four pins, two obviously for power + and -, the other two for status indication? What exactly are these other two pins for and what is the best way to deal with them for using these batteries in a hobbyist, DIY setting?

EDIT Additionally, I would just want to use the batteries for low current purposes (few hundred mA at most) and most of them are unused from unused phones.


Those batteries do have built-in protections, but not the kind of ones you see on sparkfun batteries. They have thermal protection required for charging the battery and sometimes low charge cutout device which prevents discharging too much the battery. However, there is no overcurrent protection, overvoltage and noob-proof abuse protections...

You have two pins for + and - (trivial) and a temperature pin to detect if the temperature rise is in normal levels. This makes sure that the battery do not explode when charging it. In practice, you read it as a variable resistance. Some batteries also has a fourth pin tied to a voltage divider allowing the cellphone to detect the battery capacity. If I remember well, Samsung is using such a thing to allow the cellphone's firmware to detect the capacity of the battery when there are many variants that could fit in the battery socket of the phone. This feature is not used often, because they usually try to max out the capacity of the battery, especially with modern smartphone, but it wasn't so rare with old not-so-intelligent flip phones such as mine :)

Obviously, charging LiPo batteries is a risky thing if you don't have a proper setup. Disclaimer: I do NOT recommend you charging your batteries without an approved charger.

That being said, one could charge it with any LiPo compatible charger. Cell phone batteries typically have only one cell so it is pretty easy to recharge. It should check the battery voltage and divide by 3.5 to 4. This will indicate the amount of cells in series. Then using a non balancing charger to recharge the cells to appropriate levels would work. If charging the first time, one would hypothetically want to check with a multimeter if the batteries are getting hot with a remote circuit. Also, that one might want to do that outside at a place where a battery catching fire or exploding would not hurt anyone around.

Seriously, just drop them to your nearest electronics store's recycling bin. Its much safer. You can buy cheap LiPOs at some RC hobby store such as Hobby King or whatever with an approved charger for like 30$. It is really not worth the risks of injury. Also if those batteries were taken out of a phone they are probably already at the end of their practical life, so...


To complete on your question about your management IC: basically, yes you could use it. It looks like it can charge up to 500mA, which is typical for cellphone batteries. But... make sure to configure your IC to recharge at correct voltage levels, which may end up being trial and error since not all batteries are created at exact same target voltage. If I were you, I would charge to one of the lower options first (4.35V or 4.4V maximum) and limit the charging to 7.5%-10% for the first one. IF it goes well and the battery doesn't overheat, you can try other settings. If it looks hot, revert back to a lower settings. I ever saw a LiPo overcharging and while it didn't explode, gas formed and the battery inflated a little. If you see the battery inflate, disconnect power and dispose of the battery. If you take great care and you know what you are doing, it looks ok.

  • \$\begingroup\$ I should have mentioned that these are batteries form unused phones: outdated stock that did not sell basically. I would feel bad throwing away nice rechargeable batteries without at least trying to make use of them. \$\endgroup\$ – Shubham Jul 17 '14 at 5:36
  • \$\begingroup\$ When you say "LiPo compatible charger", do you mean any charge management IC like the one I mentioned? Or like a black box charger that you plug into the wall and the battery plugs into? \$\endgroup\$ – Shubham Jul 17 '14 at 5:37
  • \$\begingroup\$ I didn't look for you management IC. I was talking about a black box charger. But wait a few minutes, I'll take a look at your chip. \$\endgroup\$ – Mishyoshi Jul 17 '14 at 5:48

I would try to get one of these unsold phones and the wall power supply. Than use the phone as charger. So you eliminate any mechanical problems, you have near to perfect contact and you can be sure, that no lipo is burned. As a bonus you get a quite reasonable Display, if the lipo is still charging. I use an old Nokia 3310 for that matter for two years now and i am happy with this solution.

  • 2
    \$\begingroup\$ I don't know why this have been downvoted. It might not be what the OP expects since he may want to integrate the battery in some circuit, but this is definitely not a stupid idea. It is by far the safest and easiest way to go for charging. \$\endgroup\$ – Mishyoshi Jul 17 '14 at 21:45

I live in South Korea, currently a paradise for masses of discarded but perfectly useable 3.7V Samsung batteries - in this regard, its a paradise. I have a drawer full of Samsung batteries of all sizes and flavours, but all are the typical 3.7V.

And it is an excellent question - how to practically use them?

The first and simplest method is to make use of the separate battery charger that comes with the phone - these are very common in Korea, and from what I see, people use them more often than in other places - in other words, actually popping it out and flipping between a charged and uncharged battery.

(Cultural note: Korea has a long tradition of people opening and removing their battery during the day to swap in a new one, then charging the other. This is why Korean customers expect to get two batteries when they buy a phone, although this is decreasingly common with non-removable batteries in the newer phones. This tradition began in the time of the Chosun dynasty, more specifically, Sejong the Great. Many historians note that he may have had a Nokia 1011, but for understandable reasons, most Korean historians vehemently deny this.)

enter image description here


The great thing about these chargers is the quality circuity inside that can safely charge almost any three-pin 3.7 battery of any size. I say three pins, because these charger boards will not start until they detect the temperature pin connected. This narrows them to use for cellphone batteries only, but if you have a drawer full of them, then this is a great device. For flexibility, soldering on three color-coded wires to the battery terminals of each battery, plus three to the cellphone charger makes it very easy to swap batteries in and out.

There is a cool instructable that shows how it actually can be done: instructables.com/id/Recycled-Cellphone-Battery-Charger/

The other option is to use one of the versatile TP4056 charger boards off ebay. They are really great for a simple system, such as this mobile sensor pack....

enter image description here

As you can see, it doesn't use the temperature terminal of the battery. I am not sure what exact protection circuitry sits inside the battery, but the TP4056 board does the job and keeps the thing safe from undervoltage. I have been running tests on it, and it really is great, because I can either keep the battery fixed in place (bolted even, as you can see), or I can swap out another battery that was charged somewhere else.

One could theoretically hot-swap the batteries, and keep the system running.

The next problem is that you now have a 3.7V, and not a 5V. I solve this by connecting the LOAD output of the TP4056 board to a fixed voltage 0.9V>>>5V boost board. You string that in with it, and you get the 5V you need to run electronics like an Arduino.


i recently hard solder a LiPo charge circuit (using TP4056 chip that barely cot US$0.60 ) to an old Nokia 5c battery and to kept track of its charge using a USB power monitor system. plus temperature measuring.. works fine.. Just make sure you do the charging on a sizable ceramic surface and outdoors away from any flammable stuff as precaution the first time round.. I have personally experience nasty fire that could have burnt down the house.. so be safe always and NEVER EVER charge LiPo unattended...

For more detail on my test.. I drew power from a small 1 cell 186650 cell to deliver 5V 1A for the charge module.. the power meter register 0.7A charge at the beginning and gradually taper off as charge built up on the test battery.. it took about 45 minutes for the task to complete.. Sorry I did not know the residue charge on the battery.

Hope that answer your curiosity..


Please see the video link below. The author takes apart and claims there is a PCB with over current and thermal protection...

https://youtu.be/bwslG8S1kYs youtube author claims over current and thermal protection inside


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