DIY Extended Cell Phone Battery for Samsung Note

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This post is about setting up an extended cell phone battery for the original Samsung Note. The project is not complete and you will know what I mean if you read the rest of this post. However, before I forget the details I wanted to document the steps for this project so far.


When I bought the Samsung Note, one thing I noticed is that it was consuming a lot of power. With the original battery fully charged, I could use the phone maybe for about 6 hours or less. I ordered an extended battery which was much thicker and therefore came with its own back cover (2013 October from All4cellular). At first this battery worked fine, but after about a year the battery expanded enough that the back cover would not close any more. Plus, after it was fully charged, the battery lasted only a few hours. Obviously it needed a new battery.

I was about to order a new one and I was talking to my son Kubi about it. He said that he had a number of 18650 Lithium ion batteries recovered from old laptops. These batteries are pretty expensive, so if you have an old laptop to recycle be sure to take out the 18650 batteries out of them. Sometimes one or two will be bad, but the rest will be perfectly good.

18650 Lithium Ion Batteries

18650 batteries look like AA batteries, except that they are a little bigger. Another difference is that the plus side and the minus side are both flat. The plus side has a circular ring indentation. Below is a picture of a couple of batteries.

18650 Battery

Lithium Ion battery voltage is about 4.2 volts when fully charged and the voltage is down to about 3.6 volts when they are discharged. If the voltage goes below 2 volts, they are not good any more and there is no way to recover them (that I know of).

Here is a picture of the extended battery showing the relative size of it as compared to the 18650 batteries.

Extended Battery

Connecting to the Phone

As you can see from the picture of the extended battery, there are three connections: the plus, the minus, and the middle terminal. At first I connected the two lithium ion batteries in parallel to the plus and the minus on the phone. When I connected the charger, there was no indication that the phone was charging the batteries.

It turns out the battery has a thermistor connected from the middle pin to the ground (the minus sign) so that the charger on the phone can monitor the battery temperature. When I measured it, the DVM showed this resistance at room temperature to be 26.7 K ohm.

The closest resistor I had was 27 K ohm (red, purple, orange, silver). To fool the charger I connected this resistor from the middle pin to the ground (minus sign). The phone started charging the battery fine. Of course if something goes wrong with the battery and starts to heat up, there are no guarantees here.

The other thing I had to do was to solder a thin wire from the phone to the battery terminals. Otherwise it is really hard to line up the wires exactly to the phone battery terminals.


Back Cover Cut Out

Back Cover Cut Out

Also as you can see from the pictures, I had to cut out the back cover so that the batteries can fit into the phone properly.


After a full charge, the battery lasts me two full days. That is twice as long as the extended battery that I had bought last year. I am super happy with that kind of performance.

On the down side, the charging is an exercise in balancing the resistor just right. And the whole thing doesn’t look good. That is why this project needs more work…

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2 Responses to DIY Extended Cell Phone Battery for Samsung Note

  1. morgan says:

    i love this!!! i was trying to find a way to build a battery pack for the note 1. and i wanted to use 18650 or 18350′s. what if i dont plan to use the internal charger? can i just hook another neg to the middle post? or do i still need the resistor?
    thank you

  2. MG83 says:

    Hello !
    i am electrical engineer.

    Nice experiment but this is no safe :D if you use this for long term there is high chance of battery burn. but there is alternative way also. You directly connected batteries to the phone . But if you examine the orginal charger there is thin layer of plastic at top of the original battery where its terminals are. Remove it you will see a electronic pcb with little chip in it and probably 2 smd capacitor and resistor. That circuit is the protection circuit. remove the pcb and you will see it has 2x terminal (+ and -) that connects to “real battery” use these to connect your battery block if the voltage of your battery block and the phones orginal batter is similiar you will also have an average overcharge / short circuit / heat protection : ) (i assume they are similiar becouse you already made the phone working status)

    have fun !

    best regards

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