I am planning to build an electric bicycle Li-ion Battery pack. More people on the internet are using 18650 Li-ion battery cells found in Laptop batteries for this purpose. 18650 is a size standard, these are 3,7V cells with significant capacity. I have recently found a laptop battery in a dustbin and I though from the size of the battery that it contains 18650 cells. I have taken it home. It happens regularly that only 1 or 2 cells are faulty in the pack, but the others are still functional. Or just the control circuit shuts it down for some reason. In these cases the battery will not let to charge itself. This is the time when they get in the dustbin. But there are still some usable cells inside which can be used. Of course their capacity does not reach the manufacture capacity, but for hobby and project purposes they are still ideal. They can be also used in a flashlight, storing energy to recharge USB devices and so on. So let’s see.
As it does not have any brand it is a noname aftermarket piece. It is written on the outer shell that it contains Li-Ion cells and it has a capacity of 4400mAh and it gives 11,1V. The sticker on the side tells that it was manufactured in the middle of 2013. It has not been used for too long. Maybe it is because the cells are bad quality or the full battery capacity has decreased to a level that it was unable to give enough power to a laptop, or it would have run only for some minutes.
The case was easy to open at the joints. This process requires attention, not to damage the cells or to cause short circuit, as the cells may get overheated or set on fire. There are six cells in this battery.
They seems to be robust compared to an AAA or AA battery, they are bigger and heavier. Three are connected in series and three are connected with them in parallel. 3,7V*3=11,1V as it can be seen on the sticker. From the 4400mAh capacity indicated it is obvious that each cell is rated 2200mAh. The controlling circuit is located on the top right side. There is a temperature sensor duck taped to one of the cells.
It is simple plastic case. What is surprising is that if we take a closer look at this inner side, the initial numbers and markings can be seen from the mould which they used in the factory to manufacture this shape of plastic.
I have cut the cables connected to the controller circuit, then the strip which connected the cells together. We have to take care again, not to cause a short circuit. With the help of long nose pliers the metal strip can be torn off the cells. I used a circular motion to wind it down as I would open a tin can.
The other side of the case. The cells were glued in with hot glue.
The cells seem to be noname, nothing can be found out from their printings. I have searched with Google to SZN cells, a 2200mAh piece cost 3,6 USD in Hungary. The better quality branded ones costs around 5,3 USD each. I was wondering here about the laptop battery prices. A noname similar laptop battery costs in Hungary around 36 USD. A branded one costs around 72-108 USD. But what costs this much on the batteries? One cell costs 3,6USD. 6*3,6=21,6 USD. Plus the package and the controlling circuit. These cannot cost around 14,3USD, because in China they manufacture in huge quantities and in bulk it is way cheaper. So on each of the noname batteries, there is a profit around 10,8-14,4 USD. But I think it is much more. Now let's see the branded ones. Those cost 2-3 times the noname's price. But the branded cells are not cost this much more. Not even the controlling circuit. If we count 6USD*6=36USD if we count the best quality cell. Plus the controller. On the branded batteries there has to be more than 70USD profit on each. This is amazing.
Let's see if the cells are still in working condition or not. They say if a Li-ion cell's voltage drops below 2.5V then it is damaged unrecoverable. I have measured the voltage of these cells and all was on 4,1V! This is normal since while charging they are charged until 4,2V when they are full. So it is possible that all the cells are still functional.
The controlling circuit was made with modularity in mind. It can be seen on the left that this connector part can be changed depending on what type of laptop will it serve. So the other part of the circuit can be always the same. Of course the programming details are changing like how many cells are there, the capacity, the charging current and so on. The temperature sensor is checking if there is an overheating happening. But I think the temperature should be checked on all cells, because what if the one on the left gets too hot, until the sensor on the right senses it, it may be too late. Maybe the branded batteries have different design. The controlling chip is an Atmel MEGA406. You can find its specifications on the internet. There is one more interesting thing on the circuit. The small white cigarette shape with the white putty, it is a thermal fuse. If they reach the melting temperature they cut the circuit. Because it is intact this was not the cause of the dumping.
I have found something interesting on the internet. There is a chance to "reset" the controlling chip, so it will delete the stored information about the charging cycles and errors. It is possible that after resetting the battery would still work for some time. There is a manual about this here. In short it describes that if we search on the internet for the specifications of the controller chip, there is usually a reset leg on it, which in the mentioned case has to be connected with the battery ground wire. This would reset the chip. Please experiment on your own responsibility! Unfortunately I cannot try this on my battery, as I do not have a laptop to test it with.
To be frank I thought the batteries are more complicated inside, but they are not. They are also very expensive compared to what is inside them. I will keep the temperature sensor for future Arduino projects.
I have measured the cells capacity:
1- 659mAh
2- 51mAh
3- 1238mAh
4- 1303mAh
5- 780mAh
6- 875mAh
When they were new theoretically they had 2200mAh capacity. It is interesting that it has decreased in this magnitude in 2,5 years. We do not know it is because the battery was used too much, or the cells are bad quality cells. But the 51mAh cell proves that it had to perform very badly. I do not think it was capable to run a laptop in this state.
How to find out if the cells are in a good shape. For each cell:
1. Measure voltage, if it's less than 2.5v, do not use it.
2. If it gets hot during charging, do not use it.
3. After charging the voltage should be between 4.1v and 4.2v.
4. After 30 munutes if it's fallen less than 4v, do not use it. Note the measured voltage.
5. After 3 days, measure the voltage again, if voltage has fallen more than 0.1v from the recorded voltage, do not use it.
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