If we want to use the batteries we salvage from our vape pens we need to add a BMS to them.
BMS stands for Battery Management System. This system has the role of protecting both the battery and the users. I have made a short video how I am adding such BMS.
First thing first. How does the BMS protect the battery.
- It detects if too much current is drawn from the battery. This might be the case when we short the positive and negative terminals together or we have a load that is too much.
- It detects if too much current is put into the battery or it is exposed to a voltage that is too high. This happens when incompatible charger is used.
- It protects from over discharge. If the voltage level drops under a critical level the battery might not be able to be charged up again.
- Reverse polarity connection protection. Protect from connecting your charger backwards.
- Some BMS have a storage functionality. It is become more and more common to have devices shipped out of the factories with batteries already inside. To prevent the device from draining the batteries in transit or in storage, BMS disconnects the battery. When the user gets the product it seems dead. But when connecting a charger, BMS releases from storage mode and from then on is business as usual.
Eliminating the risks above, protects the user from LiPo batteries bursting into flames. Also guaranties that we can use our potentially very expensive devices for a long time.
When the battery is used out side of the safe parameters BMS operates two MOSFET transistors that disconnect the offending device. The BMS is always powered and monitoring. When the operation conditions are back to normal the MOSFETs are turned back on and we have power again.
There are many types of LiPo cells and different BMS for each on. So take care to go through the datasheets of both and compare parameters. This is why I am not posting the model I am using.
The most common BMS will have the following characteristics:
Overcharge detection voltage: 4.25V ± 0.05V
Overcharge recovery voltage: 4.05V ± 0.05V
Over discharge detection voltage: 2.50V ± 0.1V
Over discharge recovery voltage: 2.90V ± 0.1V
In other words the BMS will disconnect the battery if it detects voltage higher than 4.25V or lower than 2.50V. And will connect it again when it is between 4.05V and 2.50V. Simple as that. You can also see 1S or 2S 3S etc. 1S is a single cell and 2S, 3S and more are for battery packs. Last but not least is continuous discharge current. Make sure you do not draw more current than your BMS allows. If you overlook this, you may have funny behaviors, Your devices seems to work just fine and then suddenly dies for no reason.
This particular BMS you see in the video I got from my local electronics supplier. It fits nicely to the cylindrical LiPos. I have made my own protection and will add a log specially for it.
Now we have safe and reliable power source for our projects. We did that for almost no money and kept piece of electronics away from the landfill.