I'm in the process of learning to design PCBs and understanding electronics design. For a project, I need to charge a 3.7V LiPo battery. I also want to protect it from over‑charging / over‑discharging.
I've been experimenting with boards that use the TP4056 together with a DW01 battery protection IC and FS8205A dual N-channel MOSFET.
The datasheets are available here:
The pre-built modules are very cheap – here is an example on AliExpress:
They seem to work, but I would like to know what the circuit is actually doing before I use it 🙂
I've only found one schematic with these three components combined:
I'm having a hard time to figure out if this circuit is correct. If I understand correctly, the dual N-channel MOSFET is basically 2 switches in one package. Those two MOSFETs are triggered by pins 1 and 3 of the DW01, which are described as:
- DW01 pin 1: MOSFET gate connection pin for discharge control
- DW01 pin 3: MOSFET gate connection pin for charge control
So basically the two MOSFETs in the FS8205A are switching off the flow to B-, when DW01 tells them to do so.
I understand that this would work when the over-discharge control kicks in, no power would be flowing from B- to OUT-
However what I don't understand, is how this would work with the over-charge protection? When that kicks in, no power should be flowing from the charger to the battery, however, the device that is connected to OUT+ and OUT- should still be able to work, but it seems like B- would not reach OUT-
Best Answer
Protection circuits are usually distinct from charging circuits. Many battery packs are designed with the intention of being charged by a dedicated unit that will control the charging process.
The charging process may involve cell balancing, if the pack contains a large number of cells in series, generally 4+ cells in series (4S, 14.4V) nominal will require balancing, 3S and lower it's also a good idea to balance for the health and longevity of your battery, but not necessarily required. Balancing circuits can get complicated and typically involve a BMS (battery management system) composed of a dedicated IC and multiple external mosfets, There's a project on github that seeks to create an openBMS system. This might be a good resource if you are looking for more information.
The charging cycle for lithium ion batteries can be quite complex, especially in the case of multiple cells in series, but typically involves 4 basic steps:
Notes: there are ICs that will handle the bulk majority of this for you, otherwise you will need to resort to designing an MCU controlled circuit with external boost/buck converter, or linear regulator.
The protection circuit (PCM) is fairly simple and often times integrated right into the individual cells, these cells are typically labeled: protected, or unprotected. The PCM will monitor things like: input voltage, output current, cell voltage, temperature, etc. They often times are not that robust and should be considered a last resort in a critical system. Alarms should be going off if the PCM is ever triggered.
To answer your specific question: the DW01 is optimized to favor the charger in the case of overcharge, so the charger would remain connected to the circuit, suppling nessesary voltage, while the battery is disconnected, TP4046 looks like it is designed to handle up to 8V and as a linear charger it will be dissipating excess voltage as heat. These ICs are often thermally protected and will automatically shut off in the case of excess current or voltage. If there is an overcharge, it means the circuit is likely destroyed, thus it is nice to try to disconnect the battery in this case, as it would would pose a significant safety risk.
Also, remember that mosfets contain internal diodes, so even if charge protection mosfet were to be turned off, the battery would still be attached to the circuit as long as the voltage on the drain side of the mosfet is below a certain voltage.