Single cell lithium polymer battery cell is very handsome power source for many applications. Unfortunately it is quite dangerous when such battery cell is not properly protected and charged. I prepared simple 1S LiPo battery protection and Charger on two, single sided small PCB modules. I separated Protection from the charger when only protection is needed.
First, the schematic, which is quite simple.
1S protection circuit
All reliable battery protections are based on Seiko SII integrated circuits. There are many options for single cell and most commonly used is S-8241 incorporating high-accuracy voltage detection circuits and delay circuits. This IC is suitable for protection of 1-cell lithium ion/lithium polymer rechargeable battery pack from overcharge, overdischarge and overcurrent. The block diagram of the S-8241:
The S-8241 housing is manual-soldering friendly SOT23-5.
Protection circuit schematic
The protection circuits require only few external parts. Two MOSFETs have two functions. First is switching battery cell to/from external connection and second, the current is measured via the RdsON resistance.
Noise blocking capacitor is added and additional fuse in case all other protection fails.
S-8241 protection functions…
S-8241 normal operation
The S-8241 monitors the voltage of the battery connected to VDD and VSS pins and the voltage difference between VM and VSS pins to control charging and discharging. When the battery voltage is in the range from the overdischarge detection voltage (VDL) to the overcharge detection voltage (VCU), and the VM pin voltage is in the range from the charger detection voltage (VCHA) to the overcurrent 1 detection voltage (VIOV1) (the current flowing through the battery is equal to or lower than a specified value), the IC turns both the charging and discharging control FETs on.
S-8241 overcurrent detection
When the discharging current becomes equal to or higher than a specified value (the VM pin voltage is equal to or higher than the overcurrent detection voltage) during discharging under normal status and the state continues for the overcurrent detection delay time or longer, the S-8241 turns the discharging control FET off to stop discharging. The overcurrent status returns to the normal operation when the load is released.
S-8241 overcharge detection
When the battery voltage becomes higher than the overcharge detection voltage (VCU) during charging under normal status and the state continues for the overcharge detection delay time (tCU) or longer, the S-8241 turns the charging control FET off to stop charging.
S-8241 overdischarge detection
When the battery voltage drops below the overdischarge detection voltage (VDL) during discharging under normal status and it continues for the overdischarge detection delay time (tDL) or longer, the S-8241 turns the discharging control FET off and stops discharging.
S-8241 charger detection
If the VM pin voltage is lower than the charger detection voltage (VCHA) when a battery in overdischarge status is connected to a charger, overdischarge hysteresis is released, and when the battery voltage becomes equal to or higher than the overdischarge detection voltage (VDL), the overdischarge status is released (the discharging control FET is turned on).
S-8241 charge overcurrent detection
If the VM pin voltage drops below the charger detection voltage (VCHA) during charging under the normal status and it continues for the overcharge detection delay time (tCU) or longer, the S-8241 turns the charging control FET off and stops charging.
Charger is based on single chip Li-xx charger for USB or similar supply voltage. It is simple, CC/CV, 5Vin linear charger. There are many similar chips with same pinout. I put two options in the schematic and PCB. One has charging status output, which can be connected to LED to monitor the chargingt status. One of the many part numbers for such charging chip is MCP73832T. In this case use R4 and LED.
Another oiption has enable input instead of charging status output. One of many possible part numbers is MCP73811. For this option to properly operate, the enable should be driven high – use resistor R5 or drive this pin externally with the microcontroller.
The maximum charging current is set by resistor R3. For MCP73832T or MCP73811, the value for 500mA limit is set by 2kOhm resistor. Check the datasheet for other options.
The protection and charger PCBs
Both PCBs produced with toner transfer technology:
Here is PDF with 12 pairs of the charger and protection circuit. They are already mirrored, so just print to your favourite toner transfer paper in 1:1 ratio and prepare the PCBs.
First solder all components to both PCBs.
The USB connector has only +5 (Vbus) and GND connected to the charger:
Both PCBs can be assembled together back-to-back or laid flat on the battery cell. It depends what cell is connected to both circuits. For smaller cells it is easier to have “sandwitch” structure. Larger cells have more landscape area and both modules can be mounted flat on the battery surface with the little help from double sided adhesive tape.
I am using TESA 4965, which is really strong:
Please be careful not to damage the battery cell protection foil. Cell is destroyed when moisture from the air reaches the battery internal structure!
Finally, connect the cell to the protection circuit first (avoiding short circuit on the cell!). Connect the charger and protection circuit together.
Now it’s time for some measurements. Connect the voltmeter to the pack output. Probably there will be no output voltage.
Connect 5V from USB to the charger to wake up the protection circuit. The battery pack voltage should rise (if charger works properly):
At the end, just put the pack into the shrinking tube to protect the cell contacts from the direct contact, short circuit, etc…: