When talking about standalone single cell battery chargers only one popular name came into my mind that is our versatile Li-Ion/Li-Po TP4056 battery charger. Which is vastly available and has a lot of features related to battery protection. But what about the other batteries which work on a lower potential, for example the crude NiMh or NiCd (Nickel metal hydride or nickel cadmium). There are a lot of DIY options available, but because these batteries work at lower voltage (1.2V) there is not a specific and standard method to charge.

One simple method is to create a constant current power supply, and charge the batteries at 1/10C rating for 10-16 Hours, this method is used in low cost applications. But the standard method to charge the batteries with all the battery protection features can be an integrated circuit like TP4056.

Luckily I found the one solution after searching the web a lot which works the same, it is CN3085 NiMH single to three cell battery charger IC. The device contains an on-chip power MOSFET and eliminates the need for the external sense resistor and blocking diode. CN3085 requires few external components, and is suitable for portable applications. Like TP4056 Li-ion, the charge current can be set externally with a single resistor. Other features include undervoltage lockout, automatic recharge, the constant current charging, the maintenance charge mode(timer termination), charge/termination indicators and battery temperature monitoring. All the modes and battery charging process is given below with all details. Big thanks to PCBWay for sponsoring the PCBs for this project! Their high-quality manufacturing and quick turnaround made this build possible.

Features:

On-chip Power MOSFET

Voltage accuracy：1>#/span###

Precharge Conditioning for reviving deeply discharged cells

Charge Current up to 1A

Constant-Current/Constant-Temperature operation with thermal regulation

Automatic Low-Power Sleep Mode when input supply voltage is removed

Indicators for Charge and termination status

Maintenance Charge Mode(Timer Termination)

Automatic Recharge

Battery Temperature Monitoring

Circuit diagram:

Here is the typical application circuit as per the datasheet of the IC, the circuit can be modified according to the needs from 1 cell to 3 cell battery charge applications. I am designing the battery charger for 1S/2S cells. The resistor divider on the output, works as a feedback system which keeps the voltage in the range of 1.2V and sets the output voltage. Battery voltage is fed back to the CN3085 through this pin. The CN3085 determines the charge mode based on the FB pin voltage. The battery terminal voltage at BAT pin is given by : VBAT＝VFB×(1＋R3／R4) .

Moreover we need a potentiometer to adjust the output voltage as per the cell rating. The changing voltage may go up to 1.3-1.35V in this case. The potentiometer can be removed and replaced with a fixed resistor after that which eliminates its need. Or we can use a divider network as shown in the modified circuit diagram below. I eliminated the battery temperature monitoring function for now, because for that we need a NTC which is placed very near to the battery surface which is not possible with this unit.

PCB and Gerber file:

To keep the circuit simple and in low form factor, I took my motivation from the TP4056 again and designed the PCB as per that. The PCB has an input charging 5V type C port, some led indicators which monitor battery charging state and turn on/off according to whether the battery is full or not. If the charging mode is on, the indicator LED remains on otherwise off. Some power supply decoupling capacitor and a feedback loop makes this board fully compatible to charge a 1S/2S battery. The cell voltage can be changed from 1S to 2S from the onboard jumper. Otherwise a variable resistor can be used to tune the output voltage.

As I got my hands on the PCBs from PCBWAY, I soldered...