Single Circuit 3.7V 1S and 2S Battery Charger Circuit
Build a safe single-circuit 3.7 V 1S and 2S battery charger circuit. Learn CC-CV charging, TP4056 for 1S, 2S balancing options, parts, wiring, and safety tips.
Why correct charging matters
Lithium-ion and LiPo battery materials should be charged with caution: excessively high voltage can cause cell destruction and fire, whereas excessively high current will shorten the life; partial or unbalanced charging will reduce capacity and cycle life. A correct CC-CV profile is one in which the voltage across the cell is kept to 4.20 V (full voltage) at the start of the profile, and the current is maintained at a constant voltage as the current decreases. Termination takes place when the current level out to a threshold (usually C/10).
Key parameters
- Nominal cell voltage (1S): 3.7 V
- Full charge voltage (Vfloat/Vfull): 4.20 V ± 0.05 V per cell
- Cutoff voltage (discharge): ~3.0–3.2 V recommended for most cells
- Charge current (Icharge): normally 0.2C–1C (C = cell capacity). Example: for a 1000 mAh cell, 0.5C = 500 mA.
- Charge termination: when charging current falls to ~0.05–0.1 C (i.e., 5–10% of Icharge) or per IC algorithm.
1S battery charger TP4056 circuit
A Single Circuit 3.7 V 1S Battery Charger Circuit is a miniature charger of single-cell (1S) and two-cell (2S) lithium-ion / LiPo packs in small devices, power banks, internet of things (IoT) nodes, and hobby projects. The correct charging of Li-ion cells must have a constant-current/constant-voltage (CC-CV) profile and must terminate correctly, or the capacity and safety of the battery is lost. A practical, single-cell charger was discussed in this guide based on the popular TP4056 1S charging chip, and safe, realistic charging of the 2S pack was addressed either with two, isolated, 1S chargers with balancing, or with a special 2S charger/BMS IC (advocated). You will have component lists, wiring directions, sample resistor values to use to set charge current, non-ideal operation to be aware of, and understandable safety precautions that will enable you to construct and test a reliable Single Circuit 3.7 V 1S 2S Battery Charger Circuit as part of your project.
1S (3.7V) & 2S Li-ion Battery Charger Guide — TP4056 and Safe 2S Charging Methods
1S (3.7 V) Single-Cell Charger — TP4056 Circuit
The TP4056 is a low-cost, popular linear charger IC for single 3.7 V Li-ion cells. It provides CC–CV charging and a programmable charge current using a single PROG resistor.
Components Required (1S)
- TP4056 IC or module
- Rprog resistor (sets charge current)
- 5V USB or regulated 5V supply
- Fuse (PTC or 500 mA slow blow)
- Protection diode (optional)
- Battery connector (JST-PH)
- Optional DW01 + 8205A MOSFET protection board
TP4056 Pin & Schematic Explanation
- VCC (IN+): Connect to 5V
- GND: Ground
- BAT+: Battery positive
- BAT−: Battery negative
- PROG: Resistor to ground (sets charging current)
- STAT1 / STAT2: LED indicators for Charging / Full
Setting Charge Current (Rprog Calculation)
Formula: I (mA) ≈ 1200 / Rprog (kΩ)
| Charge Current | Rprog Value |
|---|---|
| 100 mA | 12 kΩ |
| 500 mA | 2.4 kΩ |
| 1 A | 1.2 kΩ |
Practical Wiring Tips (1S)
- Use a 10 µF capacitor at the input
- Keep battery wires short and thick
- Use fused USB input
- Ensure proper ventilation — TP4056 heats up at high currents
- Use a BMS for cell protection
2S (7.4 V) Li-ion Charger — Safe Options
Charging a 2S (two cells in series) battery is more complex than a 1S pack because both cells must charge evenly. Safe charging requires balancing.
Safe Charging Options
A) Two TP4056 Chargers (NOT Recommended)
Only safe if cells are disconnected from the series and charged individually.
B) Dedicated 2S Charger IC / BMS (Recommended)
Use proper 2S charging modules with balancing (best option).
C) Charge the Pack with CC–CV + BMS
Charge at 8.4V with a BMS that provides balancing.
Balancing Types
- Passive balancing: Bleeds excess charge as heat.
- Active balancing: Moves charge between cells.
Design Notes
- Use fuses and reverse polarity protection.
- Ensure proper thermal paths.
- Use correct connectors (JST-PH, XT30, etc.).
- Monitor voltage and temperature.
Troubleshooting
- Battery not charging → Check wiring and 5V input.
- Module overheating → Reduce charge current.
- Overvoltage detection → Replace BMS or charger module.
- Cell imbalance → Use proper balancing charger.
Safety Warnings
- Do NOT charge Li-ion packs without proper protection.
- Do NOT leave batteries charging unattended.
- Use matched cells in series.
- Stop charging immediately if the temperature exceeds safe limits.
Frequently Asked Questions - Single Circuit 3.7V 1S and 2S Battery Charger Circuit:
What is a Single Circuit 3.7 V 1S battery charger?
A charger designed to deliver CC-CV charging to a single 3.7V nominal Li-ion cell, commonly implemented with TP4056.
Can TP4056 charge a 2S battery pack directly?
No. TP4056 is single-cell only; charging a 2S pack requires a dedicated 2S charger or careful balancing strategy with a BMS.
How do I set the TP4056 charge current?
Use Rprog where Icharge(mA)=1200/Rprog(Ω). Example: Rprog≈2.4kΩ for 500mA.
What is CC-CV charging?
Constant-Current (CC) until cell reaches 4.2V, then Constant-Voltage (CV) at 4.2V while current tapers to cutoff.
Is a BMS required for 2S packs?
Yes. A BMS provides protection and balancing and is strongly recommended for safety and longevity.
What input voltage should I use for TP4056?
5V USB or regulated 5V DC. Keep Vin close to 5V to reduce heat dissipation.
How to monitor cell balancing on 2S?
Use balance taps to measure individual cell voltages and choose a charger/BMS that supports balancing.
Can I use two TP4056 modules to charge cells in series?
Not while cells are connected in series. Cells should be charged individually or use a properly designed balancing charger.
What temperature is safe during charging?
Cell surface temperature should typically be <45°C during normal charging; stop charging if much hotter.
What protection should I include in my charger circuit?
Include fuse/PTC, reverse-polarity protection, BMS for multi-cell packs, TVS for transients, and thermal monitoring.
