Design Automatic 2S Li-Ion Charger Control Circuit

# Hardware Engineer Needed – Automatic 2S Li-Ion Charger ON/OFF Circuit I am looking for an experienced **electronics/hardware engineer** to design a reliable automatic charger control circuit for a **2S Li-ion battery system**. ## Project Overview I have a **2S Li-ion battery pack**: * Fully charged voltage: **8.4 V** * Desired charger turn-ON voltage: **7.2 V** * Charger: **TP5100 module configured for 2S Li-ion** * Input adapter: **9 V DC, 1.5 A** * Battery is connected to the TP5100 output. * 9 V adapter is connected to the TP5100 input. Currently, the 9 V adapter remains connected to the TP5100 continuously. I want a circuit that automatically disconnects/connects the **9 V adapter from the TP5100** based on battery voltage. ## Required Operation The desired behavior is: **Battery at 8.4 V** → Charger OFF → 9 V supply disconnected from TP5100 **Battery discharges** **Battery reaches 7.2 V** → Charger ON → 9 V adapter connected to TP5100 → TP5100 charges the battery **Battery charges from 7.2 V → 8.4 V** **Battery reaches 8.4 V** → Charger OFF → 9 V adapter disconnected again The system should then remain OFF until the battery falls back to **7.2 V**. In other words: **7.2 V → ON → 8.4 V → OFF → 7.2 V → ON** This requires proper **voltage hysteresis**. ## Circuit Requirements I would like a **hardware-only solution**, preferably using: * Voltage comparator with hysteresis * MOSFET-based switching * No microcontroller * Low standby/battery consumption * Proper protection against backfeeding * Stable operation without rapid ON/OFF oscillation * Reliable switching of the **9 V / 1.5 A adapter** * Proper gate drive for the MOSFET * Appropriate filtering/decoupling The design should be suitable for **real hardware production**, not just a theoretical simulation. ## Important Requirement – Threshold Accuracy Please calculate the resistor network so that: * Charger turns **ON at approximately 7.2 V** * Charger turns **OFF at approximately 8.4 V** Please explain the hysteresis calculation and component tolerances. If you believe different thresholds would be safer or more appropriate for the TP5100/battery, please explain why before changing them. ## Deliverables I expect the following: ### 1. Complete Schematic Provide a complete electrical schematic showing: * Battery * Voltage sensing circuit * Comparator * Hysteresis feedback * MOSFET switching circuit * 9 V adapter * TP5100 * Protection components * Pull-up/pull-down resistors * Capacitors * Ground connections ### 2. Component Selection Provide exact recommended part numbers for: * Comparator IC * MOSFET * Zener/TVS if required * Diodes * Resistors * Capacitors * Any additional protection components Please explain why each important component was selected. ### 3. Calculations Provide calculations for: * 7.2 V turn-ON threshold * 8.4 V turn-OFF threshold * Hysteresis * Comparator input voltage * Resistor currents * MOSFET gate voltage * MOSFET power dissipation * Maximum adapter current * Standby current when charger is OFF ### 4. MOSFET Switching The MOSFET must be selected so that it can safely handle the **9 V / 1.5 A adapter supply**. Please specifically consider: * RDS(on) * VGS requirement * Voltage rating * Current rating * Power dissipation * Heat generation * Gate drive * Switching configuration Please also explain whether **high-side switching of the +9 V line** or **low-side switching of the adapter ground** is preferable for this application. ### 5. Backfeed Protection The circuit must prevent the battery/TP5100 side from feeding voltage back toward the disconnected adapter. Please analyze this carefully, including: * MOSFET body-diode paths * TP5100 input/output paths * Any required blocking diode or back-to-back MOSFET arrangement ### 6. Startup Behavior Please analyze what happens when the circuit is initially powered with the battery at: * 8.4 V * 8.0 V * 7.5 V * 7.2 V * Below 7.2 V The circuit must establish the correct ON/OFF state without unstable oscillation. ### 7. TP5100 Compatibility Please check whether repeatedly disconnecting/reconnecting the **9 V input to the TP5100** is safe and appropriate. Also identify any potential issue with using the TP5100 in this type of automatic charging arrangement. ### 8. Simulation If possible, provide a simulation in one of the following: * LTspice * PSpice * KiCad/ngspice The simulation should demonstrate the battery voltage crossing the 7.2 V and 8.4 V thresholds and the corresponding charger ON/OFF behavior. ### 9. BOM Provide a complete BOM containing: * Reference designator * Part number * Value * Package * Quantity * Recommended manufacturer ### 10. Optional PCB Design If you can also provide a PCB design, please mention this separately in your proposal. The PCB should be suitable for the actual components selected and the **9 V / 1.5 A** input path. ## Freelancer Requirements Please apply only if you have experience with: * Analog electronics * Voltage comparators * Hysteresis circuits * MOSFET power switching * Li-ion battery charging * DC power-path design * TP5100 or similar Li-ion charger ICs I am particularly interested in someone who can explain **why the circuit works**, rather than simply providing a schematic. ## Proposal Requirements Please include: 1. Your relevant electronics design experience. 2. Examples of similar battery/charger control circuits you have designed. 3. Which comparator and MOSFET you would initially consider for this design. 4. Whether you can provide LTspice/PSpice simulation. 5. Whether you can provide PCB design. 6. Estimated time and cost. **The final design must be practical and safe to build on real hardware.**

Project ID: 40636206