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.**