How To Identify and Diagnose Battery Charging Circuits on Any Motherboard 

Identifying the Battery Charging Circuit

The battery charging circuit is usually centered around a battery charger IC such as a BQ247xx series (from TI) or similar. Here’s how to find and test it:

🔍 How to Identify Battery Charging Circuit :

  • Look for a large 20-28 pin IC near the battery connector.
  • Common marking: BQ24725, BQ24707, ISL6255, etc.
  • Usually located close to the battery connector and has 2–4 MOSFETs nearby.
  • You’ll also see large inductors and ceramic capacitors near it.

📐 Battery Charger IC Pinout (common layout):

  • ACDET: Adapter detect
  • ACOK: Adapter OK (logic high = adapter recognized)
  • BAT+ / SYS / DCIN: Voltage input/output pins
  • SMBUS: Data lines to EC for charging control

🔎 Test Points:

  • DCIN (usually 19V) — input from adapter
  • CHG — battery charging voltage (11.1V–12.6V on 3-cell Li-ion)
  • ACOK — should go high if adapter is good

Use a multimeter to verify:

  • Adapter is detected (ACOK = 3.3V)
  • Battery voltage rising when system is on

🔧 2. Identifying the 3.3V / 5V System (Standby Rails)

These are usually produced by a power rail controller IC, such as the RT8205, PU5/PU3, TPS51125, etc.

🔍 How to Identify:

  1. ICs marked like RT8205, TPS51125, ISL6237 are dual buck converters.

  2. Look for labels like PU5, PU3, etc. in boardviews/schematics.

  3. Near the CPU socket or near the charger circuit.

  4. They usually output:

  • +3.3V_ALW

  • +5V_ALW

✅ How to Test:

  • With adapter plugged in (even without pressing power), these “ALW” rails should be present.
  • Measure voltage on output inductors of these ICs. Example: You’ll see coils with 3.3V and 5V on them.
  • No voltage? Possible short, bad IC, or missing enable signal.

🔌 3. Power Jack Testing (DC IN)

The DC jack on Dell laptops often has 3 lines:

  • +19V (Main power line)
  • ID pin (middle pin, for adapter communication)
  • GND

✅ How to Test:

  1. Check adapter voltage at jack tip: 19V between inner pin and outer sleeve.
  2. Plug into laptop and check:

  • Does 19V appear on the DCIN line? (Follow the trace from jack to the first MOSFETs)

  • Does the ID pin (middle pin) show ~3V when plugged in? If not, adapter not detected.
  • First MOSFET gate should be controlled by ACOK / ADAPTER_PRESENT logic.

🔥 First components to check from DC jack:

  • Input MOSFETs  PQ1, PQ2 style
  • DC fuse  Usually marked F1, F2, test for continuity
  • Input filter capacitors.

🎯 Pro Tips

  • Use boardview software like OpenBoardView with the E6420 schematic and boardview file (search: "LA-6592P" – it’s the common motherboard model for E6420).

  • Check for +3.3V_ALW and +5V_ALW first — if they’re missing, the system won't start.

  • If it turns off quickly, suspect:

  • Overcurrent protection (shorted rail)

  • CPU power fault (VRM short)
  • EC or BIOS issue.