NO DISPLAY REPAIR GUIDE BATLOW# SIGNAL ASSERTION

The BATLOW# signal (often labeled as PM_BATLOW_L or BATLOW_L) is a low-active signal used in laptop motherboards to indicate battery presence and health status, especially during early power states like G3 and S5.

🧠 What BATLOW# Indicates

• Active Low Signal: The # or _L suffix means the signal is active when low (0V). So when BATLOW# is low, it typically means the battery is not present, not healthy, or not supplying sufficient voltage.
• Battery Detection: It helps the Embedded Controller (EC) or System Management Controller (SMC) determine whether to proceed with power sequencing from battery or wait for adapter input.
• Charger IC Coordination: Some charging ICs (like the BQ24780S you’ve studied) use this signal to decide whether to enable charging or switch power paths.

⚙️ How It Works in the Circuit

• Pull-Up Resistor: BATLOW# is usually pulled up to a voltage rail (e.g., 3.3V) through a resistor.
• Driven Low by EC/SMC: If the battery is missing or below threshold, the EC or SMC pulls the signal low.
• Communicates with PCH: The signal may be routed to the Platform Controller Hub (PCH) to influence power state transitions (e.g., enabling S4/S3 rails).

🔍 Troubleshooting Clues

• If BATLOW# stays low even with a good battery, it could mean:
  • Faulty battery connector or sensing circuit
  • EC/SMC not releasing the signal
  • Corrupted firmware or missing battery ID
• If it’s high but the system doesn’t boot, the issue might lie elsewhere in the power sequence (e.g., PM_SLP_S5_L or DSW_PWRGD).

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Would you like to trace how BATLOW# interacts with the BQ24780S or how it affects S5-to-S3 transitions? I can walk you through the signal chain.

let’s trace how BATLOW# interacts with the BQ24780S and influences the transition from S5 to S3 state. This is where power sequencing gets really elegant.

🔗 BATLOW# and BQ24780S Interaction

The BQ24780S charger IC monitors battery presence and health using the BATPRES pin, which is often influenced by the BATLOW# signal. Here’s how they connect:

• BATLOW# Low (0V):
  • Signals that the battery is missing, low, or not ready.
  • The EC may prevent enabling charging or system startup from battery.
  • The BQ24780S may disable BATFET, blocking battery power to the system.
• BATLOW# High (3.3V):

• Indicates battery is present and healthy.
• Allows BQ24780S to enable charging and switch power paths.
• Supports hybrid boost mode if system demand exceeds adapter capacity.

The BATPRES pin on BQ24780S is typically tied to a voltage divider or comparator circuit that reflects BATLOW# status. If BATLOW# is low, BATPRES may also be pulled low, signaling the charger to hold off.

🔄 S5 to S3 Transition and BATLOW# Role

During the transition from S5 (soft-off) to S3 (sleep), the EC checks BATLOW# before asserting PM_SLP_S4_L, which enables S3 rails like memory power. Here’s the sequence:

1. Power Button or Adapter Insertion triggers EC to begin wake-up.
2. EC checks:
   • PM_RSMRST_L (resume reset)
   • PM_BATLOW_L (must be high)
   • PM_DSW_PWRGD, PP3V3_SUS, etc.
3. If BATLOW# is low, EC may block PM_SLP_S4_L → system stays in S5.
4. If BATLOW# is high, EC asserts PM_SLP_S4_L → S3 rails power up.

So BATLOW# is a gatekeeper for progressing beyond S5. If it’s stuck low, even with a good battery, the system won’t wake — classic symptom in deep troubleshooting.