What causes FANUC alarm 306?

Backup battery for the absolute pulse coder has completely discharged to zero volts. Battery was never replaced after previous low battery warnings were ignored. Machine was left powered off for an extended period draining the backup battery. Battery connector or wiring to the pulse coder battery pack is loose or disconnected. Faulty or incorrect replacement battery installed that failed prematurely.

How do I fix FANUC alarm 306?

1. Locate the APC backup battery on the servo amplifier cabinet or control unit, typically a 6V lithium battery pack, and check its voltage with a multimeter before doing anything else. 2. Replace the backup battery with the correct Fanuc-specified replacement immediately. Do this with the machine powered ON if possible to avoid losing encoder data on other axes. 3. After replacing the battery, power cycle the machine and clear the alarm. The alarm should clear once the new battery is detected with proper voltage. 4. Perform a manual reference position return on the affected axis as instructed by the alarm. The machine has lost its absolute position data and must re-establish the reference point. 5. Verify the axis returns to the correct reference position and that the machine coordinates match expected values before running any programs. 6. Check the battery connector and wiring harness for corrosion or loose connections to prevent the issue from recurring after the new battery is installed.

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FANUC Controller

Servo/Axis · Basic · 30-60 minutes

Alarm 306

Fanuc Alarm 306 - APC ALARM: n AXIS BATTERY ZERO

Fanuc 306 alarm indicates the absolute pulse coder (APC) battery voltage on the n-th axis has dropped to zero volts. The battery maintains absolute position data in the serial encoder when power is off. Loss of battery power means the absolute position reference has been lost and must be re-established through reference position return after battery replacement.

FANUCServo/AxisBasic30-60 minutes

306

Fanuc Alarm 306 - APC ALARM: n AXIS BATTERY ZERO

SERVO ALARM 306

Root Cause Summary

Fanuc 306 alarm indicates the absolute pulse coder (APC) battery voltage on the n-th axis has dropped to zero volts. The battery maintains absolute position data in the serial encoder when power is off. Loss of battery power means the absolute position reference has been lost and must be re-established through reference position return after battery replacement.

Causes and Fixes

1
Check servo amplifier LED status. αi/βi series amplifiers will show battery alarm indication (flashing BATT LED or specific LED pattern on α/β servo amplifiers).
2
Navigate to DIAGNOSIS screen > SERVO > select axis n > check battery voltage parameter (diagnosis number 300 for absolute position loss flag, or diagnosis 380-387 for individual axis battery voltage on newer systems).
3
Replace the battery immediately. For αi/βi motors, battery is typically located on motor encoder (A860 series) or amplifier battery connector (CN51 on αi/βi amplifiers). Use Fanuc specified battery only (A98L-0031-0025 or A06B-6114-K504 depending on system).
After battery replacement, absolute position data is lost. Reference position return is mandatory before operation.
4
After battery replacement, perform reference position return (G28 or manual reference return) for the affected axis. Clear alarm by pressing RESET. Verify absolute position re-established via CURRENT POSITION screen in ABSOLUTE mode.

DIY Feasiblehigh confidence

Call a technician if:

·Servo amplifier replacement required
·Encoder/motor replacement needed
·Multiple axes showing same alarm simultaneously

Prevention

Replace APC batteries every 2-3 years preventively
Monitor battery voltage monthly via APC BATTERY menu
Maintain clean, dry servo cabinet environment
Keep spare BR-2/3AGCT4A batteries in stock

Common Mistakes

Using incorrect battery type
Not performing reference return after battery replacement
Ignoring environmental contamination sources
Attempting to operate with failed battery

Related Alarms

305 308 310 401

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