A FANUC-controlled CNC machine has two separate battery groups, and work stops when either one dies: the memory backup battery powers the SRAM/CMOS memory that holds parameters, offsets and programs; the absolute pulse coder (APC) battery keeps the position of the servo motors while the power is off. This guide explains what to do when BAT blinks on the screen or an APC alarm 3n7 / 307 appears, in which order and within which time limit the batteries are replaced, and when a reference return is needed afterwards, based on the FANUC 0i-B, 0i-C and 0i-F manuals.
Which Batteries Are in a FANUC Machine?
| Battery | What it powers | Voltage | Location | What happens when it dies |
|---|---|---|---|---|
| Memory backup battery (lithium or 2× D-size alkaline) | SRAM/CMOS: parameters, offsets, programs, PMC data | 3 V DC (0i-B/0i-C manual heading) | Front, top or rear of the control unit depending on series, or an external battery case | BAT blinks; when fully flat, alarm 910/935 or a system alarm at power-on and the memory is cleared |
| Separate APC battery case (4× D-size alkaline) | Absolute pulse coders (up to six axes) | 6 V DC | Separate battery case next to the separate detector interface unit | 3n7/307 low battery; 3n6/306 battery zero and 3n0/300 reference return request |
| APC battery built into the amplifier (special lithium) | Absolute pulse coder built into the motor | 6 V (CX5X: red +6 V, black 0 V) | Front of the servo amplifier (underside on βiSV4/20) | Same APC alarms |
BAT Is Blinking: What It Means and How Long You Have
When BAT blinks in the bottom line of the LCD, the memory backup battery voltage has dropped and a battery alarm signal is output to the PMC. The machine keeps running. The manuals allow two to three weeks (0i-B and 0i-C maintenance manuals), one to two weeks (0i-C operator and 0i-F operator manuals) or one week (0i-F maintenance manual). If the voltage drops further, backup ends and at the next power-on the 0i-B/0i-C shows 935 (SRAM ECC error) or 910 (SRAM parity) and the 0i-F shows a system alarm; the memory is then cleared and must be re-entered. For this reason FANUC recommends replacing the battery once a year without waiting for the alarm. The lithium battery fitted at the factory backs up the memory for about one year.
Before You Start: Take an SRAM Backup
The only real risk in a battery change is losing the memory. The lithium battery is replaced with the power off, so only a capacitor feeds the memory and the time is limited. Take a full backup with SRAM DATA BACKUP from the BOOT screen first (memory card or USB memory on 0i-F, memory card on 0i-B/0i-C). With that file, even the worst case is recovered from the BOOT screen. Entering the BOOT screen and the backup steps are covered in our separate guide.
How to Replace the Memory Battery (0i-B and 0i-C, lithium A02B-0200-K102)
- Prepare the new lithium battery (A02B-0200-K102; FANUC specification A98L-0031-0012). The manual warns that any other type may explode.
- Switch the machine power on, wait about 30 seconds, then switch it off. This charges the memory capacitor.
- Locate the battery: on the 0i-B it is on the front panel of the control unit; on the 0i-C it is on the top of the unit (top right on a unit without option slots, between the fans on a 2- or 4-slot unit).
- Disconnect the connector first, then remove the old battery from its holder.
- Fit the new battery and plug in the connector.
- Time limit: the 0i-B manual and the 0i-TC operator manual allow 10 minutes from power-off to the new connector being plugged in; the 0i-C maintenance manual allows 30 minutes. Aim for 10 minutes; if that is not possible, back up the SRAM to a memory card first.
- Switch on and check that the parameter and offset screens still hold their data.
How to Replace the Memory Battery (0i-F, lithium A02B-0323-K102)
- The new battery must be A02B-0323-K102. The 0i-F battery has no connector; it is a latched type.
- Switch the power on, and after about 30 seconds switch it off.
- Pull the lithium battery out of the rear of the control unit while holding its latch.
- Push the new battery in until the latch locks and confirm that it is locked.
- The manual requires steps 1 to 3 to be completed within 30 minutes; longer and the SRAM contents may be lost. The 0i-F maintenance manual asks for a full SRAM backup first in every case.
Machines with an External Battery Case (2× D-size Alkaline)
On some machines the memory is backed up by two D-size alkaline cells in a case cabled to the control unit (case A02B-0236-C281 on 0i-B/0i-C; case A02B-0236-C282 with cable A02B-0323-K103 on 0i-F). Unlike the lithium battery these are replaced with the power on: prepare two new cells, switch the power on, remove the case cover, replace the cells observing polarity, refit the cover. If you must replace them with the power off, the lithium procedure and its time limit apply. The case is mounted where it can be reached with the power on, with the cable fixed within 50 cm of the connector.
APC Battery: What Alarm 3n7 / 307 Means
On axes with absolute pulse coders the control knows the motor position even with the power off; the APC battery makes that possible. On the 0i-B and 0i-C the alarms are 3n6–3n8 (n = axis number): 3n7 means the battery voltage is low, 3n0 means the position has been lost and a reference return is required. On the 0i-F the same meanings appear as 307 (APC blinks at the bottom of the screen), 306 (battery zero, ALM blinks) and 300 (reference position return request). On 3n7/307 the manuals ask for replacement within one to two weeks; at 3n6/306 the absolute position is already lost and the axes are referenced after the change.
How to Replace the APC Battery (Separate Case, 4× D-size)
- Prepare four new D-size alkaline cells (LR20) or A06B-6050-K061; the 0i-F manual requires all four to be replaced together and old and new cells never to be mixed (mixing may lose the absolute position).
- Switch the machine power on so the servo amplifier is powered. The 0i-F manual additionally requires the machine to be in emergency stop and the DC link charge LED on the servo amplifier to be off.
- Loosen the screws of the battery case and remove the cover.
- Replace the cells observing polarity. The 0i-B/0i-C manuals describe the layout as “two cells in one direction and the other two in the opposite direction”; follow the markings in the case.
- Refit the cover and switch the power off.
I Changed It with the Power Off. Is the Reference Lost?
According to the 0i-B and 0i-C manuals, yes: “replace the batteries with the power to the CNC on; if they are replaced without power, the stored absolute position is lost” and every axis is referenced after alarm 3n0. The 0i-F manual adds an exception: the pulse coders of αi/αiS series and βiS0.4–βiS22 servo motors have a built-in backup capacitor, so no reference return is needed if the change takes less than 10 minutes even with the power off. If it will take 10 minutes or longer, replace with the power on. This exception is not written in the 0i-B/0i-C manuals; on an older machine the safe route is to replace with the power on.
APC Battery Built into the Servo Amplifier
On current αi and βi amplifiers the APC battery sits inside the amplifier instead of a separate case: A06B-6114-K504 for αi amplifiers of 60/90 mm and 150/300 mm width and for two-axis βiSV, A06B-6093-K001 for βiSV4/βiSV20 and βiSV40/βiSV80. Remove the battery cover, replace the battery, refit the cover; the connector is CX5X (red +6 V, black 0 V). Warnings from the 0i-F manual: these batteries are not sold commercially and come from FANUC; when the built-in battery is used, the BATL (B3) pin of connector CXA2A/CXA2B must not be connected and two batteries must never share the same line (short circuit and heating); a short between +6 V and 0 V can cause overheating, explosion, fire or loss of position. On βiSV4/βiSV20 the battery case is on the underside of the amplifier, on the others on the front.
Safety and Disposal
- Use only the battery type named in the manual; a wrong type or reversed polarity can cause overheating, explosion or fire.
- The servo amplifier stays charged for a while after power-off; measure the residual DC link voltage, confirm that the red charge LED is off, and do not touch metal parts inside the cabinet.
- Insulate the terminals of used batteries with vinyl tape and dispose of them as industrial waste according to local regulations.
- Check the parameter, offset and program screens before and after the change, and keep the SRAM backup current at all times.
Differences by Series
| Item | 0i-B | 0i-C | 0i-F |
|---|---|---|---|
| Memory battery | A02B-0200-K102 or 2× D-size (case A02B-0236-C281) | Same | A02B-0323-K102 or 2× D-size (case A02B-0236-C282 + cable A02B-0323-K103) |
| Battery location | Front panel, with connector | Top of unit, with connector | Rear, latched |
| Time without battery | 10 min | 30 min (maintenance manual) / 10 min (operator manual) | 30 min |
| Time after BAT | 2–3 weeks | 2–3 weeks / 1–2 weeks | 1–2 weeks / 1 week |
| Alarm when memory is lost | 935 | 910 or 935 | System alarm |
| APC alarms | 3n6–3n8, 3n0 | Same | 307 (APC), 306 (ALM), 300 |
| APC case life | 6 pulse coders / 1 year | Same | 2 years (K061 or LR20); built-in K504: 1 year |
| Capacitor exception | Not in manual | Not in manual | αi/αiS, βiS0.4–22: under 10 min |
Frequently Asked Questions About FANUC Battery Replacement
BAT is blinking on the screen. Do I have to stop the machine?
No, the machine keeps running. BAT means the memory backup battery voltage has dropped; the manuals allow two to three weeks on 0i-B/0i-C and one to two weeks on 0i-F. Do not wait that long: take an SRAM backup and replace the battery as soon as possible. If the battery dies completely, alarm 910/935 or a system alarm appears at power-on and parameters, offsets and programs are lost.
Should the power be on or off when I replace the memory battery?
For the lithium battery: switch the power on, wait about 30 seconds, switch it off and replace the battery with the power off; a capacitor keeps the memory alive meanwhile, so the job must be finished within 10 minutes according to the 0i-B manual or 30 minutes according to the 0i-C maintenance and 0i-F manuals. The D-size alkaline cells in an external battery case are replaced with the power on.
The memory was lost during the battery change. What now?
Restore the SRAM backup (the file taken with SRAM DATA BACKUP) from the BOOT screen. If there is no backup, the memory is cleared completely (all clear) and the parameters, PMC parameters, offsets and programs are re-entered by hand from the machine builder's parameter list. This is why a backup must be taken before every battery change.
Do I have to reference the axes after changing the APC battery (alarm 3n7 / 307)?
If you replace it with the power on (servo amplifier powered, emergency stop pressed, DC link LED off) the absolute position is kept and no reference is needed. If it is replaced with the power off, the 0i-B/0i-C manuals state that the absolute position is lost and a reference return follows alarm 3n0. The 0i-F manual gives one exception: αi/αiS and βiS0.4–βiS22 motors have a built-in capacitor, and no reference is needed if the change takes less than 10 minutes.
Which battery do I need?
Memory battery: A02B-0200-K102 lithium for 0i-B/0i-C (or two D-size alkaline cells in case A02B-0236-C281); A02B-0323-K102 lithium for 0i-F (or two D-size cells in case A02B-0236-C282 with cable A02B-0323-K103). Separate APC battery case: four D-size alkaline cells (LR20) or A06B-6050-K061. APC battery built into the servo amplifier: A06B-6114-K504 (αi, two-axis βiSV) or A06B-6093-K001 (βiSV4/20/40/80). The manuals warn that any other battery type may explode.
How often should the batteries be replaced?
FANUC recommends replacing the memory battery once a year regardless of the alarm. The separate APC battery case is rated for one year per six pulse coders in the 0i-B/0i-C manuals and two years in the 0i-F manual; the built-in lithium A06B-6114-K504 for one year. These are estimates; put them in the annual maintenance plan.
Disclaimer
This guide is based on the FANUC Series 0i-B (B-63835TR), 0i-C (B-64115TR, B-64114TR) and 0i-F (B-64604EN, B-64605EN) manuals; order numbers, time limits and alarm numbers are given as printed there. The machine builder’s manual always takes precedence; battery type, location and procedure may differ on your machine. Take an SRAM backup before any battery change. Work inside the electrical cabinet must be done by qualified personnel. Mentor CNC cannot be held liable for damage or loss arising from the use of this information.