Most FANUC CNCs keep their parameters, part programs, tool offsets and macro variables in battery-backed memory. When that battery dies and the machine is powered down, the data can go with it. The fix is cheap and quick, as long as you replace the battery before it fails and you do it with the control powered on.

The two batteries on a FANUC CNC

A FANUC battery replacement usually means one of two batteries, and they protect different things. Know which one is asking for attention before you open the cabinet.

On Series 16i, 18i and 21i-B controls, the memory backup battery is a 3 V lithium cell that plugs into the control unit. On LCD-mounted units it sits on the back of the display assembly. Power Mate i controls use their own lithium backup battery in the same role. The absolute pulse coder (APC) battery keeps the encoders on each servo motor remembering where the axes are while the machine is off. On alpha i servo amplifiers it is a 6 V lithium pack that clips into the amplifier; older systems use a separate case of D-cell alkaline batteries.

Signs the battery is due

• BAT on the screen. When the control memory battery runs low, the status line shows a blinking BAT warning. The machine still runs, but the clock has started. Replace it before the next planned power-down.
• APC alarm 307 or 308. These are low-voltage warnings for the encoder battery. Positions are still held, so replace the battery while the machine is powered.
• APC alarm 306. The encoder battery voltage has dropped to zero. Expect to re-establish the reference position on the affected axes.
• It has been a year. FANUC maintenance manuals call for replacing both batteries on a yearly schedule, whether or not a warning has appeared. Putting it on the preventive maintenance calendar is the simplest way to never see alarm 300.

Back up before you touch anything

Even a careful swap can go wrong: a dropped connector, a bad new cell, an unexpected power cut. A current backup turns a lost-memory event into a 20-minute restore.

1	Take a full SRAM backup to a memory card. On i-series controls, power up while holding the two right-most soft keys to open the BOOT screen, choose SRAM DATA UTILITY, then SRAM BACKUP. This saves the whole memory image in one file.
2	Save parameters and programs as text files too. From the ALL IO screen, punch out the CNC parameters, part programs, tool offsets, pitch error compensation and macro variables. Text files can be read and restored selectively; the SRAM image cannot.
3	Write down the machine builder's parameter sheet location. If both backups fail, the builder's original sheet is your last resort.

Replace the control memory battery

This is the procedure for most 16i, 18i and 21i-B controls. Check your control's maintenance manual for the exact battery position on your model.

1	Have the new battery in hand. Match the connector and voltage to the old one before you start.
2	Keep the control powered on for about 30 seconds, then press E-stop. Leave control power on for the rest of the job.
3	Open the battery cover on the control unit and locate the battery and its connector.
4	Unplug the old battery and plug in the new one. Hold the connector body, not the wires, and work without a long pause between the two.
5	Seat the battery in its holder and close the cover. Make sure the lead is not pinched.
6	Confirm the BAT warning has cleared and spot-check a few parameters and a program against your backup.

Replace the APC (encoder) battery

The same rule applies: power on. On alpha i amplifiers the 6 V lithium pack sits in the front of the amplifier; on older systems the D-cells sit in a separate battery case.

1	Power on the control and put the machine in E-stop. The encoders stay powered from the amplifiers while you work.
2	Remove the battery cover on the amplifier or the battery case.
3	Swap the battery or cells. For a D-cell case, replace all four cells at once with fresh batteries from the same pack, and check the polarity.
4	Refit the cover and clear the alarm. Alarms 307 and 308 should reset once voltage is back.

If memory or position was lost

Lost parameters or programs

If the control powers up with cleared memory, alarms or a blank parameter screen, restore the SRAM image from the BOOT screen using SRAM RESTORE, then cycle power. If you only have text backups, load the parameters first, cycle power, then load offsets, pitch error compensation, macro variables and programs. Without any backup, you will need the machine builder's parameter sheet and their support line.

Alarm 300: reference return needed

If the encoder battery died or was removed with the power off, the affected axes lose their absolute position and the control raises alarm 300. Depending on how the machine was set up, that means a reference return with dogs or re-setting the reference position by hand. Follow the machine builder's procedure here: an incorrect reference position can cause a crash or an over-travel.

FAQ