Every CNC machine tool has at least one battery inside it that you never think about — until it dies and takes your parameters, offsets, or absolute encoder positions with it.
There are actually three or four different batteries in a typical CNC machining centre, and they each do a completely different job. Knowing which is which can mean the difference between a 5-minute swap and a 2-day engineer callout.
The Four CNC Batteries You Need to Know
1. Absolute Encoder Battery (Most Urgent)
If your Fanuc control shows APC alarm (Absolute Pulsecoder alarm) or your Mitsubishi shows Absolute Position Lost, your absolute encoder battery is dead. This battery maintains the position counter in the servo motor encoder so the machine knows where all axes are even after a full power-down.
What happens when it dies: The machine loses its home/reference position on every axis. You can't just re-home it — on many Fanuc and Mitsubishi controls, re-establishing absolute position requires a service procedure involving moving each axis to a reference mark, entering a parameter, and cycling power. It's not a major repair, but it will cost you a morning.
🔋 Fanuc A98L-0031-0028 3V 1750mAh Backup Battery — £23.45
This is the standard Fanuc absolute encoder battery. Fits all Alpha, Alpha i, and Beta servo motors. View battery →
2. CNC Memory Backup Battery (Parameters at Risk)
This is the most dangerous one. It maintains your CNC parameters, PMC ladder, pitch error compensation tables, and option parameters while the machine is powered down. On Fanuc controls, it's typically a BR-2/3AGCT4A 6V lithium pack mounted on the main board. On Siemens, it's often a 3V CR2477N or a lithium pack inside the CCU/NCU.
What happens when it dies: Every time you power the machine off, the control forgets its parameters. On older Fanuc controls without flash memory, you lose everything. On newer models with FROM/SRAM, you lose offsets, timers, keep relays, and active parameter settings.
Recovery requires reloading from a parameter backup — if you have one. If not, a Fanuc or Siemens engineer needs to manually re-enter hundreds of values, assuming factory records exist. This typically costs £800–2,000 and 2–5 days downtime.
🔋 Fanuc Servo Drive 6V Battery — BR-2/3AGCT4A — £39.95
The standard backup battery for Fanuc 0i, 16i, 18i, 21i, and 30i/31i/32i controls. View battery →
🔋 Siemens 6FC5247-0AA18-0AA0 CCU Box Battery Replacement — £21.50
Replacement for the 3V 1200mAh battery pack in Siemens 828D and 840D CCU boxes. View battery →
🔋 Heidenhain iTNC 530 / 640 CR2477N 3V Battery — £14.95
Factory-spec ID 315878-04 RENATA battery for Heidenhain iTNC controls. View battery →
3. PLC Backup Battery (Automation Memory)
If your machine uses a separate PLC (common on larger machines and automation cells), the PLC has its own memory backup battery. Mitsubishi PLCs — especially Q-series and FX-series — use ER6C or F2-40BL type 3.6V lithium cells. Siemens SIMATIC systems use a dedicated CMOS battery.
What happens when it dies: The PLC ladder program and data registers reset to factory defaults. On a machine with complex automated tool changers, pallet changers, or robot loading, this means the machine won't run until a controls engineer reloads the program. Recovery can be straightforward if you have a PLC programme backup on a memory card. If not, expect a site visit from a specialist.
🔋 Mitsubishi ER6C PLC Battery — 3.6V 1800mAh — £35.00
Drop-in replacement for Mitsubishi MELSEC Q, A, and FX series PLCs. View battery →
🔋 Siemens SIMATIC CMOS Battery A5E00331143 — £22.95
For Siemens S7-300/400 and SIMATIC Panel systems. View battery →
🔋 Mitsubishi PLC CR17335 ER2/3A Backup Battery — £29.95
Standard lithium cell for Mitsubishi Q-series base racks. View battery →
4. Servo Drive Battery (Less Common)
Some older Mitsubishi MR-J2 and MR-J3 servo drives use a lithium battery (A6BAT / MR-BAT) to retain absolute position data in the drive itself. This is separate from the encoder battery in the motor.
🔋 Mitsubishi MR-J2/MR-J3 Drive Battery — 3.6V — £29.95
Pack of 2. View battery →
Warning Signs Before Failure
The batteries don't just die without notice. Here's what to watch for:
| Control | Warning |
|---|---|
| Fanuc 0i/16i/18i/21i | Battery 0 or APC alarm on startup — replace immediately |
| Fanuc 30i/31i/32i | BAT LED lit on main board — you have hours, not days |
| Siemens 828D/840D | NCK battery warning in alarm log — battery symbol on HMI |
| Mitsubishi M70/M80 | Absolute Position Not Established — encoder battery low |
| Heidenhain iTNC | Battery voltage too low message at boot |
When to Replace (Preventative Schedule)
| Battery Type | Replace Every |
|---|---|
| Absolute encoder (Fanuc) | 12 months |
| CNC memory backup | 24 months |
| PLC backup | 36 months |
| Servo drive battery | 24 months |
Note: Fanuc officially recommends annual replacement of absolute encoder batteries. The battery might last longer, but the cost of a battery (£23) vs the cost of lost absolute position (hours of downtime) makes annual replacement cheap insurance.
How to Replace Safely
Golden rule: Always replace batteries with the machine powered ON. This is the most important thing to remember. When the machine is running, the batteries aren't being drained — the control's power supply takes over. Swap while powered down, and you've just wiped the memory yourself.
- Power the machine on normally — let it fully boot
- Locate the battery compartment (usually on the main board or in the electrical cabinet door)
- Note the battery orientation — take a photo with your phone
- Disconnect the old battery — the control will maintain memory from its power supply
- Connect the new battery within 30 seconds — don't leave the holder empty
- Record the date — use a label maker or marker on the battery itself
💾 CNC Parameter Backup — £7.50
If your battery has already died and you've lost your parameters, we stock parameter backups for 20+ specific machine models — Fanuc, Siemens, Heidenhain, and Mitsubishi. Browse parameter backups →
Do You Need All of These?
If you run a Fanuc-controlled machining centre, you need at minimum the absolute encoder battery and the memory backup battery. If there's a separate PLC rack, add the PLC backup battery. Check your electrical cabinet — if you see battery-shaped objects on boards, they need a replacement schedule.
We stock batteries for Fanuc, Siemens, Mitsubishi, Heidenhain, and NUM controls — and we keep all the common ones in stock for next-day UK delivery.