About memory back up batteries  
All Fanuc CNCs (with the exception of Fanuc model 6 and 11 which utilizes Bubble Memory Units) uses memory backup batteries to maintain the contents of the CNC's Random Access Memory (RAM). This is where NC, PMC parameters, part programs, tool offsets etc are stored.

Fanuc 0, 10, 15 controls uses three 1.5V D-Cell Alkaline batteries. These batteries are in an enclosure (click here to view) usually mounted on the machine where it is easily accessible from outside without having to open the electrical cabinet.

Fanuc 16, 18 (model A, B, C, i series) controls normal use lithium battery which is connected to the control unit (click here to view) and located inside the cabinet. However, I have seen on some Fanuc 16/18 controls where machine builder installed an external battery box (as described earlier for 0, 10, and 15 controls) and you can use regular Alkaline batteries. Please refer to relevant machine builder's manual and/or Fanuc manual on battery replacement procedures.

Under normal circumstances, control will generate a battery low message or alarm if the battery need replacement. However this message is nothing more than a small display on the bottom corner of the screen and says something like "batt". It is easy to overlook. Most of the time customer will realize that battery has gone low or sometimes actually went completely dead when machine is shut off for holidays or weekends. The common symptom will be that you will get a variation of "RAM parity error" or "System alarm 9xx"  depending on the control model.

Should this occur, the only way to recover from this situation is to clear memory and reload all the data. Be sure to refer to relevant Fanuc maintenance for alarm detail and procedures on how to clear and reload and data and of course you still have to replace the batteries.


It is recommended that you keep a regular schedule and replace memory backup batteries at least once a year even if you do not get a battery low alarm or message. The best time is usually before long holidays or while performing preventive maintenance on the machine. Most memory backup batteries such as three D-Cell Alkaline batteries (DO NOT use rechargeable batteries) will not cost you more than a few dollars. Even if control uses lithium batteries (commonly on seen on Fanuc 16/18/15B controls), which costs about $30-$40, this is far less then the cost of down time and cost of a service call associated with losing data and reloading them.

Memory backup procedures for various Fanuc CNCs


Control must be turned ON when memory battery backup batteries are replaced. This mean that control screen must be on and not just the machine breaker turned on. For Fanuc 0, 10, 15 controls where the battery compartment is easily accessible from outside, battery replacement is relatively easy since you do not need to open the control cabinet where high voltage present and risk of electric shock. Just be sure to pay attention to the polarity of batteries when you are taking them out and putting the new ones in.

If you have Fanuc 16/18 (model A, B, C, i) and 15 (model B, i), then replacing batteries is trickier. The location of the battery could be any where from behind the CRT/LCD display to inside the electrical cabinet. Since you will have to replace the battery while the power is applied to the control, be sure to take all necessary precautions and the person who is performing the task should be familiar with the danger involving working around high voltage. Be sure to consult all relevant machine and control manuals.

Better yet, if you have several Fanuc controls, schedule a service call to perform data backup and battery replacement (by the way, to those customers in Houston and surrounding areas, we at KFAS can schedule "CNC System Evaluation and Preventative Maintenance Recommendation" Service).