The $20 Battery That Kills a Robot's Memory: Encoder and CPU Battery Replacement Explained

Figure 1. Replacing a backup battery inside the controller: a small part with an outsize role in robot accuracy.

A plant in Poland shut its FANUC robot down for a long weekend and came back to an alarm every maintenance engineer knows: SRVO-062 BZAL. The backup battery that keeps the encoder's absolute position data alive while the controller is off had died, and the robot had forgotten where its own joints were. The team spent most of Monday remastering axes, resetting references, and re-verifying a weld program that had run perfectly the week before. The part that caused it cost about USD 20. The lost shift cost far more.

Why robots need backup batteries at all

Most six-axis robots use absolute encoders that remember each axis position even when power is off. That memory depends on a small battery in the controller or on the encoder. When the battery dies while the machine is off, the position data is lost and the robot cannot move safely until every axis is remastered to a defined home position. For welding cells, that means re-teaching references and re-validating programs, which is measured in hours, not minutes.

When to change them (before the alarm, ideally)

The interval table reflects OEM guidance across FANUC, ABB and KUKA controllers. The useful habit is simpler: change backup batteries on a fixed calendar, not when the alarm appears. Many plants tie the swap to the annual preventive service, and some use a voltage check every few months as an early signal.

The rule that saves your calibration

One rule matters more than any interval: replace the CPU and encoder batteries with the controller powered on, following the manufacturer's procedure. Doing it powered off risks losing exactly the position data the battery protects. This is the detail that separates a two-minute swap from a four-hour remaster. Every refurbished robot we ship gets fresh batteries and a verified position baseline, so the buyer starts with a known, documented state.

Exceeding expectations with a documented baseline

The Polish plant's next robot came with new batteries, a service log, and a note on the exact remaster procedure for its controller. When they eventually sold the old machine, that documentation traveled with it and supported a clean trade-in value. A battery is a small part, but the discipline around it is a window into how a machine was cared for, which is exactly what a smart buyer of used robots checks first.

Different controllers carry different battery arrangements, so the habit matters more than the model: know which batteries your robot has, mark their replacement date on the service calendar, and check the voltage during routine PM. On FANUC controllers the APC batteries sit at the encoders or in the amplifier cabinet, on ABB the SMB battery is part of the serial measurement board, and on KUKA the equivalents live in the controller. A five-minute voltage check at each service catches the decline months before the alarm.

Ask whether the backup batteries on your shortlisted robot are new, and whether the position baseline is documented. We answer both questions in writing.

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