Encoder Battery

The encoder battery is a small lithium pack (typically a 1.5 V or 3.0 V unit, model-dependent) that sits at the arm and keeps every pulsecoder's rotation counter powered when the controller is off. Some robots mount it in a case on the base casting; older FANUC arms clip the pack directly onto the J1 or J3 motor housing.

The encoder battery lives on the arm and is a distinct part from the controller battery inside the cabinet. The controller battery protects programs and mastering data in SRAM; this one protects the absolute position count inside each pulsecoder. Both can fail independently, and losing the encoder pack is the one that costs you a re-mastering pass on every affected axis.

The controller warns you before it becomes a problem. A BZAL warning (battery zero, in the SRVO-062 family) means the pack is below threshold and needs to be swapped soon; an RCAL warning (rotation counter alarm, SRVO-063 family) means the axis has already lost count and needs re-mastering. Catch the BZAL and you're doing a battery swap; miss it and you're doing a mastering job.

Selection is by robot model rather than by generic voltage. FANUC packs use a keyed cable and a specific chemistry (commonly 1.5 V lithium D-cells in a 4-cell pack, or a 3 V pack for newer arms). Aftermarket packs are fine when the vendor uses the same chemistry and pack construction; generic alkalines will droop under the pulsecoder's inrush current and cause phantom BZAL warnings.

Compatibility limits: pack voltage, connector style, and mounting location are all model-specific. An M-20iA base-mount pack will not fit an R-2000iC's motor-mount case, and the connector pinout differs by pulsecoder generation. Order by robot model and serial-number band rather than by cell size alone.

Replacement is done live. Power the robot on, unclip the old pack from the case, and connect the new one within the specified hold-up window (typically a few minutes at most; the pulsecoder's local cap holds up briefly). Verify the low-battery warning clears at the next power cycle and confirm mastering by jogging each axis to a known reference position.

Practical example: a plant that batched battery swaps once a year across a cell of ten robots re-mastered two arms after doing the swap with the cabinet off, then switched to a live-swap SOP and hasn't lost position since. If you do end up losing count, the single-axis mastering guide walks through the recovery axis by axis.