KUKA KRC2 Low Battery (Accu) — Buffering Fault and Mastering-Loss Risk
A weak KUKA KRC2 accu can break the controlled shutdown and risk mastering loss. How to replace both battery blocks and when you actually need to re-master.
Low battery (accu) — buffering fault & mastering-loss risk
The accu buffers a controlled shutdown; let it die and the next power loss can cost your mastering.
281 Check accumulator, effect “possible loss of mastering”. Replace both blocks, charge, then re-master only if lost.The buffering path (redrawn)
The messages
Buffer battery voltage low(U<22 V) → charge battery (early warning).Check battery(U<19 V) → charge / exchange.281 Check accumulator→ too old/defective; effect possible loss of mastering, cold start.284 … during last buffering→ too low to buffer last shutdown → exchange.282 Undervoltage while charging→ KPS charging side, not the battery.
Replace safely
- Back up first (incl. mastering); switch off, secure, de-energize, ESD.
- Exchange both battery blocks — never just one.
- Observe polarity — reversed damages accu, KPS600, KPS-27.
- Let the new pack charge before relying on buffering; acknowledge the message.
- Don’t keep swapping batteries if it recurs → check KPS charging.
Diagnostic / maintenance path
Sources: KUKA KRC2 ed05 Operating Instructions — p.19/33/95/104/114; KUKA KSS System Messages — p.50–51 (281/282/284); KUKA KSS 5.5 Operating & Programming Instructions — p.69–77 (mastering).
Low battery (accu) — what it buffers, and the mastering-loss risk
An aged accu is a maintenance item, not a mystery fault — but leave it and the next power loss can cost you mastering.
The question
What does the battery actually do, will the robot lose its position/mastering, and how is it replaced safely?
Short answer
What the accu does (verified)
Straight from the manual: “The robot controller is provided with an uninterruptible 24 V power supply by the batteries. The batteries ensure a controlled shutdown of the robot controller in the event of a power failure. They are backed up by the KPS600.” (Operating Instructions, §2.3.10, p.19). In other words the accu is what lets the controller ramp the drives down cleanly and save state when mains is lost — not a trivial backup.
"CMOS Checksum Error" when flat (p.104); the encoder/RDC battery shows an Encoder battery fault message. Different parts, different symptoms.What the messages mean (verified)
| Message | Cause / effect (manual) |
|---|---|
Buffer battery voltage low (U < 22 V) |
Battery undervoltage → remedy “Charge battery” (early warning) |
Check battery PMx (U < 19 V) |
Deeper undervoltage → “Charge battery / Exchange battery” |
281 Check accumulator <KPS> |
“The battery can no longer be charged correctly. The battery is too old or defective.” Effect: “Possible loss of mastering. Cold start.” Remedy: Exchange battery |
284 Accu-voltage below … during last buffering |
“The battery voltage was too low to buffer the cabinet last time it was shut down.” Effect: Possible loss of mastering. Cold start. Remedy: Exchange battery |
282 Undervoltage <KPS> while charging |
Comes directly from the KPS → check KPS supply voltages; if necessary exchange the KPS (the charging-circuit case) |
Sources: KUKA KSS System Messages — 281 (p.50), 284 (p.51), 282 (p.50); KUKA KRC2 ed05 Operating Instructions — KPS600 message window “Buffer battery voltage low / Check battery” (p.114); accu function (p.19).
Where it sits: the buffering path
Mains → KPS600 (charging) → accu → on power loss, buffers a controlled shutdown
Redrawn from the accu function & KPS600 description (Operating Instructions, §2.3.10 p.19; §2.7.1 p.33). Original diagram, not a manual reproduction.
Step 1 — replace the accu (both blocks)
Battery replacement is work inside a live-capable cabinet. Switch off and secure the controller against being switched on again, complete a back-up first, de-energize the power cable, and observe ESD guidelines (Operating Instructions, §10.10, p.95). Qualified personnel only.
- Back up first, including the current mastering data, so nothing is lost if the pack was already marginal (the procedure requires “Back-up must be completed” before you start).
- Open the cabinet, unplug the battery connection cables, press the spring clamp, and take out both battery blocks — always exchange both, never just one.
- Insert the new blocks, lock with the spring clamp, reconnect the cables. Observe polarity: the manual warns that reversed polarity “can damage the batteries, the KPS600 and the low-voltage power supply unit.”
- Power up and let the new pack charge before relying on buffering; then acknowledge the “Check accumulator” message and confirm the low-accu warning clears.
Step 2 — re-master only if mastering was actually lost
The low-accu warning is preventive: you usually keep mastering until the pack fails outright (mastering is auto-backed up in the RDC when the brakes are applied or the program stops). But if a cold start did lose it, the manual lists exactly when re-mastering is required — among them “after maintenance work during which the robot loses its mastering” (Operating & Programming Instructions, §5.2, p.70). Re-master with the EMT (electronic measuring tool) or the dial gauge:
- If old mastering data remain, delete them first (manually unmaster the axes) before a fresh mastering.
- Jog each axis in T1 to the pre-mastering position (mastering marks lined up), then run the EMT or dial-gauge procedure, axis by axis.
Re-mastering moves the robot (T1 jogging to the mastering position). Ensure the area is clear and jog at safe speed. Qualified personnel only.
Mastering purpose & when required: KSS 5.5 Operating & Programming Instructions, §5.2 (p.69–70); EMT / dial-gauge methods (§5.2.3–5.2.4, p.72–77).
If it recurs quickly after a new pack
A brand-new accu that drops low again is a charging problem, not a dead battery. Message 282 Undervoltage while charging “comes directly from the KPS” — check the KPS supply voltages for interference/failure (especially the AC supply) and, if necessary, exchange the KPS. So don’t keep replacing batteries if the charging side is the real fault.
Open points / to verify
- Charge-time figure and the exact accu part number are not in the loaded corpus (spare-parts catalog / field practice).
- The record notes the concept “also applies to KRC4”; that is out of this KRC2 corpus — the KR C4 battery/charging details would need the KR C4 manuals.
- Whether your specific warning is the accu (281/284) versus the motherboard CMOS cell or an encoder battery — confirm from the exact message text before ordering a part.
Sources: KUKA KRC2 ed05 Operating Instructions — §2.3.10 batteries (p.19), §2.7.1 KPS600 (p.33), KPS600 message window (p.114), §10.10 exchanging the batteries (p.95), motherboard/CMOS (p.104); KUKA KSS System Messages — 281/282 (p.50), 284 (p.51); KUKA KSS 5.5 Operating & Programming Instructions — §5.2 mastering (p.69–77).
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