Why did my robot lose its position after a holiday shutdown?

The backup batteries ran down while the controller was off. Change them, then re-master the axes.

How often should I change encoder batteries?

Follow the maker's interval, often one to two years, and always before a long shutdown.

Guide · 4 min read · Updated October 2026

Absolute encoder battery alarms: what they mean and what to do

Many absolute servo encoders keep count of shaft turns while the power is off by running from a small battery. When that battery runs down, the turn count is lost and the machine loses its position. Most battery alarms are not encoder failures, but a few are.

By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.

01Do this

What to do when a battery alarm appears

Do not switch off. If possible, change the battery with the control powered, so the turn count is kept.
Use the right battery. Match voltage and type; most are 3.6 V lithium cells.
Check the cable. A broken battery lead gives the same alarm as a flat battery.
Re-reference if the position was lost. Follow the machine's homing or mastering procedure.
Watch for a repeat. If the alarm comes back within weeks, the encoder's backup circuit may be drawing too much current.

02When it is the encoder

Signs the encoder is at fault

A new battery runs flat much faster than normal
The alarm appears with a good battery and a good cable
Position is lost even though the control stayed powered
The encoder also reports data or communication errors

03Help

When to send it to us

An encoder whose backup circuit drains batteries needs testing and usually repair. Send the nameplate or WhatsApp +91 97866 22233. Related: encoder alarms, encoder repair.

Free downloads for your maintenance file

Printable one-page sheets from our bench. More on the resources page.

Servo motor encoder quick reference

Encoder families, interfaces, counts per revolution and the motors they sit on: Fanuc pulsecoders (alpha A64, A1000), Mitsubishi OBA/OSA, Yaskawa UTOPH, Tamagawa, Heidenhain ECN/EQN/ERN and SICK SKS/SRS/SRM.

PDF · 2 pages · 8 KB · free, no sign-up

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Servo encoder alarm quick sheet

Common encoder and feedback alarms on Fanuc, Yaskawa, Mitsubishi and Siemens drives, what each usually points to, and what to check on the cable and connector first.

PDF · 1 page · 4 KB · free, no sign-up

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04Enquiry

Send the nameplate or encoder label

A clear photo of the motor nameplate and, if the cover is off, the encoder label is enough to start. We reply with whether it can be repaired and a quote.