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The battery that keeps the absolute pulse coder count alive is low. Nothing happens while the machine is switched on; the risk is switching it off and losing the axes' absolute position, which forces you to re-establish references and, on some machines, to recover data that is not trivial to recover. It is a warning, and it is worth dealing with inside the same week.
For reference only: alarms change with the manufacturer, the control and the machine PLC. Check the full code in the official manual before doing anything. Legal notice.
If absolute position has already been lost or you do not know the model procedure.
Reset after changing battery and confirming position/reference according to manual.
Choose the manual matching your control series, software version and machine builder. A support portal is a starting point; it does not establish applicability to every model.
An absolute pulse coder knows its position within one revolution from its own disc, but to know how many revolutions it has turned it needs a counter that must never be lost. That counter is fed from the control supply while the machine is on and from a battery while it is off. That is the whole point: the battery is not there to run the machine, it is there to remember while the machine sleeps. The control monitors that battery voltage and warns when it drops below the threshold, precisely so you have time to change it before it dies. If it dies completely with the machine switched off, the revolution counter is lost and at power-up the axis no longer knows where it is in machine coordinates: the control flags it and demands the reference be re-established. And here is the detail that catches many people out: re-establishing it is not always a button press. On machines with absolute pulse coders and no reference dog, the position is recovered with a builder procedure that may involve taking the axis to a specific physical mark and validating the value. That is why the battery is changed with the machine powered up and following the builder's procedure: if you do it with the machine off, the counter has no supply at all and you lose exactly what you were trying to keep.
In order, from the fastest and cheapest check to the most expensive.
01Read whether it is a low-battery warning or an actual loss
A low voltage warning, which lets you keep producing, is not the same as an absolute position lost message, which forces you to reference before operating. Note the exact wording and which axes are affected. If it is only a warning you have some slack; if the position is already lost, do not move the axes in automatic until the reference is re-established.
02Do not switch the machine off
With the warning active, every power-down is a chance to lose the position. If you can, leave the machine on until the battery is changed. If you have to switch it off for some unavoidable reason, warn the next shift that a reference loss may appear at power-up.
03Get the correct battery before touching anything
The type, voltage and format are specified by the machine or control builder. Do not improvise with a similar-looking cell: a different voltage or a poorer contact leaves you in the same position a month later. Bear in mind that some machines have one battery in the cabinet for all axes and others have a battery per axis next to the motor.
04Change it following the builder's procedure, with the machine powered as the manual states
That procedure exists precisely so the counter is never left without supply during the change. On many machines it is a maintenance job, because it means getting into parts of the cabinet or the headstock. If the manual is not clear, or access means opening the cabinet, let maintenance do it.
05Take a parameter backup first if you do not have a recent one
The same kind of battery that backs up pulse coders backs up control memory on some machines. A backup of parameters and programs before any battery work is cheap insurance: if something goes wrong, that copy is the difference between an hour and a day.
06Verify the positions after the change
With the new battery in, check that the machine coordinates are still consistent: take an axis to a known position and cross-check. If anything does not add up, the reference has to be re-established using the builder's procedure before producing, not after.
A battery at the end of its life: the voltage sags when cold and recovers somewhat as it warms. It is confirmed by the time-of-day pattern. Change it now; this symptom usually comes shortly before real position loss.
The battery ran down with the machine stopped, which is when the retention function draws most. Confirm it by measuring the battery and by the position lost message. It means changing the battery and re-establishing the references using the builder's procedure, never by eye.
Wrong battery, badly seated, a dirty holder contact or a degraded retention cable. Confirm it by checking the installed type against the specified one and by inspecting the holder. Maintenance looks at it.
Battery changes scheduled by calendar, typically once a year or at whatever interval the builder specifies, rather than waiting for the warning. Record the date of the last change on a visible label on the machine. Avoid leaving machines switched off for long periods without checking the battery first, especially before long shop shutdowns. Keep the parameter and reference backup saved and up to date. And stock the correct battery for each machine: when the warning appears, having it on the shelf is the difference between a calm change and a stoppage.
Fanuc/OEM warns of low battery or loss of reference in absolute encoder. It is an alarm from the Encoder / measurement area, and in the catalog it is listed as “Absolute encoder battery low”. On the machine you notice it like this: Battery warning, Risk of losing absolute zero, May require homing at start-up.
On the shop floor it almost always comes from one of these causes: Encoder battery dead, Battery change with machine off, Defective cable or battery holder. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Reset after changing battery and confirming position/reference according to manual. Resetting without fixing the cause first only hides the problem: APC / BAT trips again and, in the meantime, you keep working with the machine in bad shape.
With the machine stopped and in a safe state, start with these checks: Back up parameters if applicable, Follow OEM battery replacement procedure, Do not power down if the manual requires replacement with power on. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With APC / BAT active, avoid this: Do not bypass safety devices, Do not manipulate energized electrical cabinet, Do not continue producing if the alarm reappears. Skipping any of these points is what turns a minor stop into an expensive breakdown.
If absolute position has already been lost or you do not know the model procedure. If you are not sure how far it goes, stop the machine and report it: a repeat alarm always works out cheaper than a crash.
It is a warning: the machine carries on, but it is flagging something that will get worse if you let it go. Deal with it between parts, not at the end of the shift. Guidance reference. Always confirm the full text in the exact machine/control manual.
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