PLC Battery Replacement Guide: Types, Part Numbers & How to Replace

If your controller is flashing a BATT LED, logging a low-battery warning, or you are building a spare-parts list and are not sure which PLC battery to order, you are in the right place. This guide does two jobs: it helps you identify the exact backup battery your controller needs, and it shows you how to replace it without losing your program.

Use the brand cross-reference chart below to find your part in seconds, then follow the selection and replacement steps to get the job done safely.

What a PLC Battery Actually Does (and the Names It Goes By)

A PLC battery keeps the controller's volatile memory and real-time clock alive when main power is off. It does not power the PLC during normal operation. Its only job is to hold your program, configuration, and clock data during a power loss or while the unit is disconnected for maintenance or relocation.

You will see the same component called by several names. A CMOS battery, RTC battery, backup battery, processor battery, and memory battery all point to this one part. Different manufacturers label and package it differently, which is exactly why matching the correct part number matters more than it first appears.

Volatile vs non-volatile memory, and where the battery fits

Two memory types sit inside most controllers. Volatile memory (typically SRAM) loses its contents the moment power is removed. Non-volatile memory (flash or EEPROM) keeps the program even with no power. The backup battery powers the parts that need continuous voltage, usually the SRAM and the real-time clock.

One detail saves a lot of wasted orders: many modern PLCs store the program in flash or FRAM and do not rely on a battery at all.

Types of PLC Batteries: Chemistry and Voltage

Almost every PLC backup battery is a lithium-thionyl chloride (LiSOCl₂) cell, and the two voltages you will meet are 3.0 V and 3.6 V. Before you buy, confirm one thing: whether your CPU even uses a battery, because some store everything in FRAM or on a memory card.

Why lithium-thionyl chloride dominates

LiSOCl₂ wins on the things that matter for backup duty: very high energy density, extremely low self-discharge, a wide operating temperature range, and steady output at the tiny currents a memory chip draws. That combination is why a single cell can sit in service for years. It is also why shelf life can reach a decade, and yet you still replace on schedule rather than waiting for failure.

3.0 V coin cell vs 3.6 V lithium pack

The 3.0 V format is usually a coin cell (CR or BR types such as CR2032). The 3.6 V format is usually a wired cell or pack (ER types such as ER6V or ER14505). They are not interchangeable. A cell that physically fits can still be the wrong voltage or polarity, so never swap a 3.6 V for a 3.0 V (or the reverse) just because it slots in.

Battery vs supercapacitor vs FRAM backup

Not every controller holds memory the same way. Here is how the three common methods compare:

If your CPU uses a memory card or FRAM, you may not need a replacement battery at all. Check before ordering.

Chemistry tells you the category. What you actually need to solve is narrower: which exact battery does my controller take?

PLC Battery Cross-Reference Chart by Brand

Use this chart to match your controller to its backup battery, then confirm the exact part against the catalog or order number printed on the CPU. Battery type can change across series and firmware revisions, so treat these as common references, not absolutes.

Always confirm the battery against your CPU manual. The same chassis can use different batteries across series and firmware revisions.

Allen-Bradley / Rockwell

Keep the families straight: 1756-BA2 for ControlLogix, 1747-BA for SLC 500, and 1770-XYC for PLC-5. They are not cross-compatible. Newer ControlLogix L7 and L8 CPUs use an Energy Storage Module (ESM) instead of a battery, so check the CPU catalog number first. See our Allen-Bradley PLC parts.

Siemens

Read this one carefully. Many S7-300 CPUs use a Micro Memory Card and need no backup battery, while the S7-400 and older S5 do use one. For an S7-400, a common backup battery is the 6ES7971-0BA00, but verify the order number on your specific CPU. Browse Siemens PLC parts.

Mitsubishi

Match the series, not just the brand: Q6BAT and Q7BAT for the Q series, A6BAT for the A series, and a wired pack such as FX3U-32BL for FX. The Q6BAT cell is a 3.6 V LiSOCl₂ type. See Mitsubishi PLC parts.

Omron

Each Omron family uses its own pack with its own connector: CJ1W-BAT01 (CJ), CS1W-BAT01 (CS), CP1W-BAT01 (CP1H / CP1L), and C200H-BAT09 (C200H). The connectors are not interchangeable, so order by series. Browse Omron PLC parts.

Schneider / Modicon and ABB

Both use family-specific batteries that you should confirm against the CPU. Modicon splits across Quantum, Premium (TSX), and M340 (BMX), while ABB's AC500 PM5xx uses the TA521. Do not assume one Schneider battery covers the whole range. See Schneider PLC parts and ABB PLC parts.

FANUC CNC

FANUC systems usually carry more than one battery: a control memory battery (commonly A98L-0031-0012, 3.0 V) and separate absolute pulse coder batteries. Follow the machine manual exactly, because swapping the wrong one or cutting power at the wrong moment can wipe parameters and reference positions.

Can't find your model? Send us your CPU part number for a fast quote and we will confirm the correct battery.

The chart gives you candidates. Next, make sure what arrives is the right part, and a genuine one.

How to Choose the Right Replacement Battery

Confirm three things before you buy, and decide up front whether a compatible cell or a genuine OEM part is the right call for your site.

Read the CPU label and manual: match part number, voltage, connector

Match all three: part number, voltage, and connector (including polarity). Pull the values from the catalog or order number on the CPU and from the manual, not from what looks similar on the bench. A battery that physically fits can still be the wrong voltage or wired with reversed polarity, and either mistake can corrupt memory or damage the holder.

Quick checklist before ordering:

• Part number matches the CPU manual or label
• Voltage is correct (3.0 V vs 3.6 V)
• Connector style and polarity match
• Form factor fits the holder or clip
• For FANUC and other CNCs, you have identified every battery the machine uses

OEM vs compatible vs counterfeit: how to tell

Three categories, and only one is a real problem. A genuine OEM battery is built and labeled by the controller maker. A quality compatible cell is a correctly specified LiSOCl₂ battery from a reputable supplier, often the same underlying cell in different packaging, and it works fine for most applications. A counterfeit is a low-grade cell dressed up with OEM labeling, and that is the one to avoid.

Watch for these red flags:

• A price far below the market rate
• No visible date code or batch marking
• Sloppy printing, mismatched fonts, or a label that peels
• No traceable source or datasheet

A quality compatible cell from a trustworthy source is a sound choice. A counterfeit wearing an OEM label is the real risk. If you need verified genuine parts, talk to us; we can confirm authenticity and source before you commit, and you can read more about how we source.

Once you have the right, genuine part in hand, the priority during the swap is simple: do not lose the program.

How to Replace a PLC Battery Without Losing Your Program

Decide first whether to replace with power on or power off, because that choice is what protects (or risks) your memory.

Power-on (hot-swap) vs power-off replacement: when to use which

Many PLCs are designed for live battery replacement specifically so memory stays powered during the swap. If your manual allows a hot-swap, that is usually the safest route for your data. If you must remove power, make a verified program backup first and plan to reload afterward.

The trade-off is real on both sides. A powered-on swap avoids data loss but carries a shock risk and demands care. A powered-off swap is safer for the technician but risks memory loss if you do not have a current backup. Confirm what your manual supports before you start.

Step-by-step replacement procedure

1. Confirm the battery actually needs replacing (BATT indicator active, or a low reading on test).
2. Check the manual to see whether your CPU supports a live swap or requires power off.
3. If power off is required, back up the program and verify the backup first.
4. Locate the battery and note the connector orientation and polarity.
5. Disconnect or remove the old battery.
6. Verify the new battery against the three checks (part number, voltage, connector).
7. Connect and secure the new battery, then restore power if it was off.
8. Confirm the BATT indicator clears, then record the replacement date.

Multi-battery systems (for example, FANUC CNC)

CNC and some larger systems use separate batteries for control memory and for absolute position encoders. Replace them in the order the machine manual specifies, and keep power applied where the manual requires it. Get this wrong and you can lose machine parameters or the absolute reference position, which means a re-zero or a full reload.

After the swap (or before it), you need to know how to read battery health with a meter.

How to Test PLC Battery Voltage

Measure the cell with a multimeter and compare the reading against the thresholds below to decide whether it is healthy, marginal, or due for replacement.

Using a multimeter (DCV): quick steps

Set the meter to DC volts on a range above the cell's rating (a 20 V range is fine). Place the red probe on the positive terminal and the black probe on the negative, then read the value once it settles. Always test on the DC setting, since every backup cell runs on DC.

Voltage thresholds and when to replace

Use these as a general guide, and confirm exact thresholds in your CPU manual, since alarm points vary by brand:

A fresh 3.6 V cell often reads about 3.6 to 3.7 V. If a new cell reads slightly low, do not condemn it yet; see the passivation note below.

Reading on-board diagnostics (BATT LED and system flags)

You can often judge battery status without removing it. A BATT LED that is off usually means the cell is above threshold, while a lit, flashing, or color-changed indicator (yellow or red on many CPUs) signals low voltage. Brands surface this differently: Allen-Bradley uses a BATT LED, Siemens shows BF or SF fault states, and Mitsubishi raises a battery error in diagnostics. Because these indicators often sit inside a closed cabinet, schedule a check rather than relying on noticing the light.

A reading tells you the state right now. So how long should a cell last, and what shortens it?

How Long PLC Batteries Last and What Shortens Their Life

Plan on replacing most PLC backup batteries every 2 to 5 years, and replace on a schedule rather than waiting for the alarm.

Typical service life and replacement interval

A typical LiSOCl₂ backup cell serves 2 to 5 years in the field, depending on the processor, the environment, and how often it carries the load. A practical rule is to set a proactive replacement window at 2 to 3 years for critical equipment, well before the cell approaches end of life.

Factors that reduce life

• High operating temperature, which raises self-discharge and ages the cell faster.
• Frequent power outages, since each outage draws on the battery.
• Long periods with the system powered off, which puts the battery on full backup duty.
• General system age, where older holders and connections add resistance and risk.

Account for these when you set the interval. A hot panel that loses power often needs a shorter cycle than a climate-controlled room with stable supply.

Passivation and voltage delay in stored lithium cells

A new LiSOCl₂ cell that reads slightly low is not always a bad battery. During storage, a thin passivation layer forms on the lithium, which can briefly delay the cell's voltage when you first put it under load. A short load usually clears it and the voltage recovers. This is one reason storage time and conditions matter, which leads to the next section.

If you ignore all of this and the cell fully drains, here is what happens.

What Happens If the Battery Dies

The outcome depends entirely on whether main power is on, and your recovery path follows from that.

The takeaway is practical: a current offline program backup, plus scheduled replacement, is what keeps a dead cell from becoming downtime.

To have a spare ready at that moment, you need to store and ship the cells correctly.

Storage, Shelf Life, and Shipping Compliance

Store spare lithium cells cool, dry, and isolated, rotate stock by date code, and know the lithium shipping rules before you order across borders.

Storing spare lithium PLC batteries

Keep spares in a cool, dry place, ideally in original packaging with the terminals protected. Avoid heat, humidity, and any chance of a short, and never drop loose cells into a drawer with metal parts that could bridge the terminals. Good storage preserves both capacity and safety.

Shelf life and date-code checks

LiSOCl₂ cells have a long shelf life, often around a decade, but it is not unlimited. Check the date code on arrival, use older stock first (first in, first out), and remember the passivation effect above when a freshly unpacked cell reads a little low.

International shipping (UN3090 / UN3091): what buyers should know

Lithium metal batteries are regulated dangerous goods. Cells shipped on their own fall under UN3090, while cells packed with or inside equipment fall under UN3091, and both require compliant packaging and declaration under IATA and related rules. There are also limits on shipping a controller with its battery connected. As an exporter, we ship lithium cells under the applicable UN and IATA requirements, so your order clears customs without surprises.

Correct storage is one piece of a larger spare-parts plan.

Preventive Maintenance and Spare-Parts Planning

Turn a one-time swap into a simple plan: schedule replacements, and stock the right number of spares for your fleet.

Building a replacement schedule

Base the schedule on install date, operating environment, and how critical each machine is. Fold battery changes into your annual planned shutdown so you are not opening cabinets unnecessarily, and log the replacement date on every CPU so the next cycle is obvious.

How many spares to stock

Size your safety stock by the number of like-for-like controllers in service, the supplier lead time, and the cost of downtime if a battery fails. Long lead-time or obsolete models justify holding more, which is a good reason to line up a supplier who can hold stock and confirm parts quickly.

Planning spares across multiple sites or models? Get a bulk or MRO quote.

When you are ready to buy, here is what to look for.

Where to Buy Genuine PLC Batteries

Buy from a supplier who can match your CPU number, prove the part is genuine, hold stock across brands, and ship lithium cells compliantly. Those four points separate a clean order from a returned one.

At CHENTUO (Shenzhen Chentuo Technology), we stock PLC backup batteries and controllers across Siemens, Allen-Bradley, Mitsubishi, Omron, Schneider, and ABB. We confirm the correct part against your CPU number and handle compliant international shipping. Send your controller's catalog number and we will confirm the battery and current stock.

Request a quote or check stock, or reach us directly at jerry@szct-automation.com or on WhatsApp for a fast reply.

Conclusion

Three habits keep PLC battery work simple. First, match the exact part by number, voltage, and connector, not by appearance. Second, replace with power on when the manual allows it, and back up the program first when it does not. Third, replace on a 2 to 3 year schedule instead of waiting for the alarm. Your next step: confirm your part in the chart above, then send us the CPU number so we can verify it and check stock.

FAQ

 

 

Can PLC batteries be recharged?

In most cases, no. The majority of PLC backup batteries are non-rechargeable lithium-thionyl chloride cells, and trying to charge them is unsafe and can cause leakage or rupture. A few systems use rechargeable assemblies that charge from the PLC supply, so check your manual before assuming either way.

What voltage should a healthy PLC battery read?

A healthy 3.0 V cell reads about 3.0 V or higher, and a healthy 3.6 V cell reads about 3.6 to 3.7 V. Readings that drift toward 2.4 V (3.6 V cell) or 2.0 V (3.0 V cell) mean replacement is due. See the threshold table above for the full ranges.

Will I lose my PLC program when I change the battery?

Not if you do it correctly. If your CPU supports a live (hot-swap) replacement, memory stays powered and your program is safe. If you must power down, take a verified program backup first so you can reload if needed.

Which battery does my Allen-Bradley, Siemens, or Mitsubishi PLC use?

Start with the cross-reference chart above (for example, 1756-BA2 for ControlLogix, 6ES7971-0BA00 for many S7-400 CPUs, and Q6BAT for the Mitsubishi Q series), then confirm against the catalog number on your CPU. If you are unsure, send us the number and we will identify it.

Do all PLCs need a battery?

No. Many modern controllers store the program in flash or FRAM, or use a memory card, and need no backup battery at all. Confirm whether your CPU actually uses one before ordering a replacement, so you do not buy a part you do not need.

Where can I buy the correct PLC replacement battery?

From a multi-brand supplier who can verify the part against your CPU number, confirm it is genuine, and ship it compliantly. Request a quote with your controller's catalog number and we will confirm the battery and stock.