I've been working with Siemens PLCs in industrial automation for about six years now. In my first year, I made a classic rookie mistake: I swapped a battery on a running S7-1200 without checking the manual first. The PLC lost its program. That mistake cost us roughly $1,200 in downtime and a re-download fee from a contractor — not to mention the embarrassment of explaining to the plant manager why a line was down for 4 hours.
Since then, I've documented every battery swap, every multimeter test, and every odd behavior I've seen. This FAQ covers the questions I wish someone had answered for me back in 2019.
Put simply: a Siemens PLC (like the S7-1200, S7-1500, or the older S7-300) is an industrial computer that controls machinery. The battery keeps the real-time clock running and maintains the program in volatile memory when the main power is off. Some newer models use non-volatile memory (like the S7-1200 firmware version 4.0+), meaning the program survives a power loss. But the clock does not. If your process relies on timestamps — and most do — a dead battery means losing that data.
I should add: not all modules have a battery. The S7-1200 uses a capacitor in some revisions, not a replaceable cell. Check your specific module before ordering a battery.
It depends on the series and how often the PLC loses power. Siemens recommends replacing the battery every 3 to 5 years for the S7-300 and S7-400 series. The S7-1200 typically doesn't need a battery at all (it uses a capacitor that holds the clock for about 20 days). The S7-1500 uses a rechargeable battery for the clock, which should last the life of the module.
Here's what I've learned from experience: If the PLC is in a temperature-controlled room and rarely loses power, the battery can last 6 years. If it's on a machine that cycles power daily — like a packaging line — expect to replace it closer to the 3-year mark. We had a S7-300 in a dusty cabinet near a furnace. Battery died in 2 years. (Should mention: high heat kills lithium batteries fast.)
Short answer: yes for most models, but don't be careless. The S7-300 and S7-400 series allow hot-swapping the battery. The S7-1200—well, it doesn't have a user-replaceable battery in most versions, so that's moot. The S7-1500 also doesn't have a replaceable battery; the clock battery is internal and not meant to be swapped.
Here's where I messed up: I assumed all Siemens PLCs worked the same way. They don't. On an older S7-300, if you pull the battery and the power is lost in the next 60 seconds — sometimes less if the buffer capacitor is weak — you lose the program. I did this once. Lost a program. The machine was down for 3 hours while we reloaded it. That was the day I started using the "verify before swap" checklist.
If you're swapping a battery on a S7-300 or S7-400: do it within 30 seconds. Have the new battery ready. If you take longer, or if the PLC is old, the buffer capacitor might not hold the RAM. I recommend saving a copy of the program to an SD card or your laptop before starting — just in case.
This is a skill every automation technician should have. Here's the process I use:
A healthy 3.6V lithium battery should read around 3.6V with no load. If it reads below 3.0V, replace it — it's already weak. I once tested a battery that read 2.8V; the PLC had been running fine, but the clock was losing about 5 minutes per week. Nobody noticed until the time-stamped logs showed weird intervals. (That was a fun debugging session.)
One mistake I made early on: testing the battery while it was still connected to the PLC. The PLC draws a small current, so the reading can drop by 0.1-0.2V. If you want an accurate reading, test the battery after disconnecting it — but remember the 30-second rule from question 3.
A multi-chemistry battery charger can handle different types of rechargeable batteries — NiMH, Li-ion, LiFePO4, lead-acid, etc. In the industrial automation world, you might deal with: backup batteries for UPS systems (lead-acid or Li-ion), batteries for handheld testing equipment, or batteries for mobile robots (AGVs).
For PLC work specifically: I've never needed a multi-chemistry charger for the PLC itself, because most PLC batteries are non-rechargeable lithium cells. But I've used one to maintain the backup battery for a 24V DC power supply in a control cabinet. That battery was a 7Ah lead-acid unit. The charger I used (a Nitecore SC4, as of early 2023) handled it fine — but I had to manually set the voltage and current. If you're not careful, you can overcharge a lead-acid battery, which causes gas buildup and potential leakage. Don't ask how I know this. Let's just say: ventilation is your friend.
If you're maintaining a battery backup for a Siemens PLC rack, you're better off using a dedicated charger for that specific battery chemistry. Multi-chemistry chargers are convenient, but they're not a replacement for proper equipment.
I've seen (and made) three common errors:
Error #1: Using the wrong battery type. Siemens PLCs use specific batteries. The S7-300 typically uses a 3.6V lithium cell with a special connector. Don't substitute a generic 3.6V cell unless you verify the connector and voltage tolerance. I once saw a technician use a 1.5V AA alkaline battery because "it fit." It didn't work, obviously. The PLC showed a battery fault within 24 hours.
Error #2: Touching the contacts with bare fingers. This sounds paranoid, but it's real. Oils from your skin can corrode the contacts over time, causing intermittent battery connection failures. Use gloves or handle the battery by the edges. I learned this after a spate of battery faults on a S7-400 rack. Siemens support asked if we touched the contacts. Yep. After we cleaned them, the faults stopped.
Error #3: Not verifying the backup. Even if you do a hot-swap, verify that the program and clock are intact after the battery replacement. Power cycle the PLC (if possible) to confirm the program loads correctly. I skipped this once on a S7-300. A week later, a power loss caused the PLC to lose its program because the new battery wasn't seated properly. The error was caught when the line didn't restart. That was a Friday. The weekend shift was... not happy.
An inverter (like a 3000-watt unit) converts DC power to AC. If you're running a Siemens PLC off a battery backup — say, in a remote monitoring station or a mobile setup — you might need a DC-to-AC inverter to power the PLC's power supply, which expects 120V or 230V AC. Alternatively, you can use a DC power supply that accepts 24V DC input directly (many Siemens power supplies, like the SITOP series, accept DC input).
My experience: for a permanent installation, avoid the inverter. Use a DC power supply that matches your battery voltage. Inverters introduce noise, which can cause spurious faults on sensitive PLC inputs. I had a client who used a cheap 3000W inverter to power an S7-1200. The PLC kept showing "hardware fault" errors. We swapped the inverter for a SITOP power supply running directly off the 24V battery bank. Problem solved.
If you must use an inverter (e.g., for a temporary setup), use a pure sine wave inverter, not a modified sine wave. A 3000-watt pure sine wave inverter is sufficient for a single PLC rack plus a small HMI, but check the inrush current of the PLC power supply — some can spike at startup.
Yes and no. A voltage test tells you the open-circuit voltage, which is useful (as discussed in question 4). But it doesn't test the battery under load. A battery can show 3.6V with no load but drop to 2.5V when the PLC's clock circuit draws current. That's a failing battery.
Here's a trick I learned from a senior technician: if your multimeter has a battery test function (some Fluke models have this), use it. It applies a small load and reads the voltage. If not, measure the voltage while the battery is connected to the PLC (but be quick — the 30-second rule applies). A healthy battery should only drop by 0.1-0.2V under load. A drop of more than 0.5V means replace it.
I once tested a S7-300 battery that read 3.5V unloaded but dropped to 2.8V when connected. The PLC had been showing "battery low" warnings on and off. We replaced the battery, and the warnings stopped. The old battery? It went into my "dead battery" box for disposal. That was in Q3 2024. The new one is still going strong, as of January 2025.
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