31.10.2025
Updated on 31.10.2025
3 Min.
The Siemens SIMATIC S7-300 assemblies have been a reliable backbone of numerous production systems and machinery for many years. One of the most frequently asked questions about these PLC assemblies is: how do I replace the buffer battery? We asked our technical experts for answers and a detailed step-by-step guide. But first, let's have a brief look at the topic in general.
The buffer batteries of the SIMATIC S7-300 secure the data stored in the CPU's user memory. If an S7-300 is disconnected from the power supply without a buffer battery installed, the PLC programme will be lost unless a flash memory card is inserted, and the internal clock will stop. In addition, information such as retentive markers, counters, and timers would also be lost.
An empty buffer battery can lead to data loss in the PLC when the system is switched off. The consequences range from extended recommissioning times to complete production stoppages.
By replacing the buffer battery in good time, you can avoid these risks with your SIMATIC S7-300 assemblies. Manufacturers generally recommend replacement at intervals of around five years. In practice, however, the actual replacement interval can be significantly shorter, depending on environmental conditions and operating duration.
An empty buffer battery is indicated by the illuminated BATF LED on the front of the SIMATIC S7-300 CPU. In addition, the system generates a battery warning that can be read via the control software. The rule is simple: as soon as the assembly or software issues a warning, the battery must be replaced immediately to prevent data loss.
Before starting the battery replacement, ensure that a new 3-volt lithium battery (6ES7971-1AA00-0AA0 or equivalent specification) is available. The battery should be unused and within its specified storage period.
The replacement must be carried out with the CPU switched on to ensure that the data in the working memory (RAM) is retained.
Before opening the unit, it is advisable to take ESD protection measures to prevent damage to electronic components.
The buffer battery on all SIMATIC S7-300 CPUs is located behind the front cover, which can be opened without the need for tools. Above it, you will usually find the slot for the flash memory card.
The battery compartment is secured by a small cover that can be carefully unclipped or pulled out.
The old battery is pulled straight out of its holder. Take care to observe the polarity - the positive terminal is usually clearly marked.
The contact surfaces must not be bent or damaged. Do not lever out the battery with metal tools to avoid short circuits.
However, there are also SIMATIC S7-300 assemblies in which the battery is fitted with a cable. This cable is connected to the CPU via a polarity-protected plug and can be easily disconnected.
The new battery is inserted into the compartment with the correct polarity.
A firm fit ensures a reliable contact. Afterwards, replace the cover.
Important: the entire replacement should be carried out promptly to prevent the CPU from losing its supply from the internal buffer.
For SIMATIC S7-300 assemblies with a cabled battery, first connect the new battery to the CPU using the plug connection. Then insert the battery into the compartment and close the cover.
After inserting the battery, the BATF-LED on the CPU should go out.
If the LED remains lit, this may indicate a discharged battery, incorrect polarity, or contact problems.
After the replacement, the system clock should be checked and, if necessary, reset.
Stored markers, timers, and counters remain intact if the CPU was powered during the replacement. A brief functional test confirms that the procedure was carried out correctly.
The replacement date, the CPU serial number, and the type of battery used should be documented in the maintenance plan. This facilitates preventive maintenance and helps schedule the next replacement in good time.
The old battery should then be disposed of in an environmentally responsible manner as hazardous waste.
CPU switches to STOP Analyse the diagnostic buffer and assess the cause. Load the project consistently and perform a warm start.
EICHLER specialised in the repair, maintenance, and trading of discontinued assemblies. For Siemens SIMATIC S7-300, we provide spare parts, carry out preventive maintenance measures, and professionally repair Siemens SIMATIC S7-300 assemblies. In addition, we support you with all questions relating to obsolescence.
Customers benefit from tested spare parts, predictable costs, and planned maintenance. In this way, the service life of existing systems can be significantly extended. If you prefer not to carry out the replacement of the buffer batteries in your SIMATIC S7-300 assemblies yourself - no problem: simply send your assemblies to the EICHER Service Centre for preventive maintenance or arrange an appointment with our On-Site Service team.
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