The Allen-Bradley PLC-5 battery indicator (BATT) warns you when the battery has gone below the low threshold. This BATT LED indicator first lights when the processor has 10 days worth of battery back-up power remaining, regardless of whether the processor is powered or not.
This article focus on how to determine if the PLC-5 battery has failed and how to replace it. I will also outline what the LED looks like when the PLC-5 battery has failed.
Here are some key points about the PLC-5 battery.
- PLC-5 battery can be replaced on a live PLC (that is, without having to power cycle the PLC-5 controller)
- “BATT” LED turns solid red when the battery voltage is below the low threshold.
- When the battery voltage is above the lower threshold, the “BATT” LED goes off
- PLC-5 battery is approx 3.3 VDC
- PLC-5 battery lower threshold is approx 2.8 VDC to 2.9 VDC
See my YouTube below, which demonstrates the following points.
- Show to measure battery voltage
- What does a dead battery voltage looks like
- What a good battery voltage (right off the shelf) looks like
- The “BATT” LED states as bad battery vs good battery is plugged into the PLC-5
Purpose of The Battery
The battery connects directly to the PLC-5 CPU board and helps to retain important system settings and information such as system time, date, PLC configuration, PLC logic (program) and setpoints while the PLC-5 is turned off (remove 120VAC power supply).
If your battery is bad (below low threshold voltage), you will lose all these important information (including PLC logics and setpoints) when the PLC experience power outage.
Table below shows average battery life (table is from Classic 1785 PLC-5 Programmable Controllers, page 2-13) when power is not applied to the PLC-5.
Can PLC-5 battery be changed with 120 VAC
Yes. Allen-Bradley PLC-5 battery can be changed when the power is still applied to the PLC. Under the same note, you do not see any capacitors for PLC-5. See my YouTube video below.
After Market Battery
After market 3.6VDC battery can be used for PLC-5 but it has to be Lithium ion type. One take away from this is that soldering leads on a Lithium ion battery is NOT recommended for beginners as there is a serious danger of overheating which can damage the internal barriers and cause excessive internal leakage, which can quickly cascade into a damaged battery, or just about ready to explode. I would shrink wrap it rather than solder the lead.
I don’t work for Allen-Bradley nor Rockwell, and I am not saying you cannot use after market batteries but I would exercise extra cautious when using after market batteries. Here is one example of after market battery for PLC-5 with shrink wrapped leads.
Battery for PLC-5 From Allen-Bradley
The recommended battery catalog number is as shown in table below (table is from Classic 1785 PLC-5 Programmable Controllers, page 2-13).
The battery catalog number used in this YouTube video is “1770-XYC”. See images below.