Mitsubishi PLC FX5U vs. Dual Battery Charger Backup: What Actually Keeps Your Line Running?

If your line depends on a Mitsubishi PLC FX5U, the word 'battery' should never be a surprise. I say that because I'm the person who gets called after the surprise. My name isn't important; the pattern is. I'm a senior service engineer at an industrial automation integrator. In the past six years, I've handled 42 emergency calls involving a dead or dying PLC battery, and almost every one was avoidable.

Why This Comparison Exists

When an automated line loses power, there are two ways to protect the control system. The first is the internal battery inside the Mitsubishi PLC FX5U. The second is an external backup power setup built around a dual battery charger—usually an XS battery charger with two 12V batteries feeding a 24V DC bus. These aren't direct competitors, but they are two different answers to the same question: what happens when the mains disappears?

I'll compare them on upfront cost, actual behavior during an outage, maintenance, and the scenario where each one belongs. If you're here because you've seen the term 'PLC Mitsubishi' and want to know how to keep that PLC alive, this is for you.

Dimension 1: Upfront Cost and Complexity

The internal battery wins on paper. A replacement FX5U battery usually costs somewhere between $30 and $80, no extra wiring, no engineering. If you simply replace it every three or four years, the PLC keeps its retentive data and clock alive during a normal power-off. That's attractive, and it's the reason most Mitsubishi PLC panels leave the factory with only the internal battery.

The external dual battery charger setup costs more. An XS battery charger with two 12V batteries, a small enclosure, a DC breaker, and wiring can run you $800 to $2,000, depending on the runtime you need. It takes time to engineer and time to commission. But that's not a reason to skip it. It's a reason to compare what you're actually buying.

In March 2024, a customer had a line start 36 hours away. The PLC Mitsubishi FX5U on their label machine showed a low-battery alarm. There was no spare battery in the plant. We paid $80 for the replacement battery and the customer paid $1,200 for the emergency service call. That math isn't unusual. When I compared the true cost of a battery-only plan versus a dual battery charger plan over five years, the battery-only plan was cheaper only if nothing went wrong.

Dimension 2: Behavior During an Outage

Here's the thing: the internal battery in a Mitsubishi PLC FX5U does not keep the PLC running. According to Mitsubishi Electric's FX5 User's Manual, the battery maintains device memory and the clock while the power supply is off. It's backup for your data, not backup for your process. If the outage lasts longer than the battery's remaining capacity—or if the battery is already weak—you lose retentive values and possibly the time/date. Then you get restarts, missing recipes, and a machine that seems to have a mind of its own.

Because the FX5U is a popular PLC Mitsubishi model, there's a common assumption that a battery means the PLC will 'ride through' an outage. It won't. It's just a memory holder.

The external dual battery charger is a different animal. Maintained properly, it keeps a 24V DC bus alive during short power loss. The Mitsubishi PLC FX5U never goes down, the HMI doesn't reboot, and outputs don't drop. In 2023, I was involved in a paint booth line with poor power quality. The numbers said internal battery was fine. My gut said the plant's old compressors would cause repeated voltage sags. We added a dual battery charger backup. In summer 2024, that line went through four sags in one day and never stopped. The maintenance manager didn't even know until I showed him the event logs.

When I compare these two side by side, the conclusion is clear: if uptime matters, the internal battery is not enough. It's a memory keeper, not an uninterruptible power supply.

Dimension 3: Maintenance and Human Error

The internal battery is easier to maintain. No charger, no water levels, no capacity tests. But easier maintenance often means no one does it. I've opened panels where the battery date sticker said 2019 and the PLC was struggling in 2025. A five-minute battery check today is cheaper than a five-day correction later.

It took me 5 years and more dead-battery calls than I want to count to understand that 'battery life three years' means nothing unless someone marks the calendar. After my third avoidable call, I created a simple 12-point battery verification checklist. It has saved us an estimated $10,000 in potential rework because it forces each service visit to record the battery model, installation date, and measured voltage.

The dual battery charger requires more discipline. You have to verify the batteries, clean terminals, and make sure the charger is actually in float mode. But a good charger gives you a remote alarm. An internal battery has no alarm until it's already weak. In a plant with no preventive schedule, an alarm is worth more than a slightly lower upfront cost.

The Practical Part: How to Charge a Battery with a Battery Charger

If you decide to go the external route, you need to know how to charge a battery with a battery charger correctly. This isn't a PLC skill, but it becomes your problem when a backup battery has been drained by a long outage.

1. Turn the charger off before connecting anything. Label the positive and negative terminals on each battery.
2. Connect the charger positive to the battery positive, and charger negative to battery negative. Check the polarity with a multimeter if you have any doubt.
3. Select the correct battery chemistry and charge profile. A standard 12V lead-acid battery usually floats around 13.2 to 13.8V, while AGM batteries may need a different curve.
4. Charge in a ventilated area, especially if the batteries are flooded. Hydrogen gas is not something to ignore.
5. After charging, verify the voltage under load before reconnecting the PLC. A battery that measures 12.8V at rest can still collapse the moment the DC bus asks for current.

The XS battery charger I've used for these backup panels has separate charging channels, so each 12V battery gets its own profile. For a 24V system, that's the kind of feature I look for. A single charger trying to handle two batteries in series can undercharge or overcharge one side.

Which One Should You Choose?

There's no universal answer, and anyone who says 'always use a battery charger' isn't thinking about your site. Here's my practical rule:

Choose the internal battery alone only if you have a strict replacement program, the PLC runs in a stable electrical environment, and a short loss of retentive data is just an inconvenience. That's still a legitimate choice for simple machines with no safety-critical sequence.

Choose the dual battery charger backup if a voltage dip or short outage stops your line, causes scrap, restarts a process with dangerous edges, or triggers a long manual recovery. In those plants, the backup charger pays for itself the first time it prevents one reset.

Looking back, I should have specified the dual charger on that paint booth project from the beginning. At the time, it felt like overengineering. But after seeing the plant's power quality data, it was the right call. If I were building a new line around a Mitsubishi PLC FX5U, I'd do both: internal battery for long-term data retention, and an external dual battery charger for ride-through. Then I'd put both on a quarterly verification schedule.

Between you and me, most PLC battery emergencies aren't really about the battery. They're about the absence of a plan. The FX5U is a reliable controller. Your job is to protect its conditions for success. A few minutes of prevention—a battery date, a charger alarm test, a voltage reading—beats the kind of 2 a.m. call that starts with 'the PLC is dead and the line is down.'