The Day My Savings Plan Backfired
It was September 2017 — my first year as the lead maintenance technician for a mid‑sized packaging plant. I’d inherited a mix of Allen‑Bradley CompactLogix and ControlLogix PLCs, plus a handful of Siemens S7‑1200s on older lines. My boss told me to “keep the lights on” and gave me a modest budget for spare parts and backup power supplies.
One morning, I noticed the UPS battery indicator on our main panel was flashing red. The batteries were old sealed lead‑acid units, and they’d started to swell. I knew I had to replace them fast. But here’s where my rookie mistake began: I decided to switch to a lithium LiFePO4 chemistry because it’s lighter and lasts longer — and I found a 24V lithium battery charger on Amazon for $89. The Allen‑Bradley‑approved charging solution from our distributor was $289. To me, that extra $200 seemed like a waste. I thought, “A charger is a charger, right?” (Ugh — I still wince when I say that out loud.)
Why the Cheap Charger Looked So Good (At First)
I’m not a battery engineer, so I can’t speak to the nuances of LiFePO4 chemistry. What I can tell you from a PLC maintenance perspective is that 24V DC is the lifeblood of most industrial controls. The Allen‑Bradley PLCs in our plant require a stable 24V supply within ±10% to avoid random faults. The charger I bought had a generic “smart” algorithm that was allegedly compatible with any 12V/24V lithium pack. The seller’s description promised “multi‑stage charging” and “overvoltage protection.”
I installed it, connected a new 12V 20Ah LiFePO4 battery (two in series for 24V), and everything seemed fine for the first three weeks. I even bragged to my colleague that I’d saved the company $200. (Which, honestly, was petty cash — but I wanted to look good.)
The First Sign of Trouble
One Tuesday afternoon, the packaging line stopped. The HMI was showing a “Power Supply Error — Voltage Out of Range” alarm on the Allen‑Bradley 1756‑PB75 module. I grabbed my multimeter and measured the DC bus voltage: it was 26.8V — that’s within spec (24V ±10% = 21.6V to 26.4V). Actually, 26.8V is slightly above. I scratched my head. The charger was still plugged in, but the battery was fully charged. I decided to leave it; maybe the meter was off.
Then Thursday night, the same line went down again — this time with a processor fault on the ControlLogix L71. I rushed to the panel and measured the voltage: 22.1V. That’s below 21.6V? Wait, 22.1 is actually inside the spec, but the PLC had crashed. Turns out, the cheap charger’s float voltage was drifting. When the battery was nearly full, the charger kept pulsing current, causing the DC bus to oscillate between 25.4V and 21.8V several times per minute. The Allen‑Bradley power supply’s internal capacitors couldn’t filter fast enough, and the processor brown‑out protection kicked in.
The Real Cost of Being Cheap
That processor fault took down the line for four hours. We had to replace the power supply ($380), buy a new charger from our distributor ($289), and pay two electricians overtime ($1,200). The production delay cost us an estimated $1,500 in lost throughput. Total damage: $3,369. My $200 savings turned into a $3,169 loss — and a big black mark on my performance review.
When I called the distributor to order the correct Allen‑Bradley‑compatible charger, the technical support engineer (shout out to Mike) explained something I’d never considered: “For mission‑critical PLC systems, the charging algorithm must match the battery chemistry and the load profile. General‑purpose chargers can’t handle the dynamic current draw of a CompactLogix rack with 8 I/O modules. You need a charger that can deliver clean, regulated DC even while charging the battery.” That was my wake‑up call.
What I Learned from Allen‑Bradley PLC Training
After that disaster, I signed up for the Allen‑Bradley PLC Training Classes offered by our local Rockwell Automation office. The three‑day course covered everything from power supply sizing to grounding best practices. One module that stuck with me was “Battery Backup for ControlLogix Systems.” The instructor showed us a table comparing Siemens PLC vs Allen‑Bradley power requirements — both use 24V DC, but the recommended voltage regulation and ripple limits differ slightly. More importantly, he emphasized that cutting corners on the power chain is a false economy. (I took a photo of that slide — it now hangs above my bench.)
Here’s the key takeaway that applies to anyone working with lithium LiFePO4 battery chargers in a PLC environment:
- Choose a charger rated for industrial loads — look for low ripple (< 50 mV peak‑to‑peak) and a constant voltage/constant current profile that matches your battery bank.
- Always verify the output voltage with a multimeter under load. I now teach every new hire the how to test 12v battery with multimeter procedure — but also test the charger output while the PLC is running. If the voltage sways more than 0.5V, reject the charger.
- Don’t mix brands — use the same manufacturer for PLC, power supply, and battery management. That $289 Allen‑Bradley‑recommended charger was actually a rebranded industrial unit from a reputable vendor. The $89 Amazon special was a consumer‑grade device with loose specs.
My Checklist (So You Don’t Make the Same Mistake)
After that $3,200 lesson (I still count it as $3,200 even though the accountant rounded up), I created a pre‑installation checklist for any battery backup project. We’ve caught 47 potential errors using this checklist in the past 18 months. Here are the highlights:
- Confirm PLC voltage tolerance (Allen‑Bradley spec: 24V ±10% for most models).
- Buy a charger that specifically lists compatibility with your battery chemistry and has a published ripple spec.
- Before connecting, test the charger output with a multimeter for 30 minutes — no load, then with a dummy load simulating half the PLC current.
- Check the battery’s internal BMS (battery management system) — it should have over‑discharge protection set to cut off before your PLC’s brown‑out threshold.
- Document everything: part numbers, test results, and the date. Your future self (or the next technician) will thank you.
One More Thing About the Siemens vs Allen‑Bradley Comparison
A lot of forum posts ask “siemens plc vs allen bradley” for reliability. From my experience, both are solid — but they have different power supply quirks. Siemens S7‑1200s tend to be more tolerant of voltage dips (they can ride through ~15 ms at 20.4V). Allen‑Bradley CompactLogix is a bit stricter; I’ve seen crashes at 21.5V if the drop is fast. That’s why I now always spec an industrial‑grade 24volt battery charger for AB setups.
Final Takeaway: Value > Price
In my opinion, the cheapest option rarely saves you money. That $200 difference between a generic charger and an Allen‑Bradley‑approved unit was a tiny fraction of the total project cost — but it caused a cascade of failures that cost way more than the premium I’d tried to avoid. I’d argue that total cost of ownership (TCO) is the only metric that matters: initial price + installation + downtime + replacement. Calculate that, and you’ll never cheap out on the power side of a PLC system again.
If you’re new to industrial automation and thinking about saving a few bucks on a battery charger or backup power supply — don’t. Take an Allen‑Bradley PLC training class first. Learn the specs. Buy the right tool for the job. Your PLC will run smoothly, your plant manager will be happy, and you won’t have to write an embarrassing story like this one.
