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I have 25 locations spread out over a large area we recently had another power outage lasting 48-72 hrs.depending on the location. all locations have UPS back ups with generators. Problem:

  1. The ups does not last but 6- 8 hrs. All equipment runs on 24 vdc. at about 3-3.5 amp stedy draw.
  2. Generators are running for days to power a PLC. I have people running around check generators round the clock.

I propose remove UPS and install 2, 12vdc lead acid type batteries in series 200 amp hr.each to power the system. So the system runs off the batteries 24/7 this should have a smooth current out flow immune from spikes??Yes/No?? I also need to be charging the batteries all the time?? I hope to size the batteries to last 72 hrs or more. Can this work?? Benefits no expensive UPS ours are pricey, no wear and tear on the generators, no fueling the generators during the outage, no people checking operations round the clock during outage. need a solution.

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    \$\begingroup\$ Do the math: 3.5 A * 72 hours = 252 Ah. Clearly 200 Ah isn't enough. \$\endgroup\$
    – Dave Tweed
    Aug 27, 2020 at 12:23
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    \$\begingroup\$ Current either goes into a battery or comes out of it. It can't do both at the same time. When you're charging them you need your supply to power your equipment. \$\endgroup\$
    – Finbarr
    Aug 27, 2020 at 12:29
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    \$\begingroup\$ Using lead acid batteries, you don't get many charge cycles before they fail, if you discharge them below 50% full. So you'll need more like 600Ah to cover a 250Ah discharge. \$\endgroup\$
    – user16324
    Aug 27, 2020 at 12:45
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    \$\begingroup\$ You need an solar+battery+generator hybrid solution, there are off the shelf options in the telecom market for this. \$\endgroup\$
    – Jeroen3
    Aug 27, 2020 at 13:05
  • \$\begingroup\$ You might not be able to power these on raw battery voltage. 12V lead acid means ~10-14V depending on charge. Two in series and you have somewhere between 20-28V. PLCs and 24VDC relay coils may be picky with how you supply them. \$\endgroup\$
    – Lundin
    Aug 27, 2020 at 14:13

2 Answers 2

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If your PLC takes an AC input, give it an AC input. UPS are there to accomodate any fluctuations and surge. Cause when AC mains comes back? Bye bye PLC.

I own a UPS business and I have seen plenty of people make a silly mistake like this and it will cost you dearly. The price of your UPS will be the least of your worries.

My two cents worth.

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  • \$\begingroup\$ The PLC doesn't take AC input, it runs on 24Vdc. Usually they're buffered with their own UPS, I think that's what's missing here. \$\endgroup\$
    – Mast
    Aug 27, 2020 at 16:01
  • \$\begingroup\$ Use a DC UPS then. In my view it's important to have regualation, which would offers some sort of isolation in a scenario like that. I wouldn't take out the UPS factor. A UPS will do more than merely provide uninterrupted power. It's why they are expensive, well the type of ones an environment like yours is designed for. To connect straight from a battery is a bit of a cowboy move if you ask me. Lol. \$\endgroup\$
    – Sean
    Aug 27, 2020 at 19:15
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We (our customers) use PLCs with SCADA/telemetry to watch the diesel generators. These can utilize standard VHF/UHF, cellular or even Wi-Fi/ISM radios (on the associated frequency bands) to cover the required region. There are also PLC-less electronic monitors that can send emails/text messages over cellular radios in case of remote alarms.

That said, there are 24-volt UPSes out there that might meet your needs (they charge from 120 volts AC). Ours are usually sold with deep-cycle/marine-type batteries for longer back-up time.

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