SMX1500RM2U 12V output?

[krep_apc]

Hello! 

I have an APC SMX1500RM2U UPS. It's time for me to replace the batteries, and I'm in the process of doing that. At the same time, I noticed a majority of the components I'm powering are actually 12V DC devices. Since the UPS is well past it's warranty, and I'm an electrical engineer, I'd like to bypass the UPS's AC outlets and connect my devices through a fuse -- and possibly an efficient voltage regulator circuit -- directly to the devices, instead of going AC->DC->AC->DC and incurring so many power losses. My question is, is there already a 12V regulator circuit I could tap into in the UPS? Perhaps going into the interverter circuit? And/or perhaps if someone has a schematic of this UPS? Option B would be I just hunt for it on my own with a meter, Option C install my own regulator. Option C is easy, but I thought I'd just check to see if anyone has crossed this bridge before... 

[Brad_C]

Option C is your only realistic option, but not at all optimal. There is a 12V rail in the UPS that is generated from a little flyback DC/DC converter, but it's not going to sustain anywhere near the sort of load you are after (more than perhaps an extra 100mA).

You would need to build your own 24->12V DC to DC converter and at that point you are simply using the UPS as an expensive and relatively inefficient 24V float charger.

If you were to do that, any potential runtime estimates by the UPS on the 240V output would be out the window, so you compromise that also. The Charger is relatively limited in the current it can provide as it's entire purpose is to charge and maintain a battery. Sustained charge current is going to be a couple of amps at best. Additionally, you lose the low voltage protection, so you'd need something to prevent deep discharging the SLAs and the consequential impact on service life.

If it were me, I'd be picking up a bigger 12V SLA and equipping it with a float charger big enough to run the desired load, with a couple of amps left to charge the battery after a big outage and an undervoltage cutout.

I picked up a relatively large (5A) very efficient SMPS, put that on the UPS output and ran all the devices off that. One power supply, more efficient and I get to power all the mains powered doodads at the same time (PoE network switches, desktop machines and so on).

[krep_apc]

You're right! I realized after this post that the 4 SLA Batteries are wired in series for 48V so I would have to go even further to pull off 24V out of half that, and probably hook up a battery equalizer at the same time to equalize the battery draw... and probably easier to use some sort of simple charger on a timer and leave the UPS out of the mix. Why did Schneider charge the batteries in series with no equalizer circuitry? That seems really sloppy and hard on the batteries to me. I guess they were trying to avoid a complicated removable harness plug for when the batteries need to be replaced, and they consider the SLAs cost and time to replace negligible.

Full disclosure, my actual plan was to use LiFePO4 batteries so I won't have to replace them for some time, and not worry too much about deep discharge. But now I think I might just go with something simpler...

[Brad_C · Accepted solution]

Ahh, I missed the "X" in the part number. Yeah, 48V.

I've done a stack of tests on 48V banks hooked up to SU and SUA series UPS. Invariably by the time the batteries start to drift out of balance they are really near end of life. I have 2 identical units, both with 17Ah internals and 55Ah of externals. I left one on its own, and the other I manually applied an equalizing charge once a month. The batteries didn't last much longer (maybe a couple of months) from a roughly 5 year life. It was a long and ultimately fruitless experiment.

If you buy quality batteries and they all live in the same environment they do tend to age fairly well together. An in principle, you have 24 cells. You'd only be balancing them in bunches of 6, so you still have the potential for drift within each set of cells.

The key for longer life seems to be to make sure they are run toward the bottom of the float voltage spec.