24V and 12V UPS... can they share the same batteries?

[doublec4]

Hi all,

I have some equipment that requires 24V and 12V in the same cabinet. Trying to figure out a UPS set up that can work with both voltage levels. Ideally I don't want to have a ton of batteries and various AC/DC and then DC/DC power supplies in the same cabinet. Really trying to simplify and cut down on components.

Meanwell has the 100A and 100B UPS power supplies that are AC to DC 12 and 24V respectively: https://www.meanwell.com/Upload/PDF/DRC-100/DRC-100-SPEC.PDF

Basically my PLC is 30V tolerant so it can accept the ~27.5V from the 24V UPS. I am driving some lights with relays. The lights are 12V units so I can adjust the output of the second power supply down to 12V to suit (or leave at 13.8V if I really had to... the lights were originally automotive lights so they can tolerate more than 13V.)

So my question is... is there a way I can use two 12V UPS batteries in series and charge from the 24V UPS, but still hook up the 12V UPS such that if the AC drops, it can feed off of a single 12V battery? How would I do this such that it doesn't interfere with the 24V UPS charging of the two batteries?

The UPS supplies have relays that close when the AC goes down to trigger a PLC or some kind of warning indicator. I had the idea of feeding one of the 12V batteries through the AC OKAY relay to my 12V rail so that the battery supplies the 12V rail only when AC is down. However, those AC OKAY relays are rated at 1A and my load is pushing that limit. I could use the relay to trip a bigger relay but I am hoping for something more elegant.

Is there some way to hook up the 12V UPS to one of the batteries, and use a diode or something of that nature to prevent the 12V UPS from attempting to charge it? Then when the battery switches from a load to a source, it would allow the current to flow into the lights as intended? This isn't really clear to me as the polarity does not change so I can't envision a diode working... open to suggestions!

Thanks!

[bsdphk]

In my experience, the best idea is to make the UPS part 24V and then drive the 12V stuff with DC/DC converters.

Also:  Look into West Mountain Radios kit for good fused distribution LVDC

[doublec4]

Problem that I am having is that the more common, less expensive UPS in 24V does not support much for current. By driving a 12V DC-DC off of the 24V UPS I would need to find something larger / more expensive. So I was hoping to have a separate 12V UPS to supply the current required for the 12V loads.

[bsdphk]

Making a 24V UPS is not hard, all you really need is a 27.6V PSU and two 12V VRLA blocks.

If you want to put loads on the midpoint, you need to use two 13.8V PSUs, and you need to size the batteries proportionally.

Problem is that most VRLA batteries dont live long that way, the six cells in the 12V battery are slightly different, so some will be overcharged and some will be undercharged with the net result that the battery probably only last 3-5 years before one of the cells either fails open (harmless) or closed (hot!).

The next step up is to do "equalization charge", that's the "three step charging" you will read about, but if your power is generally good that will not be enough.

Equalization charges should be performed every 2-4 weeks, and none of those "three step chargers" do that. (Hack: put a timer on the mains, which interrupt 5 minutes every three weeks, to kick the charger out of float.)

The next step up is to use 2V cells, where you charge each one individually (either with separate chargers or with load-balancers).

There is a huge gap in available capacity between the 6Ah cylindrical and 100Ah OPzS/OPzV cells, but if you go with OPzS and refill water every other year, your batteries will last 25 years.

So the very first thing you need to do is calculate the capacity you need, and decide how often you want to buy new batteries.

I have tried the 12/24 thing myself, but it was not worth the complexity and I had trouble with cable/connector losses.  I²R rules: You need much more copper in 12V cables than 24V cables.

I have switched to pure 24V UPS and feed the 12V stuff with distributed DC/DC converters (Traco TMDC).

And whatever you do: The first thing you bolt to the battery is an ATO-fuse (For instance with something like this: https://powerwerx.com/atc-ato-inline-fuse-holder-18-gauge-red) because even small VRLA cells will deliver hundreds of amps when shorted.

Also: Siemens makes some damn good "Simatic" 24VDC UPS kit, and you can often find it cheapish on eBay.