24vdc ups

diego96

This isn't really a solar question this is more of a battery charger question.
I normally have 120VAC grid power and I will power a load with 24VDC from an AC-DC power supply (1000W). I need to design something to switch to 24V battery backup during loss of grid power, and charge the batteries when there is grid power. So basically I need a 1000W, 24VDC UPS.
Any ideas on how I should do this? Should I build something using a solar battery charger?
Will need to be something very ruggedized, and will be in a hot desert environment.

I have seen 24VDC UPS's for sale, but they aren't powerful enough and/or don't allow me to use my own external battery packs. I plan to use a large bank of AGM batteries connected in parallel to power the load for up to 48 hours. It will be many hundreds of Amp-Hrs...

diego96

Re: 24vdc ups

And also, is it ok to connect lead-acid batteries in parallel? I made a battery pack once with alkaline batteries and it exploded after about a week of use...

mike95490

Re: 24vdc ups

I'd suggest consider flooded batteries, 4V, or 6V, and build up a 48V system . avoid parallel batteries, a series string of 200AH golf cart batteries is a good beginner system
6v x 8 batteries = 48V @ 200 AH = 9600watt hours total, 4800 useable. you don't want to plan on more that 50% discharge.

diego96

Re: 24vdc ups

All the devices I'm powering take 24VDC. I need to power 750W continuous for 48 hours so that would be 36kW-Hrs. That's a lot of batteries...

mike95490

Re: 24vdc ups

diego96 wrote: »

All the devices I'm powering take 24VDC. I need to power 750W continuous for 48 hours so that would be 36kW-Hrs. That's a lot of batteries...

I'd say that's not going to be realistic. Time to think out another plan.
I've got a large, 48V 800ah battery bank I'm building up, that uses 800Ah cells, all in series. It's "only" 38,400 Wh total, 19,200wh usable.

http://tinyurl.com/LMR-NiFe

You are looking at 1,600ah @ 48V or, 3,200ah @ 24V. Going to need a haz-mat permit for something that big.

BB.

Re: 24vdc ups

What are the input specifications of the DC loads? Can they withstand a diode drop (0.2-1.x volts)?

Why not power directly from the battery bank and use a charger to keep the battery bank floating?

Also, does the power have to be 100% battery bank, or can you have other sources (obviously, a generator is used at every telephone office and on many power pole repeaters these days). At least in the olden days (when I was a kid, I don't think the phone company planned on running pure battery for 48 hours the local central office for a town of 30,000 people).

Batteries are just not that dense for stored power as you get with fossil fuels. Add the life of the battery bank (2-6 years for AGM in float service?)--And things get expensive (vs genset with a tank of propane, natural gas, etc.).

-Bill

diego96

Re: 24vdc ups

The loads can take 20-28VDC but I will connect the battery bank to a 24V DC-DC converter. That way the batteries can drop below 20V if needed.

A generator takes time to turn on, there are some electronics in the load that I don't want to be abruptly shut off, I need an instant transfer of power to and from the backup source. Also, the location is remote and difficult to get fuel to. So I don't think a generator is an option. (But I'll not rule it out yet).

Is there a battery charger that is powerful enough to charge the (huge) battery bank with and also power a 750W load? That would be a good solution.

I just spoke to a battery manufacturer and they said no problem connecting their lead acid batteries in parallel. He said they would not explode.

mike95490

Re: 24vdc ups

OK, so here's a thought
Several years ago, a client came to me, needing a 12V system, well regulated, that would not burn out LED drivers.
I suggested a medium size 300AH 48V battery bank, and built up a dozen DC-DC converters from 48V - 12V.
Used These
Construction Slideshow

If you can split your loads to be less wattage than the output of a single DC-DC, you could build up 8-10 of them, and and run 48V (DC in = 36-75V) and chose a 24V model
Does your gear have internal regulators, or if you feed it 28V, does it consume less amps compared to 24V ?

So, this is how I'd work on it. It may be that to purchase a 100A 48V charger, you may end up buying an inverter that has an internal charger. 100A @ 48V is going to be a 240VAC input. Can't get that much from a 120V outlet.

What is your main power feed ? Utility power ? genset? Be sure to check the PF (Power Factor) of the charger, that can be a major surprise.

mike95490

Re: 24vdc ups

diego96 wrote: »

...The loads can take 20-28VDC but I will connect the battery bank to a 24V DC-DC converter. That way the batteries can drop below 20V if needed...

If you drain a 24V bank, to 20V, you better shop that evening for a new bank, because the batteries are going to be ruined.

I think you will be happier with dual redundant gnesets, and a MUCH smaller battery bank.

Consider natural gas or propane for the genset.

n3qik

Re: 24vdc ups

Look at this:

http://www.solar-electric.com/ioen40amp24v.html

This will power the load and keep the batteries charged. Total power is 960 watts.

diego96

Re: 24vdc ups

mike90045 wrote: »

If you drain a 24V bank, to 20V, you better shop that evening for a new bank, because the batteries are going to be ruined.

I think you will be happier with dual redundant gnesets, and a MUCH smaller battery bank.

Consider natural gas or propane for the genset.

I think you are right. I'm going to need 2 tons of batteries. Genset is looking like a better option...

bmet

Re: 24vdc ups

diego96 wrote: »

I just spoke to a battery manufacturer and they said no problem connecting their lead acid batteries in parallel. He said they would not explode.

What did you expect them to say? Of course they are not going to promote that danger to a customer.

niel

Re: 24vdc ups

bmet wrote: »

What did you expect them to say? Of course they are not going to promote that danger to a customer.

there is no danger of explosion in paralleling like batteries at the same voltage unless there is a severe defective battery present and it could explode anyway no matter if it is paralleled or not.

bmet

Re: 24vdc ups

niel wrote: »

there is no danger of explosion in paralleling like batteries at the same voltage unless there is a severe defective battery present and it could explode anyway no matter if it is paralleled or not.

There in lies a paradox. Parallelling batteries may not present a danger of explosion, but the implementation can cause defective batteries if not done correctly.

niel

Re: 24vdc ups

same can be said for series too unless they are as i said to be alike batteries free of defects to affect the other batteries. one would not want to parallel say a 30ah battery with a 100ah battery, but just the same one would not want to put that same 30ah battery in series with that 100ah battery. when things are done correctly, damage does not occur unless there is a defect in the battery to begin with. even with a defect or mismanagement in paralleling or series placement of batteries, it would not be a guarantee of a severe danger to a customer as you had said, even though it is possible. there is a clear-cut danger to the life of your batteries though.