Looking to build battery backup using 20x sealed lead-acid

[Demolitions]

Hi all,
I'm a bit new to the DIY side of battery backup stuff, I was hoping to get some insight on inverters to upgrade my existing setup. I'm really just looking for help narrowing down my options and proof-checking my planned setup.

I have 20x 12v 9Ah sealed lead-acid batteries (16 are currently in use). These are from various 900w CyberPower UPS units I've bought over the years as a quick battery backup solution. These units are getting quite expensive and its getting very cumbersome to manage these, physically and logically.

The UPS units each run a single computer or a handful of smaller items. I was thinking I could strip the batteries from the UPS units and purchase an inverter/charger unit to act as a unified battery backup.

My setup typically draws 1,000-2,000 watts at any given time, currently the most it can draw is ~3,500 watts. I have plans to expand in the near future, at that point my maximum usage would be closer to 4,500 watts. I'm not looking for this battery backup to last that long, I very rarely have power outages. The longest I've had an outage is an hour, and I have a 6kW portable inverter generator I can use. This setup is only going to be for the brief power outages/brown outs.

I haven't decided on an inverter, I'm not sure what specs/features I really need. Here's a list of the things I'd like it to have;

• Form factor/size isn't a huge deal - I have a lot of space in the room where the UPSs currently are, and I have room next to my utility panel for a wall mounted unit
• Compatible with 24v (48v might be better) DC battery setup
• 4-5kW of AC output - 240v might be the better here, ability to pull 120v and 240v would be best
• Quality power output - I intend to use this setup for sensitive electronics, I don't want dirty power going into anything.
• Off-line or line-interactive backup - My utility power is quite clean and reliable, I likely won't benefit from an online (double conversion) system
• Very fast switching time - This setup is mostly for momentary power outages, it will need to switch in <10ms (My CyberPower UPSs advertise 4ms)

As for charging the batteries, they vary in age and usage by quite a lot. If I recall correctly, lead acids don't need a BMS for individual monitoring/balancing. However I think this might be worth exploring given the condition these batteries are in.

I'm still in the very early stages of planning this project, so I'm open to the idea of changing stuff up where it could save me money and/or effort. I appreciate any feedback on this project.

 

[MisterSandals]

I have 20x 12v 9Ah sealed lead-acid batteries

Thats 180Ah of which 50% is usable.
90Ah x 12.8V nominal = 1152Wh

My setup typically draws 1,000-2,000

This would run an hour or less than half hours depending on which draw, best case.


Not sure this is worth the effort and cost of connectors. And with "batteries, they vary in age and usage by quite a lot", they don't sound terrific.

I'm all for using what ya got as much as the next guy. But sometimes building houses out of free popsicle sticks isn't worthwhile.

 

[Demolitions]

This would run an hour or less than half hours depending on which draw, best case.

I don't need it for long, my utility power here is quite reliable. I already have my systems set to shutdown after 3 minutes on battery.

Not sure this is worth the effort and cost of connectors. And with "batteries, they vary in age and usage by quite a lot", they don't sound terrific.

Well, if these batteries don't cut it I can always swap them out for new ones. My money would be better spent on the hardware & new batteries for this setup rather than more UPS units.

 

[Demolitions]

I did some digging around for inverters that fit my needs, I knew they wouldn't be cheap but holy crap I didn't think there were so many options. I found a few that could work;

• Victron Multiplus 48/5000/70
• Victron Multiplus II 48/5000/70-50
• Victron Quattro 48/5000/70-100/100
• Victron Quattro II 48/5000/70-50
• ATO ATO-PI-LW4000
• SunGoldPower also has a handful of offerings (with no model numbers), they seem quite cheap though. Not sure what the reliability would be

I saw a ton of other options from other brands, pricing on a lot of these is up in the air. Leaning towards one of the Victron models, I've seen them recommended a few times.
I'm still not sure on the battery pack voltage yet. Expanding the pack would be easier with 24v, but I likely won't need to expand a 48v pack. I'm still up in the air on that one.

I double checked my systems, everything that has a significant load (>50w) has the ability to use 240v. While a split phase system would be cool, its not necessary. I can easily get new adapters to replace any incompatible ones. I'd also have to replace a bunch of 120v PDUs and whatnot, I'm sure that won't cost a fortune.

I think the best place for this setup is exactly where my UPSs currently sit. Its in a temperature controlled room, very close to the load and there's already a 240v 30 amp circuit. I've never built anything like this before, so I'll need to figure out what I need aside from busbars, breakers & contactors.

 

[JanisD]

Well, if these batteries don't cut it I can always swap them out for new ones.

Been here, done that - not any more!
There is no chance to reliably manage 20 mixed batteries in 4S (to make 48V and You will need balancers, too) and 5P (to make some Ah usable) for a while. You will be busy, and very likely without power in the one moment You'll need it.

I survived about 6 months on 19 such VRLA batteries in parallel. Most of them was brand new, others slightly used - I supposed it would still be bad, but was willing to learn the lesson. Result was worse - see below:
Every of them was checked by measuring real Ah (it was 6-7 Ah per piece typical), internal resistance and evaluating self discharge.
Every battery had it's 3 A fuse.
Max load was max 50 W, charge current 3 A max from sun or 5 A from mains.
After about 6 months one evening I found the "array" to loosing voltage without load. One of the bats developed partial internal short - too small to blow the fuse, but serious enough to start the transfer of the stored energy of all batteries to heat. Not what I hoped for.

Two used 40 Ah bats from big UPS took over the task for a more than a Year. And I have real story to tell and better sleep.

 

[Demolitions]

Been here, done that - not any more!
There is no chance to reliably manage 20 mixed batteries in 4S (to make 48V and You will need balancers, too) and 5P (to make some Ah usable) for a while. You will be busy, and very likely without power in the one moment You'll need it.

I survived about 6 months on 19 such VRLA batteries in parallel. Most of them was brand new, others slightly used - I supposed it would still be bad, but was willing to learn the lesson. Result was worse - see below:
Every of them was checked by measuring real Ah (it was 6-7 Ah per piece typical), internal resistance and evaluating self discharge.
Every battery had it's 3 A fuse.
Max load was max 50 W, charge current 3 A max from sun or 5 A from mains.
After about 6 months one evening I found the "array" to loosing voltage without load. One of the bats developed partial internal short - too small to blow the fuse, but serious enough to start the transfer of the stored energy of all batteries to heat. Not what I hoped for.

Two used 40 Ah bats from big UPS took over the task for a more than a Year. And I have real story to tell and better sleep.

Yeah, probably not worth the extra cost to get those batteries to work together in a single pack. And that's not even considering I might've already killed one when attempting to charge it, the other three spares have only been topped up once in 4 years. Really unfortunate though, the resale value of these UPS units is pretty much nothing.

I was thinking 4x deep cycle marine batteries for a 48v pack. Probably not the greatest $/kWh but they're readily available. But if there's better options now that lithium is getting cheaper I'm open to suggestions.

 

[outboard1196]

Yeah, probably not worth the extra cost to get those batteries to work together in a single pack. And that's not even considering I might've already killed one when attempting to charge it, the other three spares have only been topped up once in 4 years. Really unfortunate though, the resale value of these UPS units is pretty much nothing.

I was thinking 4x deep cycle marine batteries for a 48v pack. Probably not the greatest $/kWh but they're readily available. But if there's better options now that lithium is getting cheaper I'm open to suggestions.

Your option is either 12v or 48v.
12v is OK up to 3000w inverter. After that just go 48v. If you go lifepo4, don't series 12v batteries to get 48v. Just get a 48v battery.
Your asking a lot from those 9ah batteries, it's not worth the effort.
For this application, agm or gel would be a better option over flooded batteries.