"Modding" a small UPS with a high capacity AGM battery?

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will34
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"Modding" a small UPS with a high capacity AGM battery?

So at my work there are lots of voltage fluctuations and short outages and we have 3 independent UPS for the computers and networking devices. Problem with these cheap UPS is that they rely on sealed lead acid tech and the batteries go bad within a year, also these are relatively low capacity at 12V 9Ah and once fully depleted they’re pretty much done for.

I was considering modding one of the UPS and pair it with a 125Ah solar AGM battery but not sure about if the low voltage protection will trigger much sooner than what the battery can provide. I opened up the larger 1500VA one and it uses 2 SLA batteries in series. The smaller 600VA one uses a single battery so that’s what I’m modding, in theory it should extend the runtime and keep the wifi running from 10 minutes to 2 hours.

Has anyone done this before and how did it turn out?

Edited by: will34 on 09/09/2021 - 11:59
jeff51
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I have done this for years with no ill effects using deep cycle lead-acid batteries.
I’m not sure about the top off voltage of SLA vz AGM. But the charge rate of most UPSs is small compared to a big ass battery.

I make up a battery tray that sits under the UPS. The big batteries go there. I extend the leads through the bottom of the UPS.
Be sure to put a good inline fuse in the mix. Often you can steal one from the OEM UPS build.

I don’t recommend doing this to the little home type UPSs that do not have an internal cooling fan.
They are designed to give you a chance to shut down elegantly before quitting. That’s about all.
Try to run one at near capacity for an hour or more? I smell overheated electronics.

Also remember to actually test the UPS under a load every now and then.
The batteries often appear good until it’s time to – UPS – something.

I like to use a load I made up of a string of 100w light bulbs. I unplug the computer eq. Attach my load and see how well the UPS runs.
The lights give a clue as to how the output is acting.

Don’t be surprised if a modified sine wave UPS drops the light level when it switches from line to batteries.
That’s the nature of the beast.

No matter what I do, I seem to be replacing batteries every 3 years of so. There are only so many deep discharges in lead cells.
Looking forwarded to when LiFePo4 batteries get cheap enough to start using them.

All the Best,
Jeff

bansuri
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Wow Jeff, that’s a helluv an answer, nice job!

chops728
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There are lots of LifePo4 batteries that are direct replacement / compatible with SLA —- with LifePo4 you get the full rated AH not half like SLA / AGM

I would use the 24v versions of UPS — or even 48v — they run a lot more efficient

I think I’ve seen mods on these at:

diysolarforum.com
Lightbringer
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RVs and stuff use LFP battery banks, so that’s worth a look-see.

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I was running ups with car battery connected. Worked as it should.

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will34
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Thanks for all the tips, I will go ahead and do the mod.

The reason why I’m not heading towards lifepo4 is because I already have 2 AGM batteries, one 50Ah and one 125Ah from a existing solar inverter system and I wanted to re use these, and will be rebuilding my DIY solar generator with modern lifepo4 cells for a 2kwh system

chops728
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You plan on using one of those Right —- It’s not recommended to parallel different capacity batteries

will34
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chops728 wrote:
You plan on using one of those Right —- It’s not recommended to parallel different capacity batteries

Yeah just the 125Ah for the 600VA small UPS that uses single SLA, not for the other 1500VA UPS that uses 2 SLA in series

Lightbringer
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chops728 wrote:
You plan on using one of those Right —- It’s not recommended to parallel different capacity batteries

Why not? With the same float-voltage, it shouldn’t matter when charging in parallel. Even when discharging, the smaller one which might sag more would have the bulk of the current covered by the bigger one which won’t sag as much.

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chops728
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Lightbringer wrote:
chops728 wrote:
You plan on using one of those Right —- It’s not recommended to parallel different capacity batteries

Why not? With the same float-voltage, it shouldn’t matter when charging in parallel. Even when discharging, the smaller one which might sag more would have the bulk of the current covered by the bigger one which won’t sag as much.

I/ve read lots of threads not to use different capacity batteries — I think it has more to do on the charging side —- I’m no expert for sure

zoulas
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Home UPS system from APC and others are only good for 2-3 battery changes. Then the capacitors crap out. SLA works excellent but you have to get a name brand Japanese battery.

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In series, absolutely not. In fact, both (for 24V) should be independently charged/monitored.

In parallel, though, it should be a completely different story. SLAs don’t exhibit –ΔV or anything, and it’s just like expanding the area of the plates.

Maybe they have other reasons, unno.

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Lightbringer wrote:
In series, absolutely not. In fact, both (for 24V) should be independently charged/monitored.

In parallel, though, it should be a completely different story. SLAs don’t exhibit –ΔV or anything, and it’s just like expanding the area of the plates.

Maybe they have other reasons, unno.


24v UPSs charge the batteries in series.
One always eventually gets out of ballance.
Then it’s new battery time.
I always charge the new ones in parallel before installing – if possible.
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No BMS?!?

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Lightbringer wrote:
No BMS?!?

Nope, just a pair of SLAs in series. Or 4 Batts in a 48v UPS. At least in the dozen or so models I’ve had apart over the years.
When I do a big external battery conversion. I put a couple of cheap volt meters in the mix to monitor the 2 external Batts.
Helpful to indicate if one of them is getting out of balance.

Same deal on an old Roomba I have. 12 or 14 (don’t remember) NiCds in series without a BMS to be seen. Eventually one would reverse and performance would get crappy. I’d end up pulling the Batt. pack and using a hobby charger to refresh the batteries in need of it. Or replace one (or more) of the NiCds.

Eventually the whole pack got too crappy to mess with. Found a LiIon pack full of 18650s on Amazon. Must have a good BMS cause it takes care of the NiCd vs Lith charging characteristics. It’ still going strong after little more than a year. Way better than the old Roomba pack. Cheaper than an official Roomba battery by like 40% with a hight mAh rating too.

Same deal for NiCds, NiMh, and at least 1 LiIon tool battery pack I’ve had apart.
Why o’ Why not do it right the first time? Save $1 per pack x 600,000 packs???
All the Best,
Jeff

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Also hate that OEM replacement SLA packs for UPSs are like 100%-250% more than just a pair of SLAs naked batteries.
The only difference is some sort of propriety connection (usually) and a bunch of stick on tape covering up the mAh rating of the OEM batts. Making it hard to tell what to buy to replace the OEM Batts with.
No matter what I stick in there, the Batts need replacing every 3 years or so. Less if they are called on to do a few drain it till dry power runs.

If I have software looking after the UPS I set it to shut down (if possible) at 50% of battery capacity to help preserve battery life. Depending on what the UPS is protecting. Workstations – who cares as long as the shutdown is elegant (but not required). My test bench where I do data recovery? I want to squeeze every last second out of the UPS in hopes the job can finish.

I keep looking at LiFeO4 batteries and DYI Cell/BMS prices. One of these days I’m going to get fed up with messing with Lead-Acid Batteries.
All the Best,
Jeff

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Side note.  I don't know what "work" consists of here but if you're in a typical business/office situation then you may want to think twice about doing this.  At the very least, check with your insurance company(s) and also the fire marshal.  Sounds like you really need to invest in a generator although possibly talking to the service provider and/or a electrician may yield a less expensive solution.  Filters can be added at the service entrance to help with the fluctuations if the service company can't/won't do anything about that.  This sounds like the tail wagging the dog here.  I know for a fact that there is no way we'd be allowed to do this here and we unfortunately have the same issue with our service.  Plunking down the cash for a building generator is a major investment but I think we've reached that point since our collective hourly loss of income adds up really fast, and of course we are not allowed by law to let employees stay in the building during a power interruption.