LiFePO4 UPS

[danb35]

It's a little OT for this forum, as I'm not really looking to use solar power for this, but I'm really looking for something that could serve as a UPS, running a constant load of about 100W, for a few hours--call it 300Wh or so--and using LiFePO4 batteries. Pb-acid don't seem to last long in any UPS I've had, and I don't want to deal with replacing them every couple of years. But I haven't been able to find any product quite like this. I'm seeing:

• So-called "solar generators"--lots of these out there, some of them using LiFePO4 (and many using other battery chemistries). And many of Will's reviews show serious, fundamental design flaws, in many cases making them unsafe to connect to power at all. And, of course, you're paying for solar charge controller functionality that I don't intend to use--though that isn't necessarily a deal-breaker, depending on how much it is.
• One of Will's "milk crate" builds--not quite the "finished product" I'd like, but nice that you can customize it with whatever pieces you want (delete the solar charge controller, use bigger batteries, etc.). But this really isn't going to run as a "UPS"--it's going to be running the load off an inverter, and at the same time running an AC charger. You also wouldn't be able to monitor it like a proper UPS, but for my purposes that isn't too important.
• ?

Any other suggestions here?

 

[John Frum]

I made my own double conversion ups.
Mine runs a base load of ~256 watts.
It can if needed deliver 1500 watts but in reality the load never exceeds ~512 watts.
At 512 watts my worst case scenario battery backup is 7 hours.
For the base load its 14 hours.

Mine charges off of mains power.
It discharges to ~50% DOD and then charges to full because lifepo4 batteries don't like to be kept full.
Even worse is to expose them to continual voltage stress above their full resting voltage.

Are you looking to build something?

 

[Supervstech]

Lifepo4 is not suited for the charge parameters most ups boxes use.
The issue is LFP needs to set at around 50% charge during storage. It doesn't like to rest full.
Most ups boxes float 14.8 for SLA.
A better plan would be a hybrid inverter configured for battery priority, where it runs off battery, switches to ac for charging then repeats.

 

[Supervstech]

Mpp800W 120v unit might fit your needs with a 100ah lfp amperetime pack on it.

 

[MisterSandals]

The issue is LFP needs to set at around 50% charge during storage. It doesn't like to rest full.

I have not seen any numbers for degradation over time for different states of charge. Have you seen anything like that?
I am currently storing my "emergency backup LiFePO4" batteries at 13.2V as recommended. I also have the ability to solar charge them so storing them at 50% is not a major concern.

How would the 3000-5000 charge cycles and 10 year lifetime suffer if they were stored at 75% for example?

It'd be nice to be able to do a cost/benefit analysis for state of readiness and cost in battery degradation.

Too off topic? Not meaning to hijack this thread, fearing i might be... sorry in advance.

 

[Supervstech]

Good question.
@Will Prowse can you explain?

 

[danb35]

Are you looking to build something?

I'm open to building or buying. I'm willing to pay a bit of a premium to buy--it'll look much more "finished", which will help the WAF (and this will be in her domain), but that depends on there being a commercially-available product that does the job and isn't multiples more expensive than DIY. Otherwise, building is fine with me too, depending on the level of "building".

Most ups boxes float 14.8 for SLA.

Maybe I wasn't clear--I wasn't talking about taking a Pb-acid UPS and hooking up a LiFePO4 pack to it. Pity, because (if it would work) it would likely be the simplest and least expensive way of doing what I'm looking for. But no, I'm looking for either a commercially-available LiFePO4 UPS (ideal, frankly, as long as it isn't too expensive) or a way of cobbling one together myself.

 

[John Frum]

I don't know of any off the shelf solution.
As @Supervstech suggested, a diy offline ups is basically a battery + inverter/charger with integrated automatic transfer switch + joinery + box.
You have indicated your load is 100 watts.
How much battery backup would you like in hours and minutes?

 

[MisterSandals]

looking for something that could serve as a UPS,

The tricky requirement that I see is whether it NEEDS to have millisecond cutover (uninterrupted) or not.

 

[John Frum]

The tricky requirement that I see is whether it NEEDS to have millisecond cutover (uninterrupted) or not.

Most decent inverter/chargers sync with the mains and cut over within 1 cycle of AC current(~20 milliseconds).
Basically transparent to your loads.

 

[Bud Martin]

May I ask what kind of 100W load that is?

 

[John Frum]

The tricky requirement that I see is whether it NEEDS to have millisecond cutover (uninterrupted) or not.

If you are really concerned about cutover then double conversion solves that problem utterly.

 

[danb35]

How much battery backup would you like in hours and minutes?

I'd like a minimum of 2 hours, and wouldn't mind more than that. If I call it 300Wh (as I mentioned in my OP), that gives about 3 hours' runtime, ignoring conversion efficiency and such--a 12V, 25Ah pack would handle it. At that rate, battery would be the cheapest part of the system, so I'd likely upsize the battery a bit.

The tricky requirement that I see is whether it NEEDS to have millisecond cutover (uninterrupted) or not.

I don't know that it needs to be sub-millisecond, but part of the load is a computer.

May I ask what kind of 100W load that is?

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[John Frum]

I don't know that it needs to be sub-millisecond, but part of the load is a computer.

A computer should be tickety-fine with a ~20ms synchronized cut-over.
That would be a testable product requirement for the whole class of products.

 

[danb35]

So just to make sure I'm not missing anything--"double-conversion UPS" is really just line -> charger -> battery -> inverter -> load, right? Ignoring trying to get fancy with monitoring, of course.

 

[John Frum]

So just to make sure I'm not missing anything--"double-conversion UPS" is really just line -> charger -> battery -> inverter -> load, right? Ignoring trying to get fancy with monitoring, of course.

Correct, its brutally inefficient but it works.
In my case the charger is ~75% efficient and the inverter is ~90% efficient.
Which is 67.5% overall.

 

[mrzed001]

The tricky requirement that I see is whether it NEEDS to have millisecond cutover (uninterrupted) or not.

Some MPP Solar inverters have 0 msec transfer time. These are the MK and LV-MK models.
They are like an Online UPS. Have the double conversation in it


Can be used as UPS inverter too. No need to attach solar to it.

 

[Tim Dressel]

I made my own double conversion ups.
Mine runs a base load of ~256 watts.
It can if needed deliver 1500 watts but in reality the load never exceeds ~512 watts.
At 512 watts my worst case scenario battery backup is 7 hours.
For the base load its 14 hours.

Mine charges off of mains power.
It discharges to ~50% DOD and then charges to full because lifepo4 batteries don't like to be kept full.
Even worse is to expose them to continual voltage stress above their full resting voltage.

Are you looking to build something?

Not trying to hijack this thread, but @smoothJoey can you share some details about your setup? My use case/demand seem very close to what you have described. Essentially I have some networking tech that I'm looking to build my own UPS just because I can, and I'm annoyed with consumer UPS's rarely lasting more than a couple of years (not batteries failing, but actual smoke leaking out).

To the OP @danb35 did you ever end up building something?

And to the folks looking at this thread, has anyone come across an AC to AC switching power supply with DC backup? Essentially the equivalent of a UPS circuitry but in a stand alone box, just add 12/24/36V external batter?

Tim

 

[John Frum]

Not trying to hijack this thread, but @smoothJoey can you share some details about your setup?

280ah 8s lifepo4 battery
    eve cells
    jbd smart bms
inverter
    https://samlexamerica.com/products/1500-watt-pure-sine-wave-inverter-pst-1500-24/
    chosen because its designed to be controlled by external switching
    ~%90 percent efficient
battery protect
    https://www.victronenergy.com/battery_protect/smart-battery-protect
    controls the inverter
    *note the smart battery protect is not in the high current path between the inverter and the battery
converter
    https://www.iotaengineering.com/products/detail/1188445/iota/dls-27v-25a-converter-and-charger/25-amp-acdc-power-converter-and-battery-charger-for-dc-load-operation-and-24v-battery-charging
    has an rj9 port
    when pins 1-3 are shorted it delivers ~27.6 volts(boost)
    when pins 1-3 are open it delivers ~26.4 volts(float)
    ~%80 percent efficient
raspi
    runs a tiny perl script wrapped around jbdutil(thanks to @melkier)
    talks to the bms via rs-232
    controls the converter via a solid state relay
        toggles boost at 1:00am daily
        toggles float mode when the tail current gets below 12 amps, just before the daytime electrity rates kick in.
notes
    overall efficiency is ~75%
    has a lot of surface detail but I consider it a proof of concept
    certain questions won't be answered to protect the innocent
    base load ~10amps
    typical load <20 amps
    reserve battery capacity ~180amp hours

This is the stats for the end of charge this morning.

2021-09-22 06:49:28 Low=3.389V Average=3.411V High=3.431V Delta=0.042V Balance=01010001 Cell=27.8C Ambient=26.8C Rate=11.8A boost 05:49:22
2021-09-22 06:49:34 Low=3.382V Average=3.400V High=3.419V Delta=0.037V Balance=00000000 Cell=27.8C Ambient=26.8C Rate=0.0A float 00:00:06

 

[danb35]

To the OP @danb35 did you ever end up building something?

Yeah, I should have updated the thread. I adapted one of Will's milk crate builds, leaving out the solar charge controller, and using 4x 100Ah packs from batteryhookup.com. Way overkill for the power capacity, but it's working well so far.