DIY UPS with battery, solar charge controller, and inverter?

[krby]

I was thinking about putting together a UPS-type thing based mostly on gear I happen to have already and I wanted feedback on a few things. My load is some home networking gear in a rack. It will be no more than ~220W of AC, probably more like 100W on average.

In a basic solar setup, you have panels providing DC to a solar controller, then the battery and inverter hanging off the solar controller. In effect, the solar controller is keeping the battery charged and then providing extra current to the inverter. So, let's say I replace the solar panels with a 24V DC power supply that can handle the max power of the load. I think what I've got now is a double conversion UPS right? From other projects, I already have a PSU, a battery or two I could use, and an inverter.

Do I have this right? Assuming the PSU and solar controller can support 250W, will this work? It's like a solar setup where it never gets dark.

Followup questions:

1. Since I'm going to have steady source of DC, do I need a solar controller? Can I just use a battery charger? I've looked at a few, and I've noticed that 20-30A battery chargers seem more expensive than equivalent solar controllers and from what I have read I'm not sure they'll still supply 20A if they're just in float or trickle mode.
2. If you all tell me I need a solar controller, I think I can just get PWM, no need for MPPT if the input is a steady 24V, right?
3. Can I do this with a LiFePO4 battery? My instinct is I can't because unlike a real solar setup, in this case, the battery will be charged nearly all of the time. From what I know, an SLA will handle being full for months or years on end better than a LFE. I wouldn't consider the cost of a LFE, but I have one already hand from a HAM radio project.
4. As I understand it, the load and battery are just wired in parallel, but with some disconnect logic. Right? Are their separate terminals to do low voltage cutoff and similar protection?

 

[tricky74]

I have a similar thing but have an actual UPS and I have removed the faulty ups batteries and installed an anderson plug where they were.
Removed the buzzer too.
I connect it to my solar setup for 240v power as I have no inverter.

What do you have for a power supply?
I think lithium you need to keep at a lower voltage than 100% if staying on charge long time.
Probably a BMS is a good idea.
The lithium brain crew can advise better.

 

[krby]

What do you have for a power supply?

Meanwell 24V 350W.

I think lithium you need to keep at a lower voltage than 100% if staying on charge long time.
Probably a BMS is a good idea.
The lithium brain crew can advise better.

RE Keeping it lower than 100%...Yea I think so too. The LFE battery is a drop in 12V 30Ah Bioenno brand. It has a built in BMS

 

[krby]

What I'm really hoping for by posting this is some advice on whether I can use a solar controller like I have described, or whether I could use a battery charger (which would do AC to DC for me) and if there are specific battery chargers that would do better at this constant power output use case than others.

 

[tictag]

Do I have this right? Assuming the PSU and solar controller can support 250W, will this work? It's like a solar setup where it never gets dark.

You definitely do not need a solar charge controller if you have no solar panels. That's kind-of a given. Yes, the power supply will power your loads but this isn't a UPS ... when power is removed from the PSU, your load (networking rack) dies. In order to provide a UPS function, you need a form of energy storage, such as a battery, to continue powering the load if the PSU dies. The trouble is a PSU will not charge your battery so once on battery power the battery will discharge and, even if you then reconnect the PSU, the battery will remain discharged.



Can you get away with powering all of your networking devices from DC? I mean you can buy DC-DC converters if they require a voltage dissimilar to that of your battery? If so, then all you need is a battery charger:



If you are unable to run everything off DC, then an inverter will be required for backup power with an ATS to switch between grid and backup supplies:



Points to note:

1. If external supply (grid/genset) is lost, ATS switches over to backup power from inverter
2. Battery charger is only supplied from external supply to avoid inverter powering battery charger
3. No circuit protection included for simplicity
4. On backup, load is only powered for capacity of battery - only means of charging is from external supply

To answer your follow-up questions:

Since I'm going to have steady source of DC, do I need a solar controller? Can I just use a battery charger? I've looked at a few, and I've noticed that 20-30A battery chargers seem more expensive than equivalent solar controllers and from what I have read I'm not sure they'll still supply 20A if they're just in float or trickle mode.

No, you do not need a solar controller.

If you all tell me I need a solar controller, I think I can just get PWM, no need for MPPT if the input is a steady 24V, right?

Not required.

Can I do this with a LiFePO4 battery? My instinct is I can't because unlike a real solar setup, in this case, the battery will be charged nearly all of the time. From what I know, an SLA will handle being full for months or years on end better than a LFE. I wouldn't consider the cost of a LFE, but I have one already hand from a HAM radio project.

An LFP will be fine, a good, if somewhat expensive, choice. Just ensure the battery charger has a LFP profile, or is at least programmable.

As I understand it, the load and battery are just wired in parallel, but with some disconnect logic. Right? Are their separate terminals to do low voltage cutoff and similar protection?

If you can power everything from DC, then yes, the load will be in parallel with the battery. If AC is required, then obviously not.

If using LFP then you could just run the battery down to zero SoC when the BMS will disconnect it, this won't harm it. If using lead-acid then you shouldn't regularly discharge them below 50% SoC so you can either carefully monitor SoC or install a programmable battery monitor to disconnect the inverter/battery at 50% SoC.

p.s. You can buy inverters with built-in battery chargers and ATS.

Edit: answered follow-up questions. Added postscript.

 

[tricky74]

OP states he has a battery
The LFE battery is a drop in 12V 30Ah Bioenno brand. It has a built in BMS

 

[krby]

Thanks for the answers. Let m be more clear on my thought process here.

I'm trying to do this with a bunch parts I have on hand because I think it would be a fun project and it's not really worth it to me to spend a bunch more. I'm familiar with inverter/chargers with built in ATS'. I have built a sort of portable backup box based on a LiFePO4

I have:
* Batts: LFE drop-in, I can get my hands on an 12V SLA AGM drop in if that ends up being a better fit
* Inverter: 12V 500W inverter. When I say "load", I mean just the inverter. Everything I want to power is AC, but I'll think about using DC-DC converters in the future.
* AC-DC PSU, 24V 350W (this may not be needed, see below)

So looking at the parts I have, I don't have a good way to charge the battery and also supply the load (the inverter) when grid AC is available. An inverter/charger is an obvious choice for this, but I they're expensive for this project. I need about 250-300W and those seem to start at 1000W and are priced accordingly.

If I could get a battery charger that provides 12V 20A output, this basically 2nd diagram @tictag posted, but with "inverter" in place of DC-DC converter as the load for the circuit. 20A would be enough to supply the load while charging the battery. I expect 220W max, given the labels on the gear, but it's much more likely around 100W of AC most of the time.

Before I posted the original post, I went looking for 12V 20A SLA or LFE chargers, and they're more rare and more expensive than I expected. It needs to be running all the time, something I'm not sure most SLA chargers can do. I looked at computerized chargers from the R/C hobby world because I have a lot of experience with those. Those will charge LFE and SLA chemistries correctly and the ones I am familiar with are very very configurable, but they seem to have a max session timeout. So I don't know if they're built to run 24/7, even if it it was well below their rated power.

So...then I remembered this forum, and then I had the idea of using a solar controller. To me, a solar controller is a DC-DC converter and battery charger all in one box, some of them even have LVC and other basic protection. They're intended to top up batteries or supply load, so I assume running 24/7 feels like it part of the design. It's possible to get at 20A or more controller for less than $100. The problem with using a solar controller is that it needs DC input...but I happen to have a 24V 250W AC-DC PSU already.

To be clear, I'm not set on using a solar controller for this, I just thought it was possible and low cost given the parts I already have on hand. I was asking if this would actually work. It does make the overall circuit more complex. The simplest to me seems to be a that 2nd diagram @tictag posted. 20A battery charger with AC input, a battery I have, and the inverter I have as the load. I just couldn't find a charger that would do that for not much money. Alternatively. a battery charger that needed DC input would work, because I have the AC-DC PSU.

 

[krby]

Putting this in a separate post because the other one was too long already. I'm also wondering if I should just do a AC relay that switches between grid power and the output of the inverter. If I do that, the the battery charger can be anything, just a simple trickle/tender charger would work. I guess where the above ideas are effectively a double conversion UPS, this AC relay idea is an offline UPS. I've seen AC relays with a 15ms maximum switch time, I'd have to test if that was fast enough to keep everything up.

 

[tictag]

Thanks for the clarification.

Seems to me that you just need to swap out your PSU with a battery charger then. You're inverter will need to handle the standard output of the battery charger i.e. up to 14.6V but that is typical. Plenty of LFP battery chargers out there for example CTEK, Sterling and Victron. Once the battery charger has finished it's charge cycle it will enter a 'Float' mode, typically 13.5V, where it will supply all you DC loads. If you actually want it to supply your inverter, you'll obviously need one capable of delivering the inverters load, which at 500W would be around 42A.

 

[JoeHam]

First, don’t even consider buying a SLA battery to replace your LiFePO4 that you already have.

I have the Bioenno 60 Ah battery and it has separate connections for charge and discharge. I gather from your post that the 30 Ah doesn’t??

Also, at the time I got it years ago Bioenno sold the charger for $10. Guessing you didn’t get one?

If you just need a charger that will be an easy fix. No need for a 20 amp charger for a 30 Ah battery in my opinion. Smaller will do fine.

Since you are a Ham I would bet you already have a 12v nominal power supply or two laying around?

Let’s get the battery side sorted out and move on from there. If you can confirm or deny my assumptions that will help.

 

[krby]

Thanks for the clarification.

Seems to me that you just need to swap out your PSU with a battery charger then. You're inverter will need to handle the standard output of the battery charger i.e. up to 14.6V but that is typical. Plenty of LFP battery chargers out there for example CTEK, Sterling and Victron. Once the battery charger has finished it's charge cycle it will enter a 'Float' mode, typically 13.5V, where it will supply all you DC loads. If you actually want it to supply your inverter, you'll obviously need one capable of delivering the inverters load, which at 500W would be around 42A.

Agree, I want the charger to "act like a DC PSU" when the battery is full, from the charger's point of view the load will be the inverter and battery wired in parallel. I have a 500W inverter, but the actual load I'm going to run in this specific case is max 220W and will usually be around 100W, so I've been looking for 12V 20A chargers (which seem sort of hard to find and expensive). You have confirmed one thing I wasn't sure of, that once a typical battery charger enters float mode it will still supply its full current if the load (batt + inverter combined in my case) is drawing that. From a few of the descriptions that I have read, it looked like the charger had a current limit once it got to maintain/float mode.

A charger would be the simplest, but was hoping to spend about $100 or less for it, those brands you mentioned are all pretty expensive. For my mobile battery backup project, I have a Victron Multiplus 3000VA and love it. But it is overkill for this DIY UPS project.

 

[krby]

First, don’t even consider buying a SLA battery to replace your LiFePO4 that you already have.

I wouldn't throw away the LFE, it would just go to being my "backpack mobile HAM" battery. The reason I was being flexible on the SLA vs LFE from what I know about LFE's a UPS isn't a great use case. It'll be kept at full all the time, except when the grid AC goes away (rarely). From my research on my previous project, LFE's don't like to be floated near 100% all the time. I don't need the weight savings. The usable capacity of LFE is useful for a UPS, but I'd go for an SLA (which will be free or basically free from a friend) if I found a not-expensive charger that did SLA only.

I have the Bioenno 60 Ah battery and it has separate connections for charge and discharge. I gather from your post that the 30 Ah doesn’t??

Also, at the time I got it years ago Bioenno sold the charger for $10. Guessing you didn’t get one? If you just need a charger that will be an easy fix. No need for a 20 amp charger for a 30 Ah battery in my opinion. Smaller will do fine.

Since you are a Ham I would bet you already have a 12v nominal power supply or two laying around?

Let’s get the battery side sorted out and move on from there. If you can confirm or deny my assumptions that will help.

No separate charge/discharge terminals. Kust pos and neg. I do have a 4A charger from Bioenna for it.

Charging this LFE and keeping it charged is easy with gear I have on hand. I have many many chargers!! In addition to 4A one from Bioenno, I have two different configurable multi-profile chargers intended for R/C Hobby batteries. Those will charge a variety of things as fast or slow as I need. Those R/C chargers take DC input, but being an electronics amatuer and a HAM guy, I have at least two AC-DC PSUs (13.8V and 24V) I could use to feed those.

The problem I'm trying to solve is keeping the battery charged AND supplying and inverter (basically a double conversion UPS). This is a personal project mostly to see if I can do it mostly with gear I already have. I'm not motivated to spend a bunch of money on it. If I was, the simplest way to build this would be a low power inverter/charger and a battery. But from what I can tell, those start at 1000VA and at least $400

 

[tictag]

it looked like the charger had a current limit once it got to maintain/float mode.

They definitely will, hence if you do need to permanently supply an inverter with it (at least whilst it is plugged in), you'd be better buying one that is capable of supplying the rated output of the inverter. The Victron BlueSmart 12V|17A 120V charger is only $133 on Amazon, and that's top-tier, I don't see why you can't buy a sub-$100 version of it.

 

[krby]

They definitely will, hence if you do need to permanently supply an inverter with it (at least whilst it is plugged in), you'd be better buying one that is capable of supplying the rated output of the inverter. The Victron BlueSmart 12V|17A 120V charger is only $133 on Amazon, and that's top-tier, I don't see why you can't buy a sub-$100 version of it.

Ok. I'll look harder. I was planning on just a fuse or breaker on the input side of the inverter to enforce the max DC current l expect given the load.

 

[JoeHam]

How many amps is your power supply rated at?

Can you adjust the 13.8v?

Give the make and model please as most are adjustable.

I like your idea of making this work with gear you already own.

 

[krby]

How many amps is your power supply rated at?
Can you adjust the 13.8v?
Give the make and model please as most are adjustable.

Two PSUs not being used for anything else:
- PowerWerx SS-30DV. It's 14.1VDC (I assumed 13.8V above) 25A continuous. It's not adjustable
- Unknown brand 24V 10A. I don't remember seeing an adjustable pot on it, so likely fixed.
- MeanWell LRS-350-24. Spec sheet says 14.6A. Adjustable 21.6-28.8V. I'm using this for another project, but could swap it in for this UPS. Or pick up another one from a friend.

 

[JoeHam]

Well I think you are good to go if you accept some compromises. And this assumes I understand what you want which I believe is a UPS type setup.

First off, your Bioenno battery has a BMS in it to protect itself. Note that Bioenno charges these with a simple constant voltage supply. If you want to confirm this just call Kevin at Bioenno using the phone number on their website, he is quite approachable.

Second, it sounds like you will be getting, or have an inverter that can handle your 100-220w load. The rest of the stuff you have on hand.

If you want to use the PowerWerx unit it should handle the load since it is capable of 25 amps continuous at 13.8v (345w less inefficiencies).

The 13.8v is a bit high to keep your LiFePO4 battery at but not by much. True confession here: I have often stored my Bioenno 60 Ah battery at (gasp outloud) 100% SOC for at least 5 years now. I know it is not good for it but to be honest I did it for years before I learned better and will continue using it, perhaps at diminished capacity for as long as I have it. I will not be surprised if I get another decade out of it in some capacity.

IF my lazy butt were worried about dropping the voltage to the Bioenno I would just add 20-50 feet of wire I have laying around to feed the Bioenno and find a length that drops the voltage down to whatever you find acceptable.

To get a little more sophisticated just add a relay that will use the PowerWerx when AC is present and then switch to the Bioenno when AC drops out.

Like I said, some compromises but you probably have everything you need to get this done in an hour or two without a Solar Charge Controller. Your choice.