Need help designing a small 4S 12v UPS

[dpcman]

Hi! I'm pretty new to this whole thing and I'd appreciate some advice on building something that can power my desktop PC during blackouts. At first I considered getting a portable power station like an Ecoflow or Bluetti but I realized I'd get a better bang for my buck if I were to build one on my own plus I don't need the portability and other fancy features.

Power draw from the watt meter is 300w (rounded up) and I'd like it to last 8hrs so that comes to 2.4kwhr. While the blackouts are often, they rarely last more than a day so this is all I need. Can't use solar at the moment since I live in a rented space. With that in mind, this is currently my part list:

• 4 x EVE LF230 2.3v 230AH
• JBD BMS 100A w/ bluetooth
• Pure sine-wave 2000w inverter

That's all I got for now haha. I'll try to formalize my questions:

1. Many of the inverters I could find locally look dubious at best, is there anything I should look out for? How do I know if an inverter is any good? I should say that I already have a UPS that I will plug into the inverter.
2. Should I use fuses? How can I calculate what fuse I would need?
3. I'm planning to charge the battery via the AC wall outlet. What would be the best way to go about this?
4. Is it possible to plug the battery along with my PC into the wall outlet indefinitely and have the battery kick in when the power goes out? Are there any downsides to that? Since I have a UPS I don't particularly mind manually switching the plug but it would be convenient.

That's all the questions that has me stumped at the moment, thanks for any insight on this.

 

[BentleyJ]

If you already have a traditional UPS why wouldn't you just use it and expand the battery pack if it has that capability. Many of them do have a connector on the back for Aux battery packs.

As to your specific questions if the above is not feasible.
1) Samlex or AIMS??
2) Yes, the battery to inverter connection should have a fuse based on the current draw of the particular inverter chosen. (see #4)
3) Stand alone Li battery charger that plugs into a std 120V outlet.
4) If you choose a good quality sine wave inverter, 1000W would be enough to power the 300W load. This would save you some $$. An appropriate fuse for 1000W, 12V inverter would be 100A.

 

[dpcman]

If you already have a traditional UPS why wouldn't you just use it and expand the battery pack if it has that capability. Many of them do have a connector on the back for Aux battery packs.

That's a good point haha, but alas, I just checked and sadly it does not have any aux ports. Either way, I'd rather have the option to plug something else in case there's a need for it.

1) Samlex or AIMS??

I should have gave context that I live in the Philippines so the market here isn't as varied compared to the rest of the world. Plus the power grid here uses 220v/60hz. Thanks for taking the time to reply, I think I've got a good idea what to do now, except for #4.

I meant if it was okay setting up my devices like this: AC outlet > battery > UPS > PC. I'm worried if there's any long-term side effects if the battery is kept plugged 24/7 while supplying power. I imagine it wouldn't be feasible if I were to use a battery charger. Is it possible to use some sort of power bypass method? If not I'll just stick with the charger and manually connect the battery when the need arises.

 

[brum]

There may be long-term side effects of constantly charging the battery. This depends on the charging approach. If you keep cells for a long time above 3.4V they will degrade.

Another thing that will impact cell lifespan is temperature. Avoid low (below zero is a no-go zone, preferably above 10C) and high (above 35C) temperatures.

I'll be modifying soon an of the shelf UPS and swapping its 7Ah AGM with 22Ah LiFePO4 cells. I'll take care to properly document what and why I'm doing and create a topic on that here.

 

[BentleyJ]

I meant if it was okay setting up my devices like this: AC outlet > battery > UPS > PC. I'm worried if there's any long-term side effects if the battery is kept plugged 24/7 while supplying power. I imagine it wouldn't be feasible if I were to use a battery charger. Is it possible to use some sort of power bypass method? If not I'll just stick with the charger and manually connect the battery when the need arises.

It depends on the battery charger and how your system is configured.
The battery charger should be chosen so you can enable or disable Float and also have the ability to set the bulk and absorption voltages. These features imply a midrange priced charger rather than whatever is cheapest.

As you mentioned there are 2 ways to connect everything.
1) UPS plugged into the grid outlet, computer plugged into the UPS. Inverter Off, sitting on shelf. In this case the batteries should not have constant float charging. Probably just turn on the charger once every 2 weeks for 30min to keep them topped up. Or set up a plug in timer to do it.
2) UPS plugged into Inverter, Inverter On all the time powering the UPS and computer. In this case the batteries can have constant float as long as the voltage is kept low as per the post above from brum. This is the most convenient configuration.

 

[dpcman]

@brum Looking forward to your post!

@BentleyJ After looking around, I think I might have found the perfect candidate for the battery charger: Mean Well NPB series
Seems to tick all the boxes and even has a predefined charging curve specifically for Li batteries with BMS ?Looking at the spec sheet, it seems the constant float is set to 13.8v. Only drawback I can see is that I need to purchase their programmer if I wanted to configure the voltages manually.

They do have the NPP series which functions as both a charger and power supply and is adjustable via potentionmeters. It makes me wonder if that would be a better fit for my purposes.

 

[BentleyJ]

I use MeanWell low wattage DCDC converters for a few PCB designs I've worked on. I know they also make much higher output DCDC converters but wasn't aware of their battery charger products. Good find.

 

[MarcoWasRight]

There may be long-term side effects of constantly charging the battery. This depends on the charging approach. If you keep cells for a long time above 3.4V they will degrade.

Another thing that will impact cell lifespan is temperature. Avoid low (below zero is a no-go zone, preferably above 10C) and high (above 35C) temperatures.

I'll be modifying soon an of the shelf UPS and swapping its 7Ah AGM with 22Ah LiFePO4 cells. I'll take care to properly document what and why I'm doing and create a topic on that here.

Sorry to necrothread but where did you purchase your 22Ah cells. I am assuming you are talking about the Gushen cylindrical that are 5C/10C.

 

[brum]

From here - https://www.aliexpress.com/item/1005004221234736.html .

 

[MarcoWasRight]

From here - https://www.aliexpress.com/item/1005004221234736.html .

Thanks. Same cells I am looking at. Love how Varicore/LiitoKala has to lie and call them 25Ah cells when they are 22.

How accurate is the 5C continuous rating. For example A123 is a little optimistic in their claims of 50A continuous. I find 20A to be a good middle ground to run them at and keep the cell above 3.00 but much more than 35A is reallllly pushing them. If I was able to load them up with 50A I’m not sure they could take it without voltage being in the toilet. Granted they are NOS cells, maybe I should buy a new one from LithiumWerks.

I feel like the 120A burst rating would drop the voltage to 1.5V at which point you’d need a 6 or 7S pack for 12v

 

[brum]

No idea about the load rating. I'll be using them with a few amps of current. The UPS will be producing 100W at most, just to support a single low-power device.

They are good for very short pulses though. I'm using the 4S setup as a power source for the spot welder. It performs better than the 80Ah lead acid battery that was used before that.