Charging small LiFePO4 (home built 12 volt UPS)

I am wanting to build a small 12 volt UPS, specifically for and only for a household router. It runs on 12 volts.

There are some lithium ion powered 12 volt mini UPS setups out there for not a lot of money but I had wondered if I couldn't do a little better than that. Reviews on them suggest they're a lot like laptop batteries, which are generally good for a year or so and then die. LiFePO4 batteries are supposed to have a much longer life but I have not seen anything for sale with that type of battery. Maybe there is a problem with that type of battery in this application? If so, please tell me.

I don't need a lot or power. The router only draws something like 12 or 15 watts. And I don't need a super long run time. If the power goes out and stays out, there is time to put other backup systems in place and keep on going. But the router needs to be uninterruptible. (Don't wanna bore anyone with the details of why. Yes, the internet still works when the power goes out. If the router is live, laptops running on battery power would not know the power went out because the internet will keep on going just fine.)

So with that said, the LiFePO4 battery is quite easy and cheap. Most of them come with a basic BMS.

Output for the router is easy with a tightly controlled output of a very basic buck converter, cheap and easy.

What becomes more questionable for me is how to charge the battery? I would prefer to just use something like a wall wart to keep a trickle going to the battery all the time. But, I do not know if I would need to place some kind of a current limiter in that charge circuit such that the battery won't demand an output form that wall wart that would basically make it self destruct. Or would it? I'm thinking a solar panel would basically give what it had but not be damaged in any way by a battery that would like more current. A solar panel for this application would not be as easy to make happen here.

I could potentially put an additional buck converter in between the wall wart and the battery to control both the voltage the battery sees as well as the maximum current. (Correct me if I'm wrong about that.) They're cheap, especially in such a small size.

What do you think?

FWIW, I've been using a standard 120 volt UPS. It died last night, probably the SLA battery, I haven't dug into it yet. E02 code, I think. And I could potentially either fix that or just go back to one of those types of units. Kinda wanted to explore the mini homebuild, though and figured I'd run it past some of you. I know you're a pretty bright group... and my electronics education was long enough ago that too much is fading. Thanks!

Lithium batteries CAN work for years, my "almost" off grid system has been going for six years and still works fine on the original batteries.
However, they need to be treated with tender loving care and looked after.

Lithium batteries in laptops and power tools get thrashed mercilessly, are put away for long periods in the fully discharged condition and generally abused and quite naturally this shortens their life.

If you just buy a cheap Lithium battery and connect it up to a wall wart it too will probably not have a super long life. However it might still give you pretty good value for your dollar. Same with the sealed lead acid batteries. They are cheap enough to replace every year or two and just accept that they are not designed to be long life batteries.

To build a really good long life Lithium setup for your UPS can certainly be done, but it requires a good quality voltage regulated charger, with current limit, and a system to keep the individual cell voltages very close in voltage. That costs more though.
You can buy 12v Lithium battery packs that have all the internal battery control and monitoring system inbuilt.
And you can buy a decent switching power supply to keep your UPS running, set to a precise and accurate voltage (13.8v or 3.45v per cell recommended).

One very big advantage that a UPS application has, is that it goes through very few actual charge/discharge cycles. It normally just sits there at a fixed voltage. Sealed lead acid batteries still tend to dry out over time, but a Lithium battery is actually really fully sealed.

Warpspeed said:

Lithium batteries CAN work for years, my "almost" off grid system has been going for six years and still works fine on the original batteries.
However, they need to be treated with tender loving care and looked after.

Lithium batteries in laptops and power tools get thrashed mercilessly, are put away for long periods in the fully discharged condition and generally abused and quite naturally this shortens their life.

If you just buy a cheap Lithium battery and connect it up to a wall wart it too will probably not have a super long life. However it might still give you pretty good value for your dollar. Same with the sealed lead acid batteries. They are cheap enough to replace every year or two and just accept that they are not designed to be long life batteries.

To build a really good long life Lithium setup for your UPS can certainly be done, but it requires a good quality voltage regulated charger, with current limit, and a system to keep the individual cell voltages very close in voltage. That costs more though.
You can buy 12v Lithium battery packs that have all the internal battery control and monitoring system inbuilt.
And you can buy a decent switching power supply to keep your UPS running, set to a precise and accurate voltage (13.8v or 3.45v per cell recommended).

One very big advantage that a UPS application has, is that it goes through very few actual charge/discharge cycles. It normally just sits there at a fixed voltage. Sealed lead acid batteries still tend to dry out over time, but a Lithium battery is actually really fully sealed.

Click to expand...

Hmmm... OK. Thanks for the thoughts.

It sounds like the charging part of it is going to be the most demanding part of the whole thing and that my little wall wart probably isn't going to get the job done well.

I think I may have confused a BMS with an actual charge controller.

The complexity may be one reason I don't find LiFePO4 batteries in small UPS systems.

Thanks for your input.

Why not just run your router off your battery 24/7.

Get a programable charger to maintain a conservative battery voltage. If the power goes out, there is no cutover, you only lose charging. Removing he complexity of an inverter and converting power 3 times! from AC to charge DC battery to AC inverter to 12V router will be far more efficient as well.

MisterSandals said:

Why not just run your router off your battery 24/7.

Get a programable charger to maintain a conservative battery voltage.

Click to expand...

That is the intention! The router doesn't need 120vac anyway.

The second part of it, the "programmable charger" is where I am trying to learn.

It would be quite easy to just run the router off of the battery. I just need to figure out how to maintain that battery in a way that the battery likes enough to last for a very long time, even if used as emergency backup from time to time, and it will be.

Aren't LiFePO4 batteries kinda fussy about their charge parameters? Or am I making that out to be more than it really is?

Thanks!

Complexity and cost.
UPS manufacturers need to keep their products cheap to be competitive, so you generally find sealed lead acid batteries.
Nothing wrong with that, except its pretty much down hill after about a year of operation.

Charging part is not difficult. All it takes is a PROPER voltage regulated power supply that has some adjustment of output voltage.
Get something well up to the job that will not be running absolutely flat out all the time.

How about this:
13.8v, 5A, 70watts, $6.75
https://www.ebay.com.au/itm/155842875935?hash=item2448f5921f:g:bGIAAOSwMLFk7b5s&amdata=enc:AQAIAAAA8Ggqpzb/EhT50QGPm7gb32pQqR6TOUxCkarVa6X7PH72LOoavBkZpPi0O8yr6eBcfeYk0ub3FsQ1g5mUtzt0uBssTFMZXe4PJm5Swir+UXjMkUbkQJUB4Nwc6O8FQsXTwlOuAdi8MGT+aRSM8JbAf1zXCOBajdVXQdYfcBBBuE0i7umMmlcQ6gADcguKr00gmiAE01NHNbLsD34SXQEp+XC0Vx+p4IS/rCHqXHqx4vFAZ3aZq8HPDD6t9/CyU0GfdpsnDxnES42DAYwJWINDsjr3LT5eN7rpkkJnOyP0ReMQXRJYXx9u3nQ9HVlc3rIzQQ==|tkp:Bk9SR7Lwq6azYw

Double post.

Warpspeed said:

How about this:
13.8v, 5A, 70watts, $6.75

Click to expand...

That's actually a bit large for what I'm considering.

Here is an example of the battery I had in mind:


Recommended charge current on that is 2 amps. Would the power supply you mention be feeding it at a rate higher than optimal? Searching here in the US, I'm not seeing chargers that small but may be looking in the wrong place. (?)

That same supplier can supply a 2 amp thirty watt version for the same cost.

https://www.ebay.com.au/itm/1558428...MQXRJYXx9u3nQ9HVlc3rIzQQ==|tkp:Bk9SR7Lwq6azYw

It's a bit more of a search here in the US but I think I've found a supplier or two who has small power supplies like that.

Is it really as simple as just setting the output voltage on that power supply to an appropriate voltage (14.2?) and hooking the battery up directly to its output?

Yes its as simple as that.

14.2v is a bit high for lithium, you will gain hardly any extra capacity, and its much safer to stay within the flat part of the voltage curve.
Above about 3.45v per cell, the voltage begins to rise increasingly steeply, but you gain very little by doing that.
Be kind to your battery.

Thanks very much!

I have for example a dumb* buck converter in // with my grid-tied solar inverter on 38V solar cells.
* just take care that the buck converter can handle more than 45V input and has an adjustable volt / amps feature that you can set to 13V / 2A
On the DC side I have the regular power supply of the router, that delivers ~12,8V DC.
The 20 Ah battery and the power supply both feed the router over two blocking diodes.

The grid-tied solar inverter cares for MPPT and the parasitic load of the buck converter is so low that it does not interfere.
The regular power supply takes care that the batteries are never discharged below 12,5V (unless you have a very long power outage, which never happens)

I don't know if the topic is still in consideration for the OP, but I have seen a similar setup to maintain a wifi-router. He wired it to a 50AH LiFePO4, then bought a CC (MPPT of course) and paired a 100 watt panel to that; making it its own self-contained system. The only concern would be for days with very little to no sun at all. That's where I would suggest that charger Warpspeed has recommended.

I might have been foolish, but I just replaced the Cyberpower lead acid batteries with equivalent LiFePO4 batteries. From what I read, you might not get accurate readings of power remaining/discharge time, but since it spends most of the time at full power, the feedback was it wouldn't hurt the battery.

Any objections or thoughts on this?

jsmithtx said:

Any objections or thoughts on this?

Click to expand...

I'm not well-knowledged on the advanced science behind LiFePO4, but I would probably keep the SOC around 40 to 60% and only top it up if you know there will be a storm/chances of power outages. I had a EcoFlow River 2 that was always sitting at 98% (I used it for its PSW inverter and left the machine plugged in) and it now only charges my phone for 45 minutes before it shows 10%. Again... I don't know if it would be the same across the board for all LiFePO4 batteries, but just an idea.