UPS Recommendation?

[Joe_H]

Have you actually used one of these? Yes, drop-in LiIon replacements are being made, are expensive and only replace 12 V batteries. If th UPS uses 6 V batteries it is a no-go. The only reason they work as drop-ins is that internally they have circuitry to "look" similar to lead acid. A particular UPS charging circuit may not work with them. And if you are pulling a load higher than the UPS's inverter is rated for, that may shut it down as well.

[JimF]

I have at least six of the CyberPowers, and they are good (better than APC). But I am slowly letting them die off as their batteries give out on most of my machines, the ones that are on Ubuntu. That is because as the batteries become marginal, the UPS will shut down unexpectedly. You don't know whether it was a power outage, or a weak battery. They don't have a good test for that. It is some trouble to sort out.

I find that on my Ubuntu 18.04 machines with the EXT4 file system, I don't get file corruption when the power goes out anyway. And I set the motherboard BIOS to automatically start up the machines after power is returned, so I don't even know of an outage. It is a bit different on my Win 7 machine, and I will keep a UPS there for insurance.

[Rel25917]

Enjoy the house fire, lithium does not charge the same even if voltage is same.

[Kebast]

Enjoy the house fire, lithium does not charge the same even if voltage is same.

Yep, I don't recommend this

[MeeLee]

Enjoy the house fire, lithium does not charge the same even if voltage is same.

As long as you stick with Panasonic or LG cells, you're good.

[Joe_H]

And does Panasonic or LG make up the battery with their cells and protection circuitry? More important than the maker of the cells is that protection circuit. And without tearing apart a built battery from some maker who sources OEM cells from whoever, you have no way of knowing what is inside for sure.

[MeeLee]

Why do you need a protection circuit?
Most UPS systems have 12V.
I already mentioned above that the upper voltage of 3,7V charged, is higher than the UPS will be able to charge (3,7V x 4 = 14.8V; lead acid battery charging circuits top off around 14,4V).
It'll never overcharge!

And at the lower end, the cells won't ever get so low in voltage that they would end up dead. Their safe lower limit is 2,7V x 4 = 10.8V. A lead acid battery is considered dead at 11,7V. The UPS won't allow the Li batteries to go below this voltage.

The Li batteries will charge faster, thanks to their (much) lower internal resistance.
As long as you're not putting 4 cells in series, but have a minimum of 8 cells (2 rows in parallel), or more, the charging process won't easily overheat the batteries.

and in the space, there easily fit 40 cells. That's 10 rows of 4 cells in series. The charger needs to be a 20 amp charger or more to overheat that, and I highly doubt the circuit could even provide 20A charge currents!

[Joe_H]

There are many things wrong with that approach, for starters the assumption that the charging circuit of th UP will not exceed 14.4 V. Many charging circuits depend on the increasing internal resistance of the lead acid batteries approaching full charge to limit voltage and current. Your approach would put a bunch of low internal resistance LiIon cells across that instead.

As for the lower voltage, I have seen UPS batteries pulled down below that 11.7 V in more than a few models.

The protection circuit is needed to ensure that all of the cells are getting the same charge and to also monitor temperature. The correct charging current depends on that as much as the voltage.

To get the voltage mentioned, you do have to have to put 4 cells in series, so your last two paragraphs contradict each other.

The exact voltages will also depend on the specific chemistry of the rechargeable Li cells involved. Nominal voltages range from 3.2 to 3.85 V.

Essentially you are depending on such jury-rigged system to behave without all of the normal protections that should be used with Lithium cells. It might work okay with a specific design of UPS, and fail entirely with another. If that happens and causes a fire, you may have a hard time getting a fire insurance carrier to pay out on a claim.

[MeeLee]

My last paragraphs don't contradict.
Running 4 cells in series to get the 12V might draw too high amperage for the cells.
However, if you have 2 banks of these in parallel, their individual charging current halves.

I really don't see HOW it could cause a fire..
Really.
Perhaps the charging circuit of the UPS might catch fire, but as long as it can handle the lower internal resistance, the batteries won't easily catch fire.