I have an existing APC 1500 UPS, designed for use with 24v lead acid batteries and want to know if it would be possible to replace my dead lead acid battery with a 24V Lithium Iron Phosphate (LFP) battery instead. The 24V LFP battery comes with a built-in BMS, so I assume (but don't know, not being a battery expert) that the BMS should be able to regulate and protect the LFP battery. I checked with the seller and they 'think' it should be possible but are not certain. Can anybody knowledgeable help?
I would say NO. Yes 24 CFC is 24v and that would not matter but the charging circuitry for lithium ion is radically different than lead acid and using the APC battery charging circuit may cause the lithium ion battery to explode or catch fire.
If you can convert the charging circuit to a lithium ion type circuit then it could be possible.
Ed Beal is correct. The answer based on the information provided is NO. However, there are battery replacements that are advertised as "drop-in" matches for lead-acid versions. They typically have circuitry that manages the charge profile for the new chemistry. If your battery presents itself as you suggest, it's possible you've found such a drop-in replacement. Consider to edit your question to include a product link.
All of the search results I've pursued for this type of product (specifically 24v lead acid replacement) have text indicating that a lithium rated charger is to be used, not the lead acid charger. This is not going to work within a UPS unit, which charges based on the lead acid profile.
BMS terminology can be somewhat misleading. Some of them regulate charging and discharging, others manage only the charging aspect. One would expect that a drop-in replacement has at least the charging portion and has tailored it to expect the input voltage and current for the device in which it is installed. For example, in a conventional motor vehicle, the alternator can be expected to provide 14.1 vdc, which would cook a lithium battery in short order. The drop-in version would accept the voltage and massage it to keep the lithium battery comfortable.
The OP would have to discover a drop-in lithium pack that is designed to accept either 13.x or 26.x volts (typical VRSLA charging level) and adjust it to the native/healthy levels required of the lithium pack. Accordingly, the output of a lithium-based battery isn't going to match precisely the UPS VRSLA figures, but that's less of a factor, generally speaking, as such devices are supposed to handle 13.x down to about 10.x volts. The drop-in BMS has to also manage the lithium-based battery cut-off voltage independently of the UPS cut-off voltage, so it has to be smarter than the average charge-only BMS.
I've looked at it, and aside from the problems others state, it's a waste of money.
Lithium batteries are so expensive that it ends up being pearls before swine: you have this Cadillac battery bolted to a Pinto UPS.
By the time you spend that money, you might as well install a proper lithium-rated battery charger, and an inverter, and have yourself a full online UPS in effect.
MAYBE. Depending on the BMS behavior and if you know exactly what you are doing.
LiFePO4 batteries are used in cars as lead-acid replacement with no apparent ill effects and the conditions in a car are way harder.
APCs have quite "mild" charging algorithm, something like CC/CV where CC is like 13-14 ampere and CV is 27.8V. Nothing a sane 8S LiFePo4 of more than 20Ah capacity cannot handle.
They (in particular APC Smart 1500 or 1400VA models with a fan) are also quite tolerant to using them with 20x the nominal battery capacity. They don't complain, don't overheat (provided no excessive amount of dust) and don't shutoff early with the big battery almost full.
The fuse in the original APC setup is in the battery. Be sure to have one real, hardware, melting-conductor based fuse, no matter what your BMS manual happens to boast.
What can go wrong, then?
The BMS will have no means of lowering the charging current except by cutting off the charger. This means somewhat inefficient ballancing and the ballancing outright failing when the battery is deeply unballanced. Or your ballancin circuit can handle 15 ampere per cell? Good!
There are huge capacitors in the UPS, on the battery side. Be sure your BMS can handle them (hint: it is not trivial). Not sure of their exact value, but you will open the UPS anyway.
The UPS will not show neither reasonable runtime values, nor an adequate low battery warning. It discharges lead-acid batteries down to 19.5V or so. The BMS over-discharge protection will trigger first.
I have a similar setup in my motorhome (a rackmountable APC 1500VA + 8S 100Ah LiFePO4 battery), but I rarely use the charging mode of the UPS. The UPS is basically used as a foolproof inverter.