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.
The answers that reference Li-Ion are accurate, I wouldn't ever use Li-Ion with a charger that is designed for Lead Acid. But as was pointed out LiFePo4 are more tolerant (and much safer) than Li-Ion to various charging voltages. I just checked the voltage on my APC 1500 and it is about 27.35 volts. Each LiFePo4 cell can take unto 3.65V, but you'll want to charge to a lower voltage to extend life. At 27.35V that gives you 3.42V/cell, which is at a pretty good point for maintaining the batteries. The BMS will also cut off at 2.5V/cell - 20V for the pack if the UPS hasn't cut off before that point. The BMS (if you're building your own pack as I am) can be programmed for cut off voltages and currents to protect the cells, but again as pointed out USE A FUSE or high quality breaker inline with the battery! Bottom line, it is doable. Regarding the capacitors - never just hook the battery up to the UPS (or inverter for that matter) without recharging the caps. You can do this by using a high wattage 100 ohm resistor on either the positive or negative side of the battery just long enough to get them charge and reduce the chance of a massive inrush voltage. Once charged, hook up your battery without the resistor.
All that said, as suggested you may be much better off finding a reasonable charger/inverter to be sure you don't run into any issues.
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.
LFP and SLA (sealed lead-acid) batteries charge at different voltages. SLA batteries charge at 14.4 volts and LFP charge at 14.6 volts. That might not seem like much but if the battery charger will not get to 14.6 volts then the LFP battery will never completely charge. Every UPS built today will be programmed with a maintenance cycle for the battery type that comes with it. SLA batteries need to have the charging voltage cycled in a certain way to optimize battery health. This charging cycle is different from LFP in more ways than just the maximum voltage.
There are battery management systems that allow for LFP batteries to "drop-in" to something built for SLA. LFP batteries with such a BMS will advertise they are drop-in replacements for SLA because this is not something that is cheap and easy to do. One thing they'd have to do is boost up the charging voltage. They may also buck the voltage down because the 12 volt and 24 volt ratings of these batteries are a nominal voltage. The actual voltage of a fully charged "28 volt" LFP is more like 29.2 volts. https://footprinthero.com/lifepo4-battery-voltage-charts
It would not be wise to put LFP batteries in any UPS built for SLA batteries unless the LFP battery pack is explicitly labeled for such a use. The BMS may protect against things like overloads but it can't raise the charging voltage if it wasn't built to do so.