LiFePO4 batteries plus a BMS are a mostly drop in replacement for lead acid. LiFePO4 is often cheaper than SLA nowadays, especially for small packs.
Do you have proof to backup this claim? My findings show the opposite.
I bought 8 of these '7000mAh' 32700 LiFePo4 cells recently:
https://www.aliexpress.com/item/32998328673.html?spm=2114.12010615.8148356.11.5c8f5451McipjNThe 700mAh headline number is obviously fake and does them no credit but they do state 6500mAh as the real capacity. I measured the actual capacities @ 1A discharge rate:
6323, 6353, 6373, 6375, 6379, 6381, 6381, 6381mAh
That's a 1% spread. 6 out of the 8 are within +/-0.08% of each other! I wasn't expecting 6500mAh so I wasn't dissapointed having seen a couple of youtube tests and this review which I believe to be the same cell under a different brand name:
http://budgetlightforum.com/node/65598The author of the review, HJK, is a member here. Oddly the above review isn't on his website
http://lygte-info.dk/. Again the spread in capacities was very small but around 5950mAh. That was 12 months ago, so perhaps the latest cells are higher capacity or there are larger variations between batches. I have ordered some more but it will take at least 4 weeks before they arrive.
4 cells to make a 6Ah+ 12V battery currently costs £10.5 or £130/kWh ($163/kWh) which is the lowest cost Li-ion price that I know of for small quantity purchases. I don't know how much a suitable BMS and case would cost but the total cost isn't going to be much more than the £10 typical cost of a 12V 7Ah SLA. I'd be surprised if you could get below the cost of an SLA though.
You could fit 8 cells, 70mm x 32mm, into the SLA case to get 12.6Ah
Either way, I don't mind switching to LiFePO4 however it would need to be a drop in replacement for the typical 7AH lead acid batteries found in most commercial UPS units.
Would it be like building a Li-ion battery pack? Shouldn't be a problem as I have made some of those for power banks in the past.
For a drop in replacement the BMS is key. Li-ion cells must not be charged below 0C so won't be suitable for all uses; ideally a BMS with temperature sensor should be used to prevent charging below 0C but I guess most UPSs aren't installed in such environments.