The primary reason why they are so cheap is because in China there are literally multiply grades of quality for many products. There is top quality, medium, and bottom end. They make low grades available as they know there are always a niche market for it. There are so many people always looking for the cheapest buy they can get and that is where the money is.
When LiFePO4 batteries first came out mainstream about a decade ago the diy cells and also top brand batteries hardly sold at all, the chemistry was considered exotic and out of reach. No one wanted to spend 2grand on a 12v battery.
Then things started to change from the Chinese cell manufactures all the way to suppliers, they started making them for much cheaper prices and know everyone is getting them. Another secret to their success is Lifepo4 naturally has a very long lifespan and this helps to hide the cheap Chinese quality. On the other end lead Acid has the massive problem of Sulphation which means if not cycled perfectly with correct voltages and duration of elevated voltages even top of the line US batteries can die within just 150 realworld cycles. So made in china lithium is on fire and now even the producers (at least in my area) no longer assemble them, they just design them and its all made and assembled in china.
No one really knows how many cycles lifepo4 last, not without actual science anyway. So far the cycle figures are just an assertion. No real studies have been done besides a few datasets from individuals. I just brought a very pricy "well engineered" 180ah 12v and honestly I have no idea how long it will last. Could last 2000 cycles or maybe only 800. When this chemistry started to come out many armchair experts in the Lead Acid & electrical solar community were highly skeptical, most had no realworld experience with them but many ordinary people using the stuff showed they actually last alot longer than the "experts" thought. The key is making sure they are well looked after and babied, abused they dont last long.
In short the factors that determine cycle life are charge rates (higher rates degrade the battery, like 0.5C+), DoD (staying in the 10-90 window is best but this practice is misunderstood, lithium always gives the max throughput when cycled between 40-60 or 40-70), the time kept at elevated SoC/high voltages (this degrades the electrolyte and SEI layer and also speeds up side reactions that permanently reduce capacity), exposure to sustained high temps, capacity of cell (lower capacity tend to last longer especially under hard loads with no cooling), cell format (cylindricals do better at high c rates) and just dumb luck. In EVs, they actually pre-heat the cells before fast charging, then quickly cool them off after which all helps alot minimizing the degradation.
Top of line Chinese brand prismatic in the diy world are known for 1000-2000 to 60-80%DoD in realworld offgrid, there seems to be a large variation owing to the average quality but also how because of just many factors their are in operation.
Top of the line made in japan consumer products were alot more but also alot more pricy. The golden rule with all batteries is you get what you pay for, you cant rush or cheat quality, not with chemistry anyway. If you want to hammer your cells like in a cordless drill or EV you must get top quality.
Top of the line made in japan and korea conventional 3.6V cylindricals like LiNiMnCoO2 can do 400-1200 depending on use case and DoD. Back in the early mid 2000s when I got into high end flashlights it was probably half for these types of cells. Given how big lithium has gotten with RE, they are making big progress with studies showing even more cycle life than Lifepo4, this is a bigdeal as the conventional lithium ion has much higher energy density than lifepo4.
However, the same is true for Lead Acid, there are many grades of quality and on the extreme end some brands are rated for 3500 cycles to 50%DoD. I eventually tossed all my lead acid batteries due to the constant upkeep of maintaining them.
Rightnow 12V Lifepo4 batteries sit at 123-149wh/kg specific capacity on the higher end vs 35-50 for the best lead acid AGM and Flooded deepcycle. So they are an amazing chemistry and what makes this chemistry so good is its open source nature, basically any Lead Acid charging gear (at least quality gear set properly) can charge Lifepo4 no issues, all thanks to their voltage profile being very similar to Lead Acid.
