I currently have a 12 volt UPS unit that uses 4 100ah LiFePO4 batteries.  Can I add a 4 cell 200ah LiFePO4 pack in parallel to increase the power/storage to 300ah or do the additional batteries need to match the current 100ah batteries for a total of 200ah?

As long as they are parallel connected ( adding more amps) it is fine . In series ( increasing voltage) it is no good , since different wear makes the total pack perform worse , and puts additional strain on the weakest of the pack .

In parallel use , maybe 1 pack shuts down 1st but then the other pack will still go on taking on the complete load .

I do need search for it , but i think both packs need to be on different BMS also ( so 1 pack can shut down while the other 1 goes on) , but im not 100% sure .

Yup ^^^, sticking another pack of the same voltage and chemistry in parallel will work just fine.

 

Of course, the combined packs will take longer to charge assuming the UPS charge current remains the same.

Thank you for the replies.  

I found these 200ah CALB lifepo4 on Lazada for 6000 baht including a BMS and active balancer during their September 9th sale.  I actually only wanted an additional 100ah but I felt buying these would insure I would get at least that amount.

1 hour ago, GregTN said:

I found these 200ah CALB lifepo4 on Lazada for 6000 baht including a BMS and active balancer during their September 9th sale.  I actually only wanted an additional 100ah but I felt buying these would insure I would get at least that amount.

Doubt you'll get more than 100ah out of those ten year old secondhand batteries. Five months back those batteries were selling for 600bht each.

Yeah, it's very much pot luck with used cells.

 

We're doing a bit better with our used CALB 200Ah golf cart cells but they are a reasonably low-cost way in to LiFePO4 (even if they're only 100Ah they're still cheaper than the equivalent in lead-acid).

 

I've had some real junk 35650 cells from sellers who've been good in the past. If you have the (not really expensive) kit to check the capacity and get the duff ones back to the seller within the return time period you're good.

 

Sadly I missed out on that 600 baht/200ah battery deal and have been watching the market for the last six months.  I am using this system as a UPS for a couple of TV’s, fan, and a few lights and normally draw less than 175 watts unless the MIL has her old CRT TV running.  I do agree that it is a waste of money to purchase these used batteries for a solar setup that has to cycle every day.