I finally got the cells and wired this up. The battery pack looks like that because the 32mm cell spacers I got were too small for these “32140” cells. They’re more like 33140 cells and need 33mm spacers. I taped them together with kapton tape for now. Yeah, I know it looks like shit, but it works. They’re connected in parallel to the 9Ah lead acid inside the UPS. Connecting LA and LFP in parallel might seem insane but it’s actually alright and may even be better for the longevity of the LA, provided the LFP has a BMS. There’s a whole discussion about why this is (LFP’s higher resting voltage keeps LA at float voltage constantly and LFP will always drain first due to lower IR) and there are a few people online who’ve tested it for years with much bigger battery systems.
Unfortunately, I found out that the APC UPS is trying to charge the batteries at 14.62V which is 0.02V above what the BMS on the LFP cuts off the battery at. The result is that the LFP only gets charged during the float charging mode, and only up to the float voltage of the LA i.e 13.5V. On the whole though, this actually isn’t a big problem considering that is like 90%+ capacity for them. I wasn’t sure how this would impact the charging time, but they happily charged from empty to full overnight.
I tested them with a load of about 150-200W (my gaming PC and 27″ monitor at mostly idle, just playing Youtube in a chrome tab) and I got 1 hour 33 minutes of run time.
For reference, the brand new lead acid by itself only gets 33 minutes in the same test. And that was without the monitor connected to it!
Considering this is now a 9*12 + 15*12.8 = 300 Wh battery pack, I’d say that is exactly around the runtime you would expect. Neither too low nor violating the first law of thermodynamics.
After a complete discharge, the LFP was at a resting voltage of 11.21V and 11.69V for the VRLA lead acid. Both within spec. Both started charging after connecting to mains again.
So a 3x improvement in backup time with ~3.5k in parts. Easily scalable further by adding more cells. Not a bad deal, I think. @goDofWar_skr
