For electrical storage .. mainly to run the HVAC system:
Low self Discharge
Low calendar aging
High cycle life
High cycle efficiency
Ideally used or older cells (reduce / reuse/ recycle)
It's a stationary system so weight isn't as important as it would be in a mobile system.
I currently plan to use , used LTO and/or LiFePO4.
Both of which have good round trip cycle wh efficiency ~95% .. both have very low yearly self discharge less than ~5% .. both have very high useful life to both cycles and calendar aging effects .. and both are among the safest flavors in the Li family.
If I am able to eventually move my whole ~7Mwh a year of HVAC loads to a ~3COP heat pump system that would need ~2.3Mwh per year / 365 = ~6.4kwh per average 24 hr day .. of course more in winter , less in spring/fall .. winter time is my peak .. my record worst day so far was ~38kwh @ ~3COP = I would need about ~12.7kwh
I already have ~7.7kwh of used Toshiba SCiB LTO cells .. I've already tested them .. I may or may not add more on later .. we'll see .. but I think it will be at least a good 'entry' level battery back up system.
I also already have ~11kwh of LiFePO4 A123 20Ah pouch cells .. I've already tested .. 5-7kwh of that plan to go into an upgrade add on to my PHEV .. which will leave 4-6kwh of them I could also add to the house battery system .. if I do , that would bring the house up to 11.7 - 13.7kwh .. that would bring the combined system up near my worst case HVAC loads for an entire day from electrical battery power alone.
If I had to .. I could tap into the 8-10kwh that will also be in my upgraded PHEV battery .. which would bring it up to ~21.7kwh .. or worst case , If I was desperate , run the PHEV ICE as a gasoline generator , ~10Gallon tank of gasoline could provide between 73-107 kwh more electricity (depending on operating details) before needing to go ~2miles and get more gasoline.
I am not planning to go 100% off grid right away .. Soo , unless the grid is down (black out) , I really only need a small enough buffer battery to run the HVAC system while the sun comes in and out of clouds and such , and for a few hours at night .. 100% off grid would require some significant upgrades .. which I might or might not do in the future .. this system will be modular enough that such upgrades will be possible without too much trouble.
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