It’s still a work in progress but I’m building a ~13kwh battery backup for some of the circuits in my house, specifically: fridge, air handler (gas heating), water heater (gas), garage door opener, network gear and my office.
I’m building a 48v LiFePO4 battery from 16 cells (280ah each), which should give me a runtime of 24+ hours. Here you can see the cells in a temporary setup to test the BMS. I charged the pack up until one of the cells hit max voltage (3.65v), then I took the whole thing apart and put all the cells in parallel to top balance. This is an important step for ensuring I can get the most capacity out of the pack.
Next step for the battery pack is to build a box, place the cells inside and then get it all wired up. Once that’s done I should be ready to start installing inverter/charger, autotransformer and sub-panel.
I decided to use a 230V inverter for this as it allows me to power heavier circuits if needed, allows for a bit more flexibility in sub-panel selection and will make expanding the system a bit easier in the future. Of course since most of the systems I want to power require 120V I need an autotransformer, which will “split” the 230V into two 120V split-phase lines just like the normal power coming into the house. It also allows me to provide the full 5000W to either phase so I don’t have to worry too much about balancing the load.
I made a crappy mspaint diagram of how it will all be laid out. Under normal conditions the inverter will actually be in “bypass” mode and power from the main panel will go to the autotransformer and sub-panel. When power is lost the inverter automatically kicks in and starts providing power from the battery pack. This is basically a bigger version of the battery backups many people use for their desktop computers.
