The batteries are wired, the inverter is running, and there is 120 volts coming out of a box on the wall that was a stack of parts this morning. Two lithium packs, a fuse, a pair of busbars, an inverter, a charge controller. No panels connected yet — that is tomorrow, because it started raining. But the thing works.
The order matters more than the parts. You build the battery bank first and you measure it before it can touch anything else. Two packs wired in parallel should read the same voltage as one pack. If they read double, you wired them in series by mistake, and the moment you connect that to an inverter rated for half that voltage you have destroyed it. So you meter, and you look at the number, and only then do you go further. Mine read 52.5 volts at each pack and 52.5 across the pair. Correct.
Then the fuse goes in the positive line, before anything downstream exists. Then the cables up to the busbars — and you meter again, because a fuse that is not seated or a lug that is not making will show up right there as a wrong number. Still 52.5. Then the inverter’s cables, and one more polarity check at its own terminals, because reversing those is instant and unrecoverable and no fuse in the world saves you from wiring it backwards.
Flip the switch. A slight hum, one green light, no smoke. 120.8 volts at the output. That is the whole day in one reading.
Then the charge controller, which is where the real trap was. It is an older unit I am reusing, and its menus only have settings for a 12-volt system — on a 48-volt bank you enter one quarter of the number you actually want and the controller multiplies it internally. Type in what you actually want and you have told it to charge four times too high. I checked that against multiple independent sources before touching anything, because getting it wrong going into winter is not a mistake I can afford to absorb. Then I installed a different app that shows the real numbers instead of the quarter-scale ones, and confirmed every setting reads the way it should. Absorption where the battery maker says. Equalization — a lead-acid maintenance cycle that has no business running on lithium — disabled twice over, once by time and once by voltage. Temperature compensation off, which is another lead-acid feature the box does not need.
That verification took longer than the wiring did. It was worth every minute.
One sour note to end on, honestly. After the cables were made and torqued, I went back through the battery manufacturer’s manual and found a line saying they prefer the packs paralleled at a busbar rather than cabled to each other directly. Which is how I spent my afternoon doing it. The way I built it is a recognized approach and the packs are matched and the cables are short, so it is not dangerous and probably not even measurable at two batteries. But it is not the way the people who built them say to do it, and I will measure tomorrow to see if I have enough cable left to change it.
That is the thing about doing this alone. You read everything, and then you find the line you missed after the wrench is already back in the drawer. You write it down, you fix it if you can, and you keep going.
Rain is coming down steady. The garden is thrilled. Panels tomorrow.