Electronics

hella dangerous working with high voltage DC, but as far as efficiency goes you are correct.

You are right on the efficiency but as others have said working with DC voltages that high is dangerous. That many batteries in series would also be hell to monitor so you are one bad cell away from burning down your house. Generally when working with DC you want to keep it under 50V to avoid electric shock risk.

For powering a home multiple 48V arrays in parallel is the best bet. You can easily find inverters for that voltage and just bumping the voltage up to 48V avoids a ton of losses without risking electeocution or making the system overly complex. 60V and 72V inverters are also readily available but that's just risking electrocution for minimal additional benefit. You would also want to be using lithium iron phosphate batteries (LiFePo4) instead of regular lithium-ion batteries. They aren't quite as energy dense as most lithium-ion chemistries but they are much more safe, much less picky about charging characteristics, and much more long lived. You can get them in 12V cells just like lead acid batteries because they are frequently used to replace lead acid batteries in stationary power storage applications. Depending on the cells you get they can be a pretty much a better drop in replacement for lead acid batteries however it is normally still a good idea to get chargers and inverters designed for them to get the most out of them. That's because they store much more power than lead acid batteries and unlike lead acid batteries they can be charged just as quickly as they can be discharged.

Source: I did portable inverter setups for a few years.

It's not impossible. You generally want >330VDC, because you want the DC voltage to exceed the AC peak voltage, even at low state-of-charge. Expect about 90-95S.

There are some LV DC (extra-low-voltage is <120VDC) products on the market already: https://www.fronius.com/en/solar-energy/installers-partners/technical-data/all-products/inverters/fronius-symo-gen24-plus/fronius-symo-gen24-3-0-plus

Because you're doing a series-parallel transform, the stress on the battery doesn't actually change. There's less current, but there's also fewer cells in parallel to share that current. The power-per-cell is constant.

48V and higher systems are quite common in higher-power off grid setups, and that's high enough that wire sizing etc. is reasonable at typical domestic loads. The main gain against those is that you potentially get to lose the isolation stage between the battery and the mains. The inverter itself will still have non-negligible losses.

However, a floating LV DC battery is not to be trifled with. BMS design is tricky; I believe a bunch of isolated sub-BMSs handling a few cells each is common, with isolated comms between them. You also need active earth fault detection usually, because the pack can't be grounded.

EVs use a separate 12V battery to power the controls to check the system is safe and communicate with the BMS, before closing a vacuum contactor to enable the HV battery. It's likely you'd need a similar system for this.

Pricing up switches/breakers/contactors rated for 500VDC is also not very pleasant.

Hi, just FYI, a better place for questions is AskElectronics. This is a good place to showcase your finished project. Hope it goes well.

In addition to the other answers: You are mistaken regarding stress and losses on battery cell level, at least if you build the same overall batterry capacity at both voltage levels with the same cells.

Starting with the HV variant, the power is P = U * I. If you now build a LV variant, let's say at U/4, with the same capacity, you will have 4 strains in parallel. So on pack level you will need 4 * I to get the same power, but inside the battery this current will split up into the 4 parallel strains. So each cell therefore again only has the current I.

Of course this is an ideal assumption. As you would have higher losses, e.g. in the wiring to the inverter, the overall power you need to draw from the battery increases. But it's not as severe as it might seem based overall current increase on system level.

Edit: The formulas did some unintended formatting. 😅

Would what Tesla & co are doing be possible or are they not actually doing it?