I’m interested in building a power wall for home use. Typically you see lithium-ion batteries up to 14s for use in ebikes and all kinds of low-voltage DC applications. For home use you want 220V AC (or 110V AC for Americans).
An inverter would waste a lot of energy going from 12V to 220V at high wattages. The lower the voltage, the more amps are drawn and thus more energy becomes waste heat. You also need thick wires to not have them become fire hazards. The battery also gets strained when drawing high amps from them.
Would it be sensible to build a 60S battery? Transformation losses from 240VDC-> 220VAC should be quite minimal. You can put enough solar panels in series to make charging this battery sensible as well.
Of course there are obvious safety hazards with high voltages but there should be advantages as well, lower strain on battery cells and minimal DC->AC losses.
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.
hella dangerous working with high voltage DC, but as far as efficiency goes you are correct.
To add to this, a lot of large scale batteries also have built in safeties in addition to the battery cells. So even if you don’t shock yourself, you could burn down your house from a bad battery cell.
Indeed. Batteries require a BMS that can handle this amount of cells as well and they’re quite obscure. In a smaller battery pack I’ve built every individual cell has a current fuse as well as a thermal fuse that pops at 60 deg C.
Hi, just FYI, a better place for questions is AskElectronics. This is a good place to showcase your finished project. Hope it goes well.
Thanks. Didn’t notice that until it was already posted here.
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. 😅
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.
Would what Tesla & co are doing be possible or are they not actually doing it?
Well. That would be the overpriced commercial version. I’m not a fan of Tesla because of their anti repair practices and I’d rather build my own anyway. The learning experience alone makes it worth it.
Their products do work though.
And? Tesla is not exactly the only electric car manufacturer if you haven’t noticed yet. In fact, people actually build their own. https://www.youtube.com/watch?v=LDz_9iN-bUU
That’s why there is an “&co” in my post above.