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60S lithium-ion feasibility for home-battery (discuss.tchncs.de)

submitted 1 year ago by Rolive@discuss.tchncs.de to c/electronics@discuss.tchncs.de

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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.

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[–] SomeoneSomewhere@lemmy.nz 2 points 1 year ago

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.