AuroraCell's sodium-ion home battery skips lithium, but not the fine print

AuroraCell's cathode, the battery's positive electrode, is made of sodium chromium oxide, which the company shortens to NCO. It says this chemistry lasts longer, stays more stable and copes with temperature better than standard sodium-ion designs. The technology isn't limited to startups, either. CATL, the world's biggest battery maker, and Chinese automaker Changan showed off a mass-production car with sodium-ion cells in February, and CATL says those cells hold up well in deep cold.

How does it stack up against a Powerwall?

Tesla's Powerwall 3 uses lithium iron phosphate (LFP) cells, the lithium chemistry found in most home batteries today. The other Tesla figures below come from Tesla's own UK datasheet and US warranty, and the AuroraCell figures come from the company's launch specs.

Spec	AuroraCell	Tesla Powerwall 3
Chemistry	Sodium-ion (NCO)	Lithium iron phosphate
Stored energy	9.7 kWh	13.5 kWh
Warranty	20 years	10 years
Temperature range	Discharge -40°C to 60°C, charge 0°C to 60°C	-20°C to 50°C
Weight	110.5 kg (244 lb)	130 kg (287 lb)
Inverter built in	No	Yes
Price	Not disclosed	Varies by installer

The temperature row is the one to read twice. AuroraCell says the battery can supply power at -40°C (which is also -40°F), but it can only charge above freezing. Picture an unheated garage in Minnesota in January. The battery could keep your lights on through an outage, but your solar panels couldn't top it back up until the garage warmed up. Tesla's published range for the Powerwall 3 bottoms out at -20°C (-4°F).

The table also shows sodium's best-known weakness. AuroraCell's pack is lighter than a Powerwall 3 but stores about a quarter less energy, and Tesla's weight figure includes an inverter. Power output is modest too. AuroraCell lists a nominal 46.4 volts, a charging current of 105 amps (half its 210 amp-hour capacity per hour) and a maximum discharge of 150 amps. Multiply those out and you get roughly 5 kW of charging and 7 kW of discharge. That's plenty for a fridge, lights and a furnace fan. A heat pump, an oven and a dryer running at once is a harder ask.

What does the launch leave out?

Start with the price. Neither the launch coverage nor AuroraCell's own website gives one. Sodium may be cheap to dig up, but that only helps you if the finished battery is cheap too. Without a price, any claim that it beats lithium on cost is a guess.

The company also hasn't said who makes its cells. AuroraCell describes itself as a European distributor and system integrator: it buys cells from large manufacturers and designs complete battery systems around them. When the main selling point is a 20-year warranty, you want to know who will still be in business in the 2040s to honor it.

The warranty terms are thin as well. AuroraCell hasn't said how much capacity the battery has to keep to stay covered, while Tesla's US warranty promises the Powerwall 3 will keep at least 70% of its capacity after 10 years when it's used for home solar and backup. Even at one full cycle a day, it would take more than half a century to use up the cycle claim, so the fine print tells you more than the headline figure does. AuroraCell's existing home battery is advertised at "up to 30,000 cycles", which shows how much these numbers depend on how they're tested.

Then there's safety. AuroraCell promises "no fire and explosion," pv magazine reports, but it hasn't published test data to back that up. One of the standards it lists is UL 9540A, which sounds like a certificate but isn't. The Sustainable Energy Action Committee explains that it's a test method that produces a report on how a fire spreads, and that "there is no 'listing'" that comes from it. CATL says its own sodium cells didn't smoke or catch fire when crushed, drilled or sawn, but that's also a company result, not an independent one.

The battery carries Europe's CE mark, and AuroraCell's site says its home batteries are in stock there. The company hasn't announced anything for North America.

AuroraCell's battery is a real product built on a promising chemistry. Its strengths are a long rated life and the ability to deliver power in deep cold. Its weak spots are weight, power output and the fact that it can't charge below freezing. Until AuroraCell publishes a price, names its cell supplier and spells out its warranty terms, it's worth watching rather than a proven Powerwall rival. Would a 20-year warranty from a young company be enough for you to put one in your garage, or would you wait until sodium-ion has a longer track record?