Saltwater Batteries: Safe, Heavy and Hard to Find

SALT WATER ON YOUR BATTERY

The Saltwater Battery That Is Very Hard To Set On Fire

Saltwater batteries are an energy storage technology that keeps electricity in salty water instead of a liquid that burns. That makes them far harder to set on fire than a lithium ion battery, and the biggest maker said its chemistry had no thermal runaway at all.

So why doesn't every house have one? Because they're heavy, hard to buy, and the biggest maker went bankrupt in 2017. Once you see how they work, both the good and the bad news make sense.

How Saltwater Battery Technology Works

Inside every battery there's something called an electrolyte. Scientists use that word for the liquid (or paste) that carries electricity between the two plates inside.

In lithium ion chemistry, that electrolyte is flammable. That's why lithium batteries can catch fire when they're damaged or overcharged.

In a saltwater battery, the electrolyte is sodium sulfate dissolved in water. It's a simple, neutral liquid, a bit like seawater. You can't set it on fire. Aquion, the biggest maker, called its electrolyte not flammable, not toxic and not caustic.

On both sides of that liquid sits an electrode, which is just a fancy word for one of the two "plates" that electricity leaves from and comes back to. In Aquion's design, one side was manganese oxide and the other was carbon with titanium phosphate. No cobalt and nothing rare.

When the battery fills up, tiny charged bits of sodium squeeze into one electrode, like kids piling into a school bus. When it discharges, they hop back out and travel to the other side, and electrons run through your wires to balance it out. That flow is electricity.

That's the whole chemical trick. But "the liquid can't burn" is not the same as "the battery can never burn". The box is plastic, and there are cables, fuses and a control board. A short circuit on the terminals still makes heat and sparks. So a saltwater battery still requires the right fuse, good cables, a charge controller and a dry spot, like any other battery storage system.

Saltwater Vs Lithium Ion: The Weight Problem

Here's the catch, and it's a big one.

One Aquion battery stack held 2.2 kWh of energy and weighed 260 pounds. A Tesla Powerwall 2, which is a lithium ion battery, holds 13.5 kWh and weighs 251 pounds.

So for about the same weight, the lithium box holds about six times more energy. Scientists call this "energy density": how much power fits into a battery of a certain size or weight.

So you need a lot of saltwater battery to store enough power for a house. Saltwater systems take up a lot of floor space, and you're never putting one in a phone, a drone or an electric car. Those applications need the lightest battery they can find.

But for a storage battery that sits in a shed forever? Weight hardly matters.

Advantages Of Saltwater Batteries For Solar And Home Backup

The sweet spot is residential solar energy storage systems. The battery sits next to your house, fills up from the panels during the day and runs your lights at night.

Aquion's own test data shows it. In September 2014 an off-grid home hooked an 80 kWh saltwater bank up to 14 kW of solar panels. On a typical day it filled up from the panels and powered the house through the night.

What you care about in a home backup battery is:

• Safety. Aquion's batteries passed the UL 1973 flame spread test.
• Fewer nasty materials. Aquion was the first battery maker with a Cradle to Cradle certificate, a label for products made from non-toxic materials and sustainable manufacturing.
• Simple daily use. Aquion rated its batteries from 23°F to 104°F, without any cooling system.

However, the chemistry differs per brand, so check what each maker puts inside and which safety tests it passed. If you want to know how big your bank should be, start with the battery size calculator.

How Long Does A Saltwater Battery Last?

Aquion's founder told the US Congress that the electrode materials could be charged and discharged over 5,000 times without losing function. For a solar battery that fills and empties once a day, that's over 10 years.

In the company's own lab and field tests, batteries ran more than 3,000 cycles over 2.5 years and kept delivering the same daily energy. Those are the maker's numbers, so treat them as a best case for the technology.

Why You Don't See Them Everywhere: The Aquion Story

Price is the first problem. Aquion planned to make its batteries for less than $300 per kWh, but lithium ion batteries kept getting cheaper and dropped below that price by 2016.

The market was tough too. Aquion only had its first sellable product in 2014, lost money for two years, and filed for Chapter 11 bankruptcy in March 2017. Its assets were sold at a bankruptcy auction to a new owner. An Austrian company, BlueSky Energy, later sold saltwater home batteries, but it went bust too in 2022 and was closed.

So if you compare options today, ask hard questions about warranty and supply of spare parts. A battery that should last 10 years needs a maker that lasts 10 years.

Saltwater Battery Safety: The Bigger Picture

Lithium ion fires can flare up again hours, days or even weeks later, and give off toxic and flammable gases. Fire services cool them with lots of water, which the NFPA calls the most effective cooling agent.

A saltwater battery can't feed a fire like that, because its liquid doesn't burn. The case and wiring can still catch fire from outside or from a short, but there is no runaway reaction inside. For a storage battery that sits next to your house for years, that peace of mind matters.

Where Saltwater Energy Storage Is Going

Researchers keep working on water-based batteries because the safety story is so strong. For now, though, most home solar storage systems use lithium, and saltwater stays a niche solution for people who put safety above weight and price.

Want to compare other battery technology? Read how home batteries work, or go back to the original: lead-acid batteries. Whatever you pick, a good battery management system keeps it healthy.

Sources

• Aquion Energy, battery technology deck
• Whitacre, The aqueous electrolyte sodium ion battery (Sandia, 2011)
• Jay Whitacre, Testimony to the House Science Committee (2015)
• Tesla, Powerwall 2 AC datasheet
• pv magazine, Aquion files for Chapter 11 bankruptcy
• MIT Technology Review, Inside the fall and rebirth of Aquion
• Industriemagazin, Bluesky Energy in Konkurs
• NFPA, Energy storage systems fact sheet