Cold Snap, Hot Mess: Fox ESS in an Australian Winter

Images of DIY winter coats for Fox batteries – shared on FB by Australian owners

Fox ESS batteries recently became the most popular battery in Australia thanks to their nice stackable design and a price that undercuts most of the competition. When asked, my personal verdict on Fox has been: I don’t have enough real-world data to vouch for their quality in harsh Australian conditions, and I’ve consistently recommended waiting for a couple of summers before committing.

Turns out it’s not the summers I should have been watching.

It’s early May. Not even winter. Melbourne hit 8°C the other day – reasonably chilly for Melbourne, sure, but hardly Hobart in July – and Fox battery owners started posting. One reported their 10kW system throttling back to 3.8kW. Others chimed in with similar stories. The batteries aren’t broken. They’re just doing what lithium batteries do when they get cold: they get cautious, limit charge and discharge rates, and in some cases trigger forced charging to warm themselves up.

This is a known issue with lithium batteries. The solution, in cold climates, is a battery heater, a small heating element built into the unit that keeps the cells above the threshold where chemistry gets sluggish.

Fox, like many budget battery manufacturers, didn’t include heaters in some of their Australian models.

I can only think of one reason to do that: cost. Specifically, the pressure to hit a low enough price per kilowatt-hour to get as close as possible to the dollar amount per kilowatt-hour of the battery rebate. It’s a rational commercial decision. It’s also a decision that seems to have been made by someone who has never spent a winter in Hobart, or apparently Melbourne.

This is particularly bad news for owners of Fox’s larger battery systems without heaters. The big stackable configurations, some pushing 48kWh, were a huge part of Fox’s appeal. Buyers were transfixed by the idea of a massive battery at a price that would have seemed impossible two years ago. But a big battery creates a big problem in winter. Unless you have a solar array the size of a tennis court, the only realistic way to fill a 48kWh battery in winter is force charging from the grid during off-peak or free-energy windows. And when you’re doing that, you want to charge at full rate – you’ve got a limited window and a lot of kilowatt-hours to shift. If the cold is crippling your charge rate, you’re simply not going to fill the battery in time¹.

In every major Australian city except Brisbane and Darwin, winter energy demand for heating exceeds summer demand for cooling. If your battery can’t be reliably filled in winter, your return on investment takes a serious hit.

What worries me more than the performance throttling is what some owners are doing about it. Enterprising Aussies, faced with a battery that won’t charge properly on a cold morning, are improvising. We’re talking rockwool wrapped in glad wrap. Insulating board strapped around the whole stack. Full DIY overcoats on a device that contains a significant amount of stored energy and needs specific clearances for cooling.

Fox has actually published guidance on this for their UK customers, who ran into the same problem. It specifies flame-retardant materials, minimum clearances from electrical components, what not to block. It also includes this note: improper insulation may lead to elevated internal temperatures, accelerated degradation, or even thermal runaway. You assume all associated risks.

That scares the shit out of me. Because the Australian owners improvising with glad wrap may not have read it. What happens when a Melbourne winter turns into a Melbourne summer, or we have on of those freak warm winter days and someone forgets to remove the insulating board they strapped on? Nothing good.

A few caveats worth stating clearly. Fox will not be the only battery on sale in Australia with this problem – they’re just the ones we’re hearing about, and that’s largely because they’ve become the best-selling battery in the country. When something goes wrong with the most popular product in a market, you hear about it first and loudest. Other brands may have the same cold-weather limitation and we simply don’t know yet because their sales volumes are lower.

It’s also worth noting that before the Cheaper Home Battery Scheme and before batteries got cheap enough that 50kWh systems were suddenly within reach of ordinary households, almost nobody needed to charge their battery at full tilt in winter. Smaller batteries, slower charging, milder use cases – if the issue existed not many people would even notice. The rebate changed everything.

I’m updating my verdict on Fox ESS batteries. It used to be: wait and see how they handle a couple of Australian summers. I’m adding to it: we also need to see how they manage a couple of Australian winters. Early May in Melbourne shouldn’t be breaking a sweat.