When Donut Lab first announced its solid-state battery earlier this year, there was some speculation around whether the Finnish startup had actually produced a supercapacitor, which is a much shorter-term form of electrical storage. Supercapacitors can charge and discharge quickly, similar to Donut Lab’s claimed solid-state battery. But supercapacitors are totally different from batteries in terms of energy storage capacity and long-term use. Was Donut Lab’s battery actually just a supercapacitor in disguise?

The startup says no, and it has the independent test results to prove it. The test, which was yet again conducted by state-owned VTT Technical Research Centre of Finland, “evaluates its charge retention over an extended idle period.” This is the third test in a series of independent examinations ordered by Donut Lab to evaluate its claims about its new battery.

“Many have said that the specifications of Donut battery can only be achieved with supercapacitor,” the company’s CEO Marko Lehtimäki said in a video. “Well, today we are proving that is not the case.”

Basically, VTT hooked up Donut Lab’s to a battery tester and left it idle for 10 days, while measuring the voltage every 10 seconds. According to the results, the cell demonstrated high stability, retaining 97.7 percent of its energy during the 10-day idle period.

There was some minor fluctuation detected, with VTT reporting a drop in voltage from an initial 3861 mili-volts to 3722mV (a change of -128mV) within the first hour due to chemical stabilization. But otherwise, the cell kept the majority of its energy, which Donut Lab says disproves the theories that its battery is actually a supercapacitor.

Over a similar idle period, supercapacitors will lose a significant portion of their stored energy through self-discharge. That’s because supercapacitors are not designed for long-term energy storage and, unlike batteries, will see their voltage drop substantially within a few days or weeks. The test confirms “normal battery-type charge retention – not the rapid linear discharge characteristic of supercapacitors,” the lab states.

Donut Lab says its important to disprove these claims to clear the air about its solid-state battery. And the reason for that is because most of what the startup is claiming sounds so unbelievable to many battery experts. Solid-state batteries are often held up as the “holy grail” of battery technology, allowing electric vehicles to travel farther and charge faster without any of the issues that current EV batteries face around fires and thermal runaways.

Donut Lab has claimed that its battery delivers 400 watt-hours per kilogram. By comparison, most lithium-ion batteries today range from 200 to 300 Wh-per-kg. The company also says the battery can charge in under 10 minutes and last 100,000 cycles. Current lithium-ion batteries typically last between 1,500 and 3,000 cycles.

There’s still a lot we don’t know about Donut Lab’s battery. There hasn’t been an independent test to confirm the chemistry of the pack. And we still haven’t seen any explanation for how the startup is handling the “dendrite issue,” in which microscopic stalagmites growing from anode to cathode across the solid-state electrolyte can cause electrical shorts. Perhaps Donut Labs will address that in subsequent independent test results, which it is promising as part of its “I Donut Believe” series.