Put a volt meter on it, then log the results. If it’s not a voltage REGULATED device, the clicks will come later, and later.
Yep, what Rusty said. Fortunately, the click is not as dependent on the voltage sag. The click is fairly consistent regardless of the rate of heating.
Thank you both for the feedback.
I don't plan to open any of them up to attach a volt meter. Considering the nature of the units, and their batteries, I am going to assume that a performance drop will accompany a reduction in battery charge. The assumption just means that I have to use a fully charged battery when testing, which will result in more frequent charging. I understand that there is a finite number of charging cycles in any of the batteries, so this would shorten the life span of the devices. At this price point I am not too concerned about that. And for future use I know there will be better induction heaters coming to market.
Even before this conversation I felt like the UFO heated more aggressively with a fully charged battery, so I think my assumption is pretty safe.