A few years ago there was some new company (I vaguely recall that it may have been a Kickstarter or similar) touting something like this built into a shell that you could put an AA battery in, and the shell would still fit in most AA slots.<p>They were way overselling its benefits, claiming you would get something like 6 times as much runtime out of your batteries, and it got a lot of negative reviews and debunking at EE tech sites.<p>I wish someone would make one of these with that sleeve approach, not as some sort of miracle energy extended scam but rather as a voltage curve adjuster.<p>A common non-rechargeable alkaline battery starts at about 1.5 volts and over its useful life drops fairly steadily to a little over 1 volt, and then rapidly drops to near zero.<p>A NiMH rechargeable starts at around 1.4, fairly quickly drops to about 1.3, then over most of its life drops fairly smoothly to about 1.2, then starts dropping faster to around 1.1, then rapidly to near zero.<p>This is why you can use NiMH in devices designed for alkaline batteries even though nominally alkaline batteries are higher voltage. The device has to actually be designed to handle 1 to 1.5 volts, and the NiMH is in that range for nearly all of its discharge curve.<p>This is also why some devices designed for alkaline report low battery on NiMH long before the batteries actually need changing, and still report low but useful battery level right up until the device stops working. They are estimating battery life by looking at the voltage and fitting that to the alkaline discharge curve to estimate how far the battery has discharged, and it makes the batter level meter on many devices close to useless if you use NiMH.<p>It would be great if there were a sleeve you could put around your NiMH batteries that would dynamically raise or lower the voltage as the battery discharges to make it match the alkaline curve. Then your battery level indicator on your devices that were designed only for alkaline would work.