So a C battery just croaked, and all you have handy is an AA. With the $3-$4 Ziotek Battery Upsizer kit, you place the AA in the C-sized plastic holder, and you’re back in business (although for a shorter time because AA batteries cannot supply the current available from C batteries: see battery details). Likewise, you can “upsize” an AAA to an AA using the small plastic tube, a C to a D using the large tube, or go all the way from an AAA to a D by nesting all the tubes.
The manufacturer cautions: “Do not use Battery Upsizers in a battery charger. If you use multiple battery upsizers, do not use batteries of different types at once, or new and used batteries together.”
Has anyone used these? Would they be handy in an emergency?