Once upon a time she-who-must-be-obeyed procured some LED holiday lights from a huge scandinavian furniture retailer.
Those had (apart from being made the cheapest way possible, obviously) the problem of being dimensioned for primary cells. And I had no intention on wasting our stockpile of alkaline manganese Mignon/AA primary cells every „season“.
Building a boost converter nowadays means taking the „Typical Application“ out of the datasheet and apart from very efficient integrated circuits, like the MAX172x, there are also simple and cheap versions like the MCP1640.
The circuit (in case of the ‚1640: page 2 of the 03/25/14 datasheet) easily fits in an empty slot in the battery compartment:
The brown cable gets the „ground“ potential to the circuit board, the output voltage goes directly to proper contact of the battery compartment. The contacts are from Keystone. If you need physical dimensions of the batteries, take a look at http://data.energizer.com/.
As long as there is charge left in the single accumulator, the LEDs get their nominal voltage now. When they start to blink it’s time to charge it again.
In case you didn’t think of this yourself: If a device has a power gauge or battery warning feature you will render it unseless for sure by using such a circuit.
(Original post: http://www.metalforum-owl.de/viewtopic.php?f=21&t=138&start=140#p44944, February 28th 2012)