Based on information from Panasonic I have discovered the following in regard to battery run time:
Using the same load for each size alkaline battery....
1 D cell = 3 C cells
1 D cell = 7 AA cells
1 D cell = 18 AAA cells
6 D cells = 200 9 volt batteries.
Think of the cost savings replacing 72 AAA batteries with 4 D cells.
To greatly increase my run time I have made some D cell battery backs as well as modifying an existing product to do double duty as a battery pack.
(More pictures to follow later this week)
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Step 1: Unviversal 1.5V, 3V, 4.5V, 6V External Battery Pack
The basic battery pack was an easy project, It is a plastic enclosure with a 4 D cell battery holder,a switch and some RCA connectors. I have soldered a wire from the positive end of 3 D cells to the negative ends. These were dead batteries that were depleted down to 0.2 volts or less. These are used to switch the battery pack over to a lower voltage than 6 volts. If you need 4.5 volts, take out one D cell and replace it with a shorted D cell. Two for 3 volts, 3 for 1.5 volts. The connectors I used are RCA type but any type should be fine. I have dedicated power cables for different devices. The one pictured is for my ancient little short wave radio that runs on 3 AA cells. So my run time should be increased 7X . My mini RadioShack 9 volt powered mini amplifier should run about 50 times longer running on 6 volts.
Other additions to the battery back could be a fuse or a power resistor ( for current limiting).
Step 2: Portable Fan / Batterey Pack
I added a connector to the outside world to one of the two 6 volt battery compartments on this portable fan. I am planning to hack a boom box soon.