If you ever needed to fit a square peg into a round hole on short notice, then this Instructable is for you!

With my parents over for the weekend, and the hour of midnight rapidly approaching, we needed to blow up the ol' air mattress and get to bed. But, lo and behold, one of the D batteries inside the electric pump had leaked, spewing battery acid everywhere inside the pump, and (of course) we had no more D batteries! What to do, what to do....

Since alkaline batteries, regardless of their size (e.e.g, AAA, AA, C, D), all have the same voltage (1.5 V), I realized that I needed to make a AA (or a couple of AA's), of which I had many, fit in the pump in place of the failed D battery.

Electrically, one AA could do this job since it runs at the same voltage as a D battery. However, it was easier to combine 4 AA's into one battery to simulate the size of a D battery and effectively replace it. This also gave the DIY battery more energy (i.e., longer run time) and likely reduced the strain on each individual AA by drawing current from all four AA's at once.

Read on for the detailed construction tips! (It's really easy.)

This Instructable creates a simple, it-will-do solution. If you want a more permanent solution, check out these Instructables: AA to D Cell Adapter 1, AA to D Cell Adapter 2, and Easy Conversion: AA to C or Any Other Cell.

It is also important to note that this arrangement (i.e., batteries in parallel) can potentially damage the batteries if they discharge into each other; however, this problem can be mitigated in part by using batteries that are all at the same charge level (e.g., fully charged) or using diodes and small resistors in series with the batteries.

Step 1: Materials and Tools

The materials for this Instructable should be just laying around your house. Nothing special at all.

2 two-inch squares of aluminum foil, any thickness
Tape (electrical, duct, masking....it's all good.)

The tools I used included the following

A pair of scissors
A multimeter to verify connections (not required)
Some q-tips
A little isopropyl alcohol (water will do in a pinch)

That's it!

Step 2: Make the Battery! Quick!

After cleaning off the battery contacts in the pump with the q-tips and the alcohol (or water) and letting everything dry, I started making my emergency D battery.

Tape the 4 AA's across the middle to hold them together as shown. Make sure their contacts are at the same heights to ensure they will make electrical contact in the next two steps.
Place a square of aluminum over the negative contacts of the batteries, fold it smartly down along the sides of the batteries and tape it in place. Make sure that the aluminum won't overlap with the second square of aluminum that you will secure in the next step!
Place the second square of aluminum over the positive contacts of the batteries, as in step 2.
Ensure that the aluminum squares are not touching! If they are, you will short out your batteries and potentially cause a fire.

This emergency battery is based on the concept of voltage sources in parallel. Check out here for lots of great information on this topic!

Step 3: Test Fit

Test out the fit of your emergency D battery in your device. Mine was too short to make electrical contact with my pump's battery contacts. So, I bodged together a make-shift extender by balling up some aluminum foil to fill the gap. I made it a little too big so that it would smash a bit when I put the cover on.

Step 4: Turn the Switch on and Gloat

Voila! You saved the day.

(It actually took a little fiddling to make sure electrical contact was made, but the third time was the charm! I leave this little detail out when I tell this heroic story at parties.)

Enjoy and happy bodging!