(be sure to check out the last step for some updated info and a how to for this method using 4 batteries, using four would increase the life span. i had to use three for the sake of saving space.)
hack that battery pack!! we have all seen those 4 double a battery holders sold at radioshack, online, etc. the following involves modifying that same battery pack. But what can you do to it? first before I answer that i would like to say that I needed to make this to meet a few needs in a small 8vdc booster circuit i made. The circuit can use a voltage input as small as 1.2 volts and amp it up to 8. So i needed a power supply, I was planning on using just one double a battery but the run time on that would be so short, also once the battery voltage drops lower than 1.2 it would quit working. It would last maybe two hours if I used one battery. SO after some thinking I decided to use a battery pack, but all 4 double a battery packs are designed for series wich would give me a higher voltage output but not a longer runtime, so I hacked my pack! with a few simple modifications I turned it into a series parallel battery pack. Whats this and why? If you havent heard of series or parallel before then I will explain the terms now. batteries that run in series outputs a higher voltage while maintaining the same mah as one of the batteries, some devices that use this set up are tv remotes, radios, flash lights etc. its where batteries are set up in a way where the positive terminal of one battery runs into the negative terminal of the next battery. Parallel however, now when you set up batteries in parallel you will have the same voltage output as one battery but a longer run time. So the batteries used in parallel would be setup with all the positive terminals and negative terminals connected. I know this can be confusing thats why I included a few pictures to show you what series and parallel look like(see next step for a look at the pics). Now this battery pack hack is modified to use series parallel. (you will notice I cut off one of the battery holders, turning the 4pack into a 3 pack) If you have a good understanding of parallel and series then you can probably figure out what both combined does. If not I shall explain! I hacked my pack to use two batteries in parallel then in series with one battery, so I am using two double a batteries in parallel while using another in series of the two parallel. this way I can have the voltage output of two batteries but the run time of three.
Update! Its been brought to my attention, if you use two batteries in parallel in series with two more batteries in parallel the performance would be better! see last step for a picture.
in short. Series creates a higher voltage output. (4 double a batteries would be 6volts. 1.5x4)
parallel creates a higher mah. longer run time (2000 mah per battery, 4 double a batteries is 8000 mah)
Step 1: Batteries in Series and Parallel
this is what series and parrallel look like. I made these pictures because I find it easier to understand something if you can see it.
Both of these setups are very easy to understand and are used a lot around the world.
Step 2: Series Parallel Using 3 Double a Batteries
I used 3 batteries for one reason. space. if I would have used four the pack would be too big for my project box. so thats why i included a picture of this set up. it is possible to use 4 batteries! I could only use 3.
in the picture you can see two batteries in parallel then in series with another battery, what this does is it increases the amperage (mah) and it doubles the voltage output (3volts)
Step 3: Hacking That Pack!
so if you need a series parallel battery pack like mine then there are a few things you need to do, now since I used three batteries I will only be covering how to do this with three batteries. I cut off one of the battery holders turning the 4 battery holder into a 3 battery holder. Now since the battery pack is designed for series we will need to break all the connections connecting the batteries. basically all you do is find the metal wire connecting one battery to the next, simply cut that. you are basicaly making each battery holder individual. its 3 individual battery holders combined. Since I made the battery pack into a 3 battery holder there is now a middle. The middle battery holder would be the battery in series with the two in parallel. I soldered a jumper wire from both negative battery connections to the middles positive. I then soldered a jumper wire connecting the two batteries in parallel positive connections, this ended up being the postive output. the middle battery holders negative connection ended up being the negative output. see the pictures for small notes.
the setup in the pack ended up being reversed to the picture I showed in the previous step. its one battery in series with the two parallel. It still works the same, just thought I would point that out :) I also included a picture so be sure to view it for some clarification!
Step 4: Using Four Batteries (UPDATE)
I cant take credit for this. A instructable member just pointed something out. They asked "wouldnt the single battery die before the two in parallel?". I havent noticed anything of the sort yet with my 3 battery set up, but just incase I chose to show this battery pack hack using four batteries. This setup uses two batteries in parallel in series with two batteries in parallel. That way the batteries all have the same capacity while still have the same doubled voltage and increase mah. the voltage output would 3 volts (if using 1.5 batteries). Using this setup ensures that the batteries run a full cycle. A million thanks to the instructables member who pointed this out!