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I recently had a Sansa C250 mp3 player with a battery go out.  When I went looking for a replacement Poly Ion battery, I was shocked how much even a third party battery costs.  It was about half as much as I had paid for the entire player!

Knowing that buying no name ion batteries can be a crap shoot and there was no way I was going to "pony up" what a Sansa branded battery would cost, I decided to try something different.  I decided to find out if I could build a piggyback power pack which would run on 3 AAA rechargeable NIMH batteries.  Since I have two of these players, if it worked on the first one, that would mean I could build a second when the other player battery died.

The good news is, it works perfect.

Cons: It makes the Sansa a little larger and heavier.

Pros: The new pack is 850 mah vs the 530 mah of the Poly ion battery (which means it should last 50% longer between charges). I'm still testing the charge level with the Sansa.  The battery pack can be charged by the Sansa player via USB or by an external charger by removing the batteries.  The Sansa displays remaining battery power in the pack via it's bar graph.  The best benefit of all is losing the proprietary Poly Ion battery For Good.

Although I did this on a Sansa C250, it will likely work just as well on others which require a 3.6 volt poly battery.

THINGS YOU WILL NEED
3 1.2 volt NIMH rechargeable batteries (I used AAA for a smaller package)
Battery box with wiring.
Soldering iron, solder and flux
Drill
Sandpaper
Glue- I used gorilla glue and Krazy glue

Recommended
Center punch
Hot Glue

Although I did not take images of every step.  You can clearly see from the images what I am talking about in the instructions.

Steps:

1. Remove Sansa Battery compartment door.

2. Line up your new battery pack how it will mount on the door.  We are making this so the battery door on the sansa still opens, the pack is just connected to the door.  That way, we can always get to the wiring in case of an issue.

3. Once you have lined up where the pack will mount to the door, take your battery box and figure out how you will rerun the wiring.  These boxes generally have the wiring running out one of the sides.  We do not want wiring running externally, we are keeping this as neat as possible.  Once you have decided where to drop the wire down on the bottom of the box, use your center punch to make a dent and then drill out the new wiring hole.

4. Line up the battery pack to the sansa door again and then use the center punch to make a matching dent on the door, drill out the hole.

5. Before we start running the wire, we are going to use the sand paper on both the battery box and the sansa door.  This will help the glue to stick to the plastic.  Use Medium to rough grit, we are not trying to make it smooth, we need something for the glue to grab on to.

6. After the sides are sanded, go ahead and run your wires through the bottom of the battery box and down through the matching hole on the Sansa door.

7. Slightly sand the inside battery terminals inside the Sansa battery compartment.  On the C250, the far right terminal is + and the far left is  -.  Use some flux on them and then solder.  Solder the wires from the battery pack and then carefully solder the wires onto the terminals.  Make sure you have a good connection.  Since we are mounting the pack to the Sansa door, we will still be able to open everything back up later though.

8. Since we don't want a pull or shock to mess with the soldered terminals, after soldering,  glue a length of the power wires down inside the Sansa battery compartment (Sand the place where you glue to help it stick).  I used some Krazy glue because I couldn't find my hot glue.  I would have rather used Hot Glue, as it can be removed easier.

9. After the wiring is soldered and then a length is glued down, position your remaining wiring inside the battery compartment (this was the hardest step for me as it was a longer length of wire).  Snap close the Sansa door.

10. Pull up the battery pack from the Sansa door just enough to glue, I used Gorilla Glue and put a small weight on top of it.  The Gorilla Glue expanded a little more than I expected which left a small amount showing on the sides.  Doesn't bother me but I'm sure a more careful application would eliminate any trace of glue, making it a little more OEM looking.  Someone could also try Velcro, would probably work pretty well.

BTW: After using the unit, I believe the only way to get the full increased capacity out of the battery pack is to use an external charger. The Sansa itself doesn't seem to charge it up to 100%, other than that it works like a charm.

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