Sure, your electronics project is awesome, but wouldn't it be cooler more than 10 feet away from a power outlet?

I designed this add-on Battery pack because I program my Propeller Platform around the house, and I hate having to drag a Power Adapter everywhere I go. With this module, I just drop it on and go. When I do use a Power Adapter, I don't need to remove it, I can flip the switch on the battery pack.

My design uses common AAA batteries as the power source. The Propeller Platform has very flexible power consumption capabilities, so the Battery Module can run for many, many hours:

40 Hours 3 cogs running @ 20MHz
17 Hours 3 cogs running @ 80MHz
8 Hours 8 cogs running @ 80MHz

Flip to the next step and I'll give you some more technical details on the Battery Module and show you how to use it. I've also put hi-res photos for this howto on flickr.

The Battery Module is also available as a kit from Gadget Gangster.

Step 1: FAQ

Design Goals
I designed the Battery Module with a few objectives;

It should be cheap. Cheap to buy, cheap to use
It should be easy to recharge / replace the batteries
It should be easy to build, repair, and modify.

I did a first rev. of the design and put it up for comment on the Parallax forums. Got feedback and made a few changes to get to the final version. I think it meets all of the design objectives;

1 - Cheap
Small PCB, uses inexpensive AAA batteries so charging circuitry doesn't have to be on-board. It takes advantage of the Propeller Platform's on-board regulator to provide 3.3v power. Liquidware's Power shield is awesome, uses a lightweight battery, and has built-in charging, but $40 is more than I wanted to spend.

2 - Easy to charge / replace batteries
Compared to LiPo or or Li-ion, nimh AAA batteries are incredibly easy to charge and replace. There are a million nimh chargers already available, and you could even use standard Alkaline batteries in a pinch. It's easy to swap out dead AAA batteries with a fresh set, and when the batteries stop holding a charge, you can pick up replacements at the local 7-11.

AAA nimh cells aren't as flashy as a proprietary LiPo cell, but given my design goals, I think it's a better choice.

3 - Easy to build / modify / repair
Everything is through-hole, and the design is available under the MIT license. It's a simple circuit but I want to keep it as open and hackable as possible. Links to download the layout files are on the last page of this howto.

Is it Easy To Build?
Yes. If this is your first time soldering, it will probably take 15 minutes. If you're a seasoned pro, it should take longer for your soldering iron to warm up than assemble.

Step 2: Preparation: Tools

This is a great project to learn how to solder. There are a ton of great instructables on how to solder, I also did a little demo video here.

Tools
You'll need a few tools to assemble the project;

1 - Soldering Iron and solder. Leaded solder is easier to work with, and a 15-40 watt iron is just fine. A conical or chisel tip works well.

2 - Dikes. Diagonal cutters are used to trim the excess leads from components after soldering them down.

If this is your first electronics project, I suggest you start with an inexpensive soldering iron. Why? Because you'll be able to get a feel for doing electronics projects without spending a lot of money. If you enjoy yourself, then you can upgrade to better equipment and give your 1st iron to someone else who is just starting out. I offer an Elenco kit that includes a 25 Watt iron, stand, wick, and a solder sucker for $25 (pictured below). You can also get a pretty nice Weller iron on Amazon that includes extra tips and solder (but no wick or solder sucker) for $15.

Step 3: Preparation: Parts List

Flip on your soldering iron and let it warm up! While it's warming up, check to make sure you have the following parts. If you've ordered a kit, double check to make sure your package has all the parts listed. If there's anything missing, just email us at info@gadgetgangster.com;

AAA Battery Holder
Mouser Part # 12BH412-GR
Qty: 4

R/A SPDT Slide Switch
Mouser Part # 506-SLS121RA04
Qty: 1

Battery Module PCB
Source: Gadget Gangster or download the design & make your own (diptrace format)
Qty: 1

40 pin header strip
Qty: 1

Step 4: Make: Switch

Start with the Power Switch. Use one finger to hold it in place, Flip the board over and 'dangle' a bit of solder over one of the pins. Use use your soldering iron to lock down one pin.

If you want to adjust the placement, re-heat that one pin and re-position with your finger on the other side.

Once you're happy with the placement, you can set the board down and solder the other pins.

Step 5: Make: Battery Holders

Add the battery holders. Note that the spring sides all go on the side of the board marked "minus".

Insert the holder, splay the leads apart, flip the board over, solder them down and trim off the excess leads.

Same deal with all 4 battery holders.

Step 6: Make: Headers and Sockets

Let's add the pin headers so we can connect it to the Propeller Platform / protoboard / breadboard.

The easiest way to do that is to take the pin headers and insert them into a breadboard / Propeller Platform, then drop the Battery Platform PCB on top. This will ensure the pin headers are lined up and straight. But before you solder the pin headers to the Battery Platform, it's time to make a decision:

To connect the battery module on top of the Propeller Platform, you'll probably want to add the pin headers on the outside row, pointing down.

To connect the battery module below the Propeller Platform, you'll want to add pin SOCKETS to the inside row, pointing up. You'll add pin headers to the inside row of the Propeller Platform, pointing down.

I've decided to add it on top of the Propeller Platform, so I dropped the board on top, and now I'll solder the pin headers to the Battery Platform pcb.

That's it!

Step 7: Usage and Downloads

Usage

Using it is very straightforward - just flip the power switch on the Battery Platform to the on position.

Note that you don't want to have the Propeller Platform Plugged into a wall adapter, flipped on, and also flip on the Battery Platform. You can keep the Battery Platform on the Propeller Platform while the using a Power Adapter, just be sure to keep the Battery Module switched off.

With nimh batteries, the V50 (5v) rail will provide 5v - 4.5v, about 90% of the time it will be about 4.8v. Alkaline batteries will work, they'll provide 3.3v to the V33 rail, but they'll provide 6v - 5v on the V50 rail (most of the time it will be about 5.6v). If you you alkalines, keep that in mind.

The V50 and V33 rails will each be smoothed by the 47uF caps on the Propeller Platform.

Power Optimization with the Propeller

Just a brief overview to get you started if you're interested in extending the battery time of the Propeller:

Shutdown unneeded cores if you're not using them A lot of objects have built-in stop methods, so if you're using a TV for a while, but stop using it, just turn it off (freeing the cog) with tv.stop
Consider your clock frequency The Propeller can run from 13kHz up to 80MHz. When you start your program you'll define the clock speed in the CON section, but you can change clockspeed on-the-fly with the CLKSET command.
Assembly is 10x more power efficient than Spin Spin is easier to program, but if your program spends a lot of time waiting for a state change, you might consider writing that portion of the program in assembly.

There are more Power Optimization tips in the Propeller Manual (pdf), too.