One important thing LiPo batteries need is a protection circuit. A major downside to LiPo batteries is that they must never be overcharged or overdischarged, and a protection circuit helps prevent that. Combine the following circuit with a smart charger that is also designed to prevent overcharging, and you have a (relatively) safe setup that is small, lightweight, and can power your portable system for hours and hours.
This guide details a typical layout for a 7.4v system, which consists of two standard 3.7v LiPo batteries wired in series.
This is what a typical protection circuit looks like (obviously much larger than life size):
Image taken from batteryspace.com
It's not rocket science. Simply wire the batteries' terminals to the appropriately labeled pads, making sure your polarity is correct. P+ and P- are what you connect your charging jack and system to. They are your protection circuit's output.
Perhaps this (not to scale) diagram will help:
Please note that how the batteries are connected to the system and power switch is just my suggestion - there are other ways to do it but I have found this to be an effective and simple setup.
Keep in mind that if your system needs 5v to operate, you will need a linear or switching regulator to drop the voltage down to what you need.
Now that you know how to wire the protection circuit, it's time to determine which protection circuit you will need. Your battery will be rated for a maximum amount of discharge current (rated in Amps, A, or milliamps, mA) that it can handle. Select a protection circuit that matches or is under that value.
A website I highly recommend is http://www.batteryspace.com - they usually have very fair prices and all the parts you will need to make your very own LiPo battery pack. There are other online stores as well - Google is your friend here. There are tons and tons of cheap single LiPo cells and pre-assembled LiPo packs on eBay, but whether or not you trust Hong Kong craftsmanship sold at discount prices is up to you.
Warnings, Disclaimers, etc.
Be very careful when working with lithium polymer cells. These are very energy dense batteries that you DO NOT want to mishandle. This YouTube video should provide the caution necessary when working with these cells. I know that BatterySpace includes safety information with every battery they sell, so heed their warnings! When working with the batteries, tape loose wires down so no accidental shorts can occur. After you have wired your batteries up, I suggest putting electrical tape over the batteries' contacts and then a thin layer of hot glue around the edges of the tape to make absolutely sure that nothing will slip loose once inside your portable.
An ugly, but effective demonstration:
If the idea of wiring up LiPo's makes you nervous, then I highly recommend you take a look at li-ion batteries or NiMH RC car batteries, especially if you are just getting started with electronics. The trade-off, however, is that they are generally larger and weigh more. If that does not scare you off and the promise of long-lasting, amazing batteries seems like just the thing you need for your next portable, then you now have the information you need to use lithium polymer batteries.
