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Lithium Polymer batteries are a favorite of ours. Very light weight and some of the highest energy densities available.
We affectionately call this battery the 'car battery' because it's huge. Not physically (it's 110 grams!) but because this 6Ah LiPo in capable of outputting 6 Amps over the period of one hour! This is a very large amount of power in our low-power embedded world. Please don't confuse this with a real car battery!
Please: Never ever charge a LiPo with anything but a special LiPo charger!
SparkFun offers two chargers to go with our LiPo cells. Our standard charger based on the old MAX1555 IC and our faster charger based on the newer MCP73831 IC. The MAX1555 was great but its limited charging current (300mA) and diminishing market availability is pushing us towards creating chargers based on the MCP73831 (500mA per hour charging rate).
Here you can see a 6000mAh brick correctly attached to the USB LiPoly charger. But how do you detach the battery from the charger? If you've ever tried to pull a JST connector from a JST socket you'll find it's incredibly difficult. This is because JST connectors were designed to be used deep inside electronics where a battery or component may need to be changed only once or twice within the lifetime of the product. A very strong, firm connection was more important than ease of removal. We use JST connectors specifically because they have such great holding and are so compact! So how do you detach the battery from the charger or your embedded device?
Note: Pulling on the wires will actually damage the connector! You need to get a mechanical advantage. Wire cutters have thin jaws that allow you to grip the head of the connector instead of the wires. Be careful not to cut the wires. Don't go squeezing hard on the wire cutters either. They are simply there to grip the edge of the connector. Needle nose pliers may work, but you really need to get behind the flanges of the connector.
Use the edge of the PCB as a fulcrum point and lever out the connector, pulling gently sideways. After a few inserts and removals, the friction points on the JST connector will wear down slightly to the point that you will be able to remove the connector by hand.
One of the down sides to using these LiPo batteries is their fragile power connections. These smaller, more fragile connectors make sense if you're embedding the battery into a hardened portable device like an iPod. But after building a few projects with LiPos, I found that under my day-to-day use the power wires would break from the terminals on the safety circuit built into the battery. Luckily for us, a little electrical tape goes a long way!
The red and black wires on a LiPo will tend to wear out and break off if you swing the battery around too much. This fix is so simple, but I find that many people don't realize what a little stress-relief can do. On your LiPo battery, fold the red and black wires to the side. With electrical tape, start in the middle of the battery and tape over the top.
Use a little bit of tension on the tape as you go over the top of the battery.
I use one and a half full wraps around the battery.
I then cut the tape with wire cutters or knife to make a clean cut. Tearing the tape can leave ugly ripples in the tape.
This one or two wraps around the battery serve as a simple stress relief. All the mechanical stress is transferred to the tape instead of to the soldered terminals inside the top of the battery. Now when you use (or mis-use) the battery, you won't have to worry about breaking the red and black wires from the top of the battery!
I found I had a surprising collection of LiPos for various projects (they just seem to be replicating at this point!). A problem that came up a few months ago was identifying the battery. What was I doing with this battery? What torture had I put it through? A sharpie marker is perfect for marking your battery. A name such as 'heater' or 'costume' makes a world of difference. Dating the battery will let you know when you first used it. All my LiPos are still working swimmingly, but after a few years I assume these batteries will start to lose some of their capacity. A date code helps to indicate that general health of a cell.
Remember, a safety circuit is built into every pack that we sell. The safety circuit will prevent over voltage (over charging), over current (the battery will turn off if there is a short in your system), and under voltage (the battery will turn off before it runs down too low). This safety circuit prevents most misuse of the battery and we have definitely misused some batteries. You can be re-assured the safety circuit will protect you and the battery.