# Laser Ball

## Step 6: Battery

Overview:
Battery technology comes in a lot of different varieties but the kinds that run your smartphones and ipods are the rechargeable lithium-ion variety. I'm no battery expert but for consumer-style electronics it seems this is the only way to go. Just be careful with the charging method because no one wants a flaming battery on their hands...

How to pick the battery?
Of course the Laser Ball could be driven with another style of battery but it's important to weigh out the characteristics and make a smart decision based on different trade-offs they offer. Let's think through the important factors in choosing a battery for the Laser Ball.
• It needs to fit in a tennis ball
• It needs to provide sufficient voltage (3.3V-5V) for the Teensy
• It needs sufficient capacity (amp-hours) to provide a reasonable battery life
• If needs to have a large enough discharge to provide sufficient current for the lasers and Teensy
• It needs to be rechargeable... trying to remove the battery pack from a fully-assembled laser ball is like trying to machete your way through a tropical jungle... bad joke, i know, but I think it paints the right picture.
Understanding the specs...:
I chose to use the 3.7V 850mAh Polymer Li-ion Battery from Sparkfun so lets see why...
• It will just barely fit in the tennis ball
• 3.7V gives me enough voltage for the lasers (3.2V) and the Teensy (3.3V)
• Its 850mAh capacity means a couple hours of battery life are possible if the Laser Ball draws ~300mA (850/300=2.8hr)
• It's max discharge rate is 2C which will provide enough current to run the Laser Ball. The C-value is just the capacity divided by an hour, so in this case a 2C rate is equivalent to 1700mA. The nominal discharge rate is 0.2C which is 170mA so the Laser Ball requirement (300mA) sits nicely within this range.
Get a charger:
Keep it simple and buy a basic Lithium-ion charger. There are lots available in the open-source community. They are simple circuits that can regulate the charging conditions for the lithium-ion batteries and provide a nice simple USB interface. Try this one from Sparkfun: LiPo Charger Basic