There are three critical things to think about when building an electric bike. While the other things are important they're not nearly as important as the motor you choose, the controller and the batteries you choose. The most dangerous part of course is the batteries because over/under charging or over drawing current from them can cause catastrophic failures.

I didn't mean to scare you with that last sentence, but any battery (yes even AA's) has the capacity to explode. AA's don't usually because the things they're used in normally don't draw that much power.

There are several different types of batteries that you can use in your electric bike and I'll be going over the pros and cons of each one.

Lead Acid

The most common type of battery available is the lead acid battery. They're cheap and they hold a decent amount of power. The only problem is they really aren't designed to be deep discharged and they weight a lot for the amount of power that they store. You'll find that in an electric bike application lead acid batteries are only going to last a year before you have to replace them.

NiCad / NiMh

Another very popular battery are NiCads (nickel cadmium) or NiMh (nickel metal hydride). These batteries store a little bit more than lead acids and they have quite a few more charge/discharge cycles. A charge cycle is simply charging the battery and discharging it. That counts as one charge cycle. These batteries seem to be the perfect fit for our electric bike; however, there have been some horror stories. Like lead acids these batteries are susceptible to exploding if you charge them too fast, discharge them too fast etc. I've actually talked to a guy who did have these batteries explode on him... luckily he wasn't on the bike at the time.

Lithium Polymer

These were my initial pick for powering the electric bike. These are the highest capacity batteries available on the market today. Quite a few of the new electric car designs are using these batteries because of how much power they hold. The only problem is their very touchy about how fast you charge/discharge them. If these batteries get to hot they can release gases that are very toxic to humans. For me that was enough to keep looking.

Lithium Iron Phosphate

You'll notice that these batteries are very similar to the lithium polymer batteries except there's iron in them. These batteries are referred to as Life or LiFePO4 batteries. They hold pretty much the same amount of energy as lithium polymer batteries; however, they have almost 4X the number of charge/discharge cycles (upwards of 2000). That means these batteries will last at least 5 years. They are very tolerant of charging/discharging and the best part... they won't explode. If for some reason something gets shorted and a fire starts in the battery the iron in the battery simply puts it out (simplified version).

You can find the LiFePO4 batteries on ebay:

LiFePO4 Batteries On Ebay

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