The power system, somewhat obviously, consists of everything that controls powering the scooter. This is primarily the batteries, but also includes power switches, wiring, and the battery charger.
To begin with we'll cover the batteries, and firstly the two variables you'll need to consider: power and voltage.
- For our purposes, power is the number of amp hours (or just amps) that a battery can store. More amps mean a longer run time between charges and more torque for the motors. There is basically no such thing as having too much power available, though I wouldn't recommend having less than 10-15 amp-hours.
- The voltage is just that, the voltage the batteries can supply. More voltage generally equates to more RPMs and speed from your motors, but only up to the motor's rated voltage. You'll generally want your battery voltage to be at or near the voltage required by your motors, as that is where they will run most efficiently. However most DC motors have a pretty broad operating voltage range (as much as ±18 volts) allowing you quite a bit of leeway.
You can use nearly any type of rechargeable battery to power your scooter, the decision of what to use largely comes down to how much money you're willing to spend. But there are three main types typically used in building a self-balancing scooter:
- Sealed Lead-Acid (SLA, pictured top left): SLA batteries are typically used in power wheelchairs, scooters, cars, and motorcycles. They're easy to find, come in a huge variety of sizes and powers, and relatively cheap, but are also exceptionally heavy for the amount of power they can store. This is definitely the simplest and cheapest battery option for a self-balancing scooter.
- Nickel-Cadmium (NiCd, pictured top center) or Nickel-Metal Hydride (NiMH): Both of these kinds of batteries are commonly used for remote control cars and trucks. They're reasonably cheap and small and can be purchased online from a variety of sources. They also hold approximately double the power of a SLA battery for the same weight. However they generally come in the small sizes used by R/C hobbyists, meaning you'll probably need to wire multiple batteries together to get a reasonable amount of power.
- Lithium Polymer (LiPo, pictured top right) or Lithium Ion (Li-ion): These batteries are the most expensive options, and are commonly used in laptops and to drive high power motors (such as those used by electric bicycles). But they provide the greatest power to weight ratio by far (2-4 times greater than NiCd or NiMH, about 8 times an SLA battery). They also come in a wide variety of sizes and power ratings allowing you to get exactly what you want for your scooter. But they also need a special type of charger and can be very dangerous if handled incorrectly.
Once you've decided on a battery type, voltage, and power, it's time to select a switch. This may seem somewhat insignificant compared to the actual batteries, but a bad switch can lead to the inability to turn off your scooter immediately if something starts to go wrong, which can lead to damage or even injury.
The one factor you need to worry about is current handling capacity. There are a few simple steps you can follow to determine if a switch is capable of handling the power you'll need it to:
- Get power ratings (in watts) for your motors and switch:
- If the power rating for your motors isn't given (it usually is), multiply the current draw of your motors times two, to get the total current draw, and multiply that times your total battery voltage to get your power in watts.
- Get the power rating of your switch (again, if not given it's max voltage times max current).
- Compare the total power ratings for your motors with the power rating of your switch.
- Your switch's power rating should be greater than that of both your motors combined.
Not having a power switch that can't handle the power drawn by the motors can cause it to actually fuse shut, causing it to stay on even when it's switched off. Trust me when I say this is very, very bad, as the only other method of turning off your scooter then becomes cutting the battery wires.
But that covers about everything related to the power system! Next, let's move on to the parts that make sure you stay balanced: the control system.