Once you’ve decided on your motors you can now choose your batteries. To choose a battery you need to know the maximum voltage that your motors can handle. The voltage will be effect the maximum RPM that the motor can operate at. Next you need determine how much amps the motor or motors on the circuit will draw from the battery. Now typically I like to choose motors that operate on the same voltage so that I can have them on one circuit. If you do this, to determine the maximum amp draw, I will take the peak amperage of each motor and add them together to get your total amperage draw from the battery. The reason I choose peak amps and not run amps is because the run amps can change depending on the resistance the motor faces as it is running.
Determining Your Battery
Once you know what the amp draw is on the battery, and what the required voltage is, you can choose your battery. For the voltage, all you need to do is find a battery supplies that voltage the your motors require or is close to the voltage the your motors require to obtain their maximum RPM. However, dealing with amp draw is a little more complicated, since this will be determine roughly on how long that battery will last before it needs to be charged. To calculate the batteries life you need to look at two things. You need to know what the batteries discharge rate is which is represented by C, and you need to know what the batteries capacity rating is which is normally in the units of milli amp hours (mAh). By knowing what the capacity rating is and what the discharge rating is you can determine how much amps can be drawn from that battery without damaging the battery, and how long that battery will last by using the following equations.
First, let me show you how to calculate the maximum current draw in milliamps that can be drawn from the battery. To do that you would use the following equation.
$(discharge~rating)$$(capacity~rating)$$=maximum~current~draw$
Once you know what the maximum current draw is you can calculate how long the battery would last at this maximum level by using following equation.
$\frac{discharge~rating}{maximum~current~draw/60}$$=time~in~minutes$
Now that you know how to calculate the maximum current draw, and how long that battery will last if the maximum current is drawn, you can use this information to determine if the battery will work for your application. From the first equation you can determine if that battery can handle the current that you want drawn from it. Basically, if the current that your motors draw is more than the current that results from this equation, you will damage the battery, and won’t be as efficient. Next, you can modify the second equation using your own current draw to determine the time it will take for the battery to discharge by using the following equation.
$\frac{discharge~rating}{motor~current~draw/60}$$=time~in~minutes$
Finally, to use these equations you must have everything in the right units. These equations assume that your discharge rate is in milli amp hours (mAh), and your current draw is in milliamps (mA). If your current draw isn’t milliamps, but instead amps (A), your discharge rate must be in amp hours (Ah).
Types of Batteries
There are three common types of batteries that are used for RC devices. These are LIPO batteries, A123 / LIFE batteries, and NICAD batteries. You could also use lead based batteries, which are similar to your car battery, but considering the weight of them, most likely you would only use them for large battlebots; so I’m not going to discuss them here.
LIPO Batteries
LIPO batteries are traditionally lithium polymer batteries, but lithium ion batteries are commonly put in this category also. The difference between a lithium polymer battery and a lithium ion battery is that lithium ion battery are gel batteries while lithium polymer are not.
There are a lot of advantages to using a LIPO battery in comparison to a NICAD battery, which is an older technology. First, LIPO batteries are light weight, and despite their size they can hold a huge amount of power. LIPO batteries also have very high discharge rate in comparison to other batteries.
The disadvantage of using LIPO batteries is as follows. LIPO batteries can be expensive in comparison to other batteries. However, I have noticed their price has been coming down. Finally, LIPO batteries can be unstable if they are punctured, or if you discharge from them to fast. Since LIPO batteries can be unstable there is the possibility that they can explode or catch on fire. This has caused some competitions to ban LIPO batteries if the facility that the competition is hosted at doesn’t have the equipment to deal with one of these batteries catching on fire or exploding.
A123 / LiFe
A123 / LiFe Batteries are nanophosphate batteries. They are a fairly new technology and are currently the type of batteries that hybrid cars use.
A123 / LiFe batteries are closely related to LIPO batteries, meaning they share a lot of the same benefits with a few additional benefits. As with LIPO batteries A123 / LiFe batteries are lightweight. They however, can have a higher current or peak power rating. They also are much more stable then LIPO batteries, meaning its harder to make them explode. Finally, they have a flat discharge curve rate. By having a flat discharge curve, these batteries will have the same voltage output through most of its cycle, until it suddenly drops off near the end of the cycle.
The main disadvantage of these batteries is there cost. Due to the fact that they are a fairly new technology, they are generally more expensive. Also, since they are still a fairly new technology, there isn’t as much information about them, as there is for other battery types.
NICAD / NIMH Batteries
NICAD batteries, or Nickel-Cadmium batteries, have been around the longest of the battery types that I have discussed. In general they aren’t bad batteries, and I have used them, but you need to keep something in mind when you are using them. First, NICAD batteries have a memory. When a battery has a memory it means that if you charge the battery before you should it will have a large voltage drop before its suppose to, or in other words it will lose its charge before it is suppose to. To solve this problem you have to discharge these batteries the correct amount before you charge them. Another issue with these batteries is if you discharge them too much, some of the cells could reverse polarity, which is called cell reversal. If this happens that battery will not be as efficient, and it could ruin the battery. Finally, these batteries are much heavier the LIPO batteries or A123/LIFE batteries. They are however the cheapest out of the batteries I have discussed.