You have been mulling the design of a new combat robot for a while. You just know you have the plans for a bot that’s gonna take names and kick some serious butt in your local combat robotics arena.
You’ve made a list of parts needed and you check it twice. A frame and armor (either ordering or 3D printing it) – check. Motors and wheels – check. A great weapon that will shred your friends’ bots – check. A receiver and ESCs (electronic speed controllers) – check. Wires, connectors, belts and switches – check. But it seems like something is missing. Hmmm… Yikes! No battery!
You have thoroughly researched all the other parts, but you totally forgot about a battery. How’s your little destruction machine supposed to come to life without any juice? Time to jump online for a little more digging.
LiPo Batteries
You find out LiPo batteries (Lithium polymer, sometimes also abbreviated as LiPoly) are the batteries most used for smaller bots due to weight and size. You go to your favorite supplier to order one, but the site has a bunch of them. Then you start noticing all the specifications: S… V… mAh… A… C… mm… Holy crap! How do you know which battery you need?
Voltage and Cells
You notice the battery above is listed as 2S 7.4V. You figure the V stands for voltage. But what is the 2S? With this LiPo, the 2 is the number of cells in the battery, S tells us the cells are wired in series, which means the voltage of each cell is added together to determine the total voltage of the battery. A single LiPo cell has a nominal voltage of 3.7V. A two cell (2S) LiPo has a total voltage of 7.4 volts, three cells (3S) at 11.1 volts, four cells (4S) would be 14.8 volts, and so on.
Capacity of LiPo Batteries
The capacity of a battery is typically specified in milliamp-hours (mAh), which indicates how much power it can produce. In the above case, the battery is rated with a capacity of 250mAh. But we don’t stop there. Next we need to multiply the capacity by the continuous C rate listed on the battery, which is the capacity multiplier. In the case of the battery above, we would multiply 250mAh by 35, which would give us 8,750mAh or 8.7 Amps. Will that be enough to power our bot? Maybe, but we need to do a couple more calculations just to make sure.
Time For More Math
To make this simple, let’s say you are building a antweight wedge with no weapon. You have two FingerTech “Silver Spark” 16mm gearmotors to use for the bot.
Checking the specifications for the motors, usually listed in the seller or manufacturer’s item description on their website, look for the stall current. This is the maximum current draw (in Amps) that the motor will pull when it is completely stalled. We find the the motor is rated at 1.6A each. So 1.6 + 1.6 (two motors) = 3.2 total amps for both motors. If our battery can deliver 8.7 Amps, it seems like that would be more than enough for our needs. But would it be enough for the duration of a three-minute match?
Now we divide capacity by current – 250mAh / 3.2A (or .25Ah / 3.2A for even dimensions), which is .078A per hour. Multiply that by 60 (seconds) and we find we can draw 3.2A for 4.7 minutes. Enough for our needs.
That would work for our antweight wedge. But if you want to add a weapon, you may need to find a battery with a little higher capacity. You just add the weapon’s motor or servo’s stall current to those of the motors. A 300mAh battery would probably work just fine, but do the math to make sure.
Size and Weight of LiPo Batteries
Size matters when it comes to the smaller competition classes (fairyweight, antweight, beetleweight). Real estate is at a premium with less area to store the battery. So don’t forget to factor in size and weight of the battery when planning a new build. TIP: you need an accurate set of scales to make sure your bot is in compliance with the weight class you plan on competing in. A good set of digital kitchen scales should work just fine.
Conclusion
Now you know how to determine what battery you need when planning a new combat robotics build. Battery safety and correct charging and discharging is also important when working with LiPos, but I’ll save that for a later blog.
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