Batteries
Almost all robots who compete in combat robots derive the majority of their power from Batteries. A decade ago it was popular to see Sealed Lead Acid batteries as the primary battery in many robots. Today the vast majority of Robots use Lithium Polymer or LiPo batteries. Lithium Batteries allow for a large amount of power to be stored in a relatively small volume. This article will only focus on Lithium Batteries as they are the most common.
How To Pick the Right Battery
LiPo Batteries come in a variety of sizes and capacities. Finding the right battery for your robot will depend on how much power you need and how much space and weight you have.
Cells
First up is picking the number of cells in your battery. The number of cells also defines the voltage range your battery operates at. Most robots in the 3lb weight class use 3 or 4 cells. This is abbreviated as 3S or 4S. The standard nominal voltage is 3.7 Volts per cell.
Nominal voltage is the voltage when the battery is about 40% state of charge. A typical nominal voltage is 3.7 Volts per cell. The nominal voltage is the voltage that is usually printed on the battery. This is also the best voltage to keep the battery at for long term (months) storage.
Empty Voltage is when the battery has no more usable capacity left. This voltage is typically 3.0 volts per cell. If you continue draw power from an empty battery the voltage will rapidly drop to 0 volts. Batteries discharged below 3.0 volts per cell (will be damaged, and you may not be able to charge them again. Even if recharged, these batteries will likely have a higher internal resistance and a reduced capacity.
Full Voltage is the voltage when the battery is charged to is maximum capacity. This voltage is typically 4.2 Volts per cell. Charging the battery beyond this voltage will damage the battery and may cause a fire!
| Cells | Nominal Voltage | Empty Voltage | Full Voltage |
|---|---|---|---|
| 1S | 3.7 Volts | 3.0 Volts | 4.2 Volts |
| 2S | 7.4 Volts | 6.0 Volts | 8.4 Volts |
| 3S | 11.1 Volts | 9.0 Volts | 12.6 Volts |
| 4S | 14.8 Volts | 12.0 Volts | 16.8 Volts |
| 5S (Rare) | 18.5 Volts | 15.0 Volts | 21 Volts |
| 6S | 22.2 Volts | 18.0 Volts | 25.2 Volts |
Mah (Milliamp Hours)
MaH or AH is a measure of how much energy a battery holds. The measurement is supposed to align the amount of power the battery could deliver at its nominal voltage for one hour. So a 3S 500 Mah battery should be able to provide 500 Milliamps of current at 11.1volts for 1 hour. For example it's a reasonable assumption that a 3S 500 Mah battery should be able to provide 5 Amps (5000 Milliamps) for 6 minutes.
C rating is maximum safe discharge rate. It would let us know if the above draw of 5 amps was okay. To determine the maximum safe amp draw it is the Capacity (C) times the C rating number. So if our 3S 500 Mah battery had a C rating of 20 the maximum safe amps we could draw would be (500 Mah x 20) 10,000 Milliamps or 10 amps. Which gives us a good safety margin around a 5 amp draw. Some batteries list a fixed and burst C rate. Battery manufactures tend to over state their C rating. It is best to ensure your robots max power draw will be safely under the C rating for your battery.
Connector
When it comes to how to connect your battery to your robot there are a lot of options. It is best to chose something that can make a firm connection and can handle the amount of current you will draw.
Some Common Connector types
- XT30
- XT60 (NHRL Favorite, Very common)
- XT90
- Deans
- JST (Not recommended.. these fall out or make poor connections easily)
Combining Batteries
You can combine multiple batteries to reach a higher voltage or capacity. One common example is for competitors to combine two 2S batteries in series, always of the same capacity to make a 4S power system. It is also possible to combine batteries in parallel to double the capacity instead of the voltage. When combining batteries it is incredibly important to always make sure the batteries have the same capacity.
