Batteries — choose the right power source for your robot
In the following article I would like to share some practical advice about supplying mobile robots with power. Let’s jump right into it.
Types of batteries used in robotics
There are many different types of batteries available on the market however to keep things simple we’ll divide them into two groups.
Batteries that are great for robots:
- Li-Ion — lithium-ion battery
- Li-Poly — lithium polymer batteries
- NiMH — nickel–metal hydride battery
And batteries that are not that great….meaning, don’t use them:
- Lead-acid battery — all types (including VRLA, SLA, gel or AGM) don’t like charging-discharging cycles and works much better as a backup power supply for stationary applications. They also have low capacity per weight unit.
- NiCd — nickel-cadmium battery — similar to NiMH, but currently they are being withdrawn from use, because of toxic cadmium and no advantages over NiMH cells.
- NiH2 — nickel-hydrogen battery — have you ever heard about that one? Probably not, unless you were building the Hubble Space Telescope…
There are couple other types, however we won’t be discussing anything expensive, difficult to buy, or difficult to charge.
A table below will give you a general overview of the battery types mentioned above (the ones suitable for robots). There are many modern Li-Ion cells, which allow discharging them with current up to 90C, but the table would be 10 times wider if I wanted to describe all of them.
*- the currents are usually defined as multiplied “C” — the capacity divided by hour; “2C” current for 2000mAh cell means “4000mA”.
**- CC, CV, ΔV/ ΔT — charging methods: constant current, constant voltage, deltaV/deltaT (detection of voltage or temperature peak)
Comparison of Li-Ion, Li-Poly and NiMH
In fact, Li-Poly batteries are the sub-group of the Li-Ion batteries. We can say that they are a special version of regular Li-Ion batteries. Why are they special? The difference is that during production, Li-Ion cells need to be pressed into a metal can (usually cylindrical) so that they remain in one piece. Li-Poly cells, which were introduced later, have different construction and can hold themselves up without the support of the external cylinder.
However, electrical parameters of Li-Ion and Li-Poly are almost identical.
The NiMH batteries are popular due to low internal resistance and good power-to-weight ratio. They are also much safer than Li- based cells. Explosions of such batteries are really rare (however I wouldn’t recommend throwing them in the fire ;)). The specific energy (energy-to-weight ratio) of NiMH is much worse than lithium cells.
The most popular form of any batteries is a cylindrical can. Used for Li-Ion, NiMH and very rarely for Li-Poly cells (they doesn’t need a can as mentioned earlier). The cells on the photo above have an 18mm diameter and are 65mm long. That’s the reason why they are called “18650” cells. So, If you find a 18650 cell, you can be almost certain that it’s a Li-Ion cell.
You can see that the yellow one has a flat “+” terminal — it is prepared for welding cells in a group to form battery packs (like the one shown below). The blue, “consumer” cell has a raised “+” terminal, so it can be easily inserted and removed from a battery holder with a spring on the “-“ terminal side. Consumer cells often have built-in protection circuits and therefore are about 2–4mm longer than regular 18650 cells.
Other popular dimensions for Li-Ions and NiMH cells are 14500 (14x50mm) widely known as AA. You can also find 18500, 26650, 16650, AAA, C, D and many other types.
This shape is used when the battery construction requires a metal can. The prismatic Li-Ion cells were commonly used in handheld devices (like cell-phones) when the Li-Poly batteries were not yet popular. Fun fact: Nokia 3310 used a NiMH battery in a prismatic shape.
As mentioned before, Li-Poly batteries, do not need a metal enclosure. They are produced in form of thin, elastic slices, which are stacked and inserted into the pouches instead of being rolled up into the can. Lack of the metal packaging is the main reason why “Li-Poly” weighs less than “Li-Ion”, providing the same power.
Pouch cells are cost-effective and are usually be very thin therefore are a perfect fit when it comes to supplying smartphones, tablets and netbooks with power.
Pouches can sometimes have protection circuits:
They can also be stacked in blocks:
These blocks are often used in drones, RC cars and other high-power toys. Li-Poly batteries for RC hobbyists don’t have any protection circuits. This is quit a unique situation if you take consumer market into consideration.
It is important to remember not to confuse Li-Polys for smartphones and Li-Polys for quadcopters! The first ones are usually high-energy types, have quite big internal resistance and low maximum discharge current (about 1C). On the other hand, batteries for RC hobbyists are prepared for discharging with high current, up to 30–90C! So…they are both Li-Polys…but completely different.
Example robots — what batteries to choose?
Robots are different, so is their appetite for power. Let’s discuss three robot examples for the purpose of this article:
- Mini-sumo robot
- Small AGV
What is a sumo robot? It is a robot designed to fight with another robot in sumo-competitions. Rules of such competitions usually specify the limit of 10 x 10 cm footprint, with any height allowed. The maximum weight is 500g. In most cases mini-sumo robots use two DC or brushless motors and some additional moving parts used for levering or confusing the opponent.
In our example, we will use two DC motors for driving and a scoop for lifting the opponent up, making it lose its grip. The scoop is placed above the robot at the beginning, so the robot footprint is within the limit. Just after the start, a scoop will move to the lower position and will be located in front of the robot. The scoop is controlled with an RC servo.
Usually competitions themselves are quite short but the robot must remain operational for several battles in a row. Let’s assume that 15 minutes is a total minimum working time.
We also assumed that we will use some low-impedance capacitors to handle the high-current peaks drawn by motors, so the batteries have to deliver about 4A in a peak. The average current used by robot will be about 2A.
So, let’s now specify the requirements for the batteries:
- Size — the batteries shall be as small as possible.
- Weight — about 150g.
- Nominal voltage — the motors have 6V nominal voltage, the electronic circuits usually need regulated 5V supply, so the absolute minimum is 7V. There must be a margin for negative voltage peaks and drop-off voltage on regulator and cables.
- Maximum discharge current — we estimated before that 5A should be enough.
- Capacity — the average current is 2A and minimum working time is 15 minutes, so the required capacity is 500mAh.
- Other — a possibility of fast charging is an advantage.
Having these parameters, we can list a few matching batteries to choose.
NiMH: 7x AAA battery pack — FDK HR-4U or similar
- Dimensions: 74 x 45 x 11.5 mm
- Nom. voltage: 8.4V (1.2V per cell)
- Min. voltage: 7V
- Max. discharge current: unknown, datasheet specifies the capacity for 2A (2C) discharge, NiMH usually allow 3C-5C for a short time
- Volume: 38.3 ccm (cubic centimeters)
- Weight: 100g
- Capacity: 930mAh
- Price: about $12-$25
Comment: Easy to charge and safe. Long charge time, but can be used as replaceable batteries with the battery holder (needs more space). Only 100g of weight. The lowest capacity-to-price ratio.
Li-Ion: 3x18650 battery pack: Tenergy 31012, 11.1V 2200mAh with protection circuit
- Dimensions: 68 x 56 x 19 mm
- Nom. voltage: 11.1V (3.7V per cell)
- Min. voltage: 8.4V
- Max. discharge current: 2C = 4.4A
- Volume: 72.4 ccm
- Weight: 140g
- Capacity: 2200mAh
- Price: about $27
Comment: More difficult to charge, but charging is much quicker. Integrates a protection circuit to avoid overcharging, over-discharging and short circuits. Bigger than NiMH but has over twice of its capacity, so you will not worry about charging or replacing it during the day.
Li-Poly: Hyperion G5 50C 2S 850mAh LiPo
- Dimensions: 75 x 27 x 14 mm
- Nom. voltage: 7.4V (3.7V per cell)
- Min. voltage: 6V
- Max. discharge current: 21A
- Volume: 28.4 ccm
- Weight: 48g
- Capacity: 850mAh
- Price: about $10
Comment: Right choice if you want the best performance and the lowest weight and volume. The drawback is the low voltage — you have to keep cables short and use the LDO regulators for electronics. It doesn’t have any protection circuit, so it’s a less safe option!
Let’s now discuss an example of a flying robot, which should be as lightweight as possible (it needs to fly right?). There are different sizes but let’s say it’s a 45 x 45 cm drone. It uses four BLDC motors for propellers, have a small but smart electronic circuits and a radio transceiver. The power source shall deliver a lot of energy in a short time.
We assumed that all calculations related to drone weight and motors power have been done and we know that we need a battery with nominal voltage of about 14–15V.
The requirements for the batteries are:
- Size — not critical.
- Weight — below 650g.
- Nominal voltage — about 14–15V.
- Maximum discharge current — min. 60A.
- Capacity — the average current is 22A and minimum working time is 10 minutes, so the required capacity is ~3700mAh.
- Other — replacing the battery should be easy. The protection circuit is not welcome because it can cause fatal drone accident if it cuts-off the battery.
Examples of batteries that match these requirements:
NiMH: not this time — we need a lot of energy from a weight unit; Li- based cells are much better
Li-Ion: 12x18650 (4S3P) battery pack made of KeepPower IMR18650 2500mAh 3,7V 20A cells (20A continuous, 35A peak)
- Dimensions: 134 x 57 x 38 mm
- Nom. voltage: 14.8V (3.7V per cell)
- Min. voltage: 8V
- Max. discharge current: 3*20A = 60A
- Volume: 290.3 ccm
- Weight: ~600g (12*48g + cables and enclosure)
- Capacity: 7500mAh
- Price: about $100
Comment: The capacity looks like much more than required but with the high current it can be even half of that. Li-Ions aren’t used in drones. As we can see, it’s not crazy to use them, but it is difficult to find and buy the high-current versions and they have much lower specific power (power-to-weight ratio) than high-current Li-Poly cells, which is critical in flying robots.
Li-Poly: Turnigy Graphene 5000mAh 4S 45C Lipo Pack w/XT90 (45C continuous, 90C peak)
- Dimensions: 144 x 51 x 41 mm
- Nom. voltage: 14.8V (3.7V per cell)
- Min. voltage: 12V
- Max. discharge current: 45C = 225A
- Volume: 301 ccm
- Weight: 589g
- Capacity: 5000mAh
- Price: about $70
Comment: Probably the parameters are overstated but it’s still the best choice because of a big margin of maximum discharge current. This should ensure a long life of a battery. The capacity-to-weight ratio is lower than for Li-Ion but only for low current discharging. In this application we won’t have low currents and the flight time will be shorter than calculated from the declared capacity.
AGV is an automated guided vehicle with a single board computer, a camera and many sensors on the board. It has 4 wheels, 2 of them are driven by DC motors. It’s application is to scan the shape of the rooms in different buildings and to create a map of the rooms with textures. The speed is not critical because focusing the camera lenses, scanning the room with LiDAR and computing or transmitting raw data takes pretty long. Therefore the robot can’t drive too fast.
This time the consumption is related to power, not to the current, because of many DC/DC converters that converts higher voltage and low current to lower voltage and high current. Then we can choose the minimum voltage for the battery — 10V — and calculate the average current for our convenience.
The requirements for the batteries are the following:
- Size — below 150 ccm.
- Weight — below 500g.
- Nominal voltage — about 10–12V.
- Maximum discharge current — 9.3A.
- Capacity — the average current is 2.1A and minimum working time is 120 minutes, so the required capacity is above 4200mAh.
- Other — low self-discharge current is an advantage.
NiMH: not this time — energy is a priority; Li- based cells are much better
Li-Ion: 6x18650 (3S2P) battery pack made of INR18650–35E Samsung 3500mAh
- Dimensions: 67 x 57 x 38 mm
- Nom. voltage: 10.8V (3.6V per cell)
- Min. voltage: 9V
- Max. discharge current: 16A
- Volume: 145 ccm
- Weight: 300g
- Capacity: 7000mAh
- Price: about $30
Comment: Very good choice. The specific energy (capacity-to-weight ratio) is much greater than required for this volume. The price is low.
Li-Poly: a 3S1P battery pack made of LP616594 4700mAh 3.7V
- Dimensions: 94 x 65 x 19 mm
- Nom. voltage: 10.8V (3.6V per cell)
- Min. voltage: 9V
- Max. discharge current: 9.5A
- Volume: 113 ccm
- Weight: 236g
- Capacity: 4700mAh
- Price: about $60
Comment: Lower weight and volume, but worse specific energy and higher price. An example of a low-current Li-Ion type — the maximum current is only 2C but it is enough for this purpose.
I wrote this article to show you the way of choosing the right battery for your robot. I hope it helps:)
- Even if you recognize that your battery is a Li-Poly LiCoO2 by its appearance, you have to look into the documentation anyway. Each chemistry can be optimized for more power or more capacity — you don’t know.
- The protection circuits increase the safety and lifespan of the battery but if your drone falls down because of triggering the protection, then the 1000 cycles of charging are not that valuable…
- Choosing the right battery size, version, high-current capability, capacity etc. is much more important than choosing between NiMH or Li-Ion. Sometimes I see robots powered from 6F22 battery. It is like using 125ccm scooter for towing a 500kg trailer.
- Charging of mentioned batteries is the topic for another article which I will publish in the near future.