Batteries

If your robot isn’t tethered somehow and it is electric, it will need batteries. Sometimes I have seen videos of walking robots that do not carry either their power supply or their battery pack. That is just sad to me.

Sometimes you can have a robot with one power system, but sometimes you have two, with one for the drive train and one for the control system. Typically I try to design around two power systems so that I can have one power system that is right for the control system and one that is right for the motors. There are two reasons why I do this. First, the voltage and current requirements for each may be so different you could separate them. Second, a lot of times you want to turn off the motors without turning off the controller. If you have them on separate circuits its easy to have two different power supplies. The potential disadvantage is that two power supplies can be more complicated, more expensive, and heavier.

All the batteries you use should be rechargeable to save yourself money and to save the environment.

Lead Acid Gel Cell

These are a great deal for power but they weigh too much for the type of small robots that you would control with a micro-controller. The reason why they are Gel Cells is that you don’t want the robot leaking acid if it is damaged in a crash.

The last time I used lead acid batteries was when I was making a 100 lb robot for a robot fighting show called Robotica. You get a lot of power storage for a low price, but also with a lot of weight. I don’t even think big combat robots use Gel Cells anymore. I think they all have gone to NiCd or better.

Lead acid batteries are pretty safe to charge and the chargers you use are inexpensive.

If you are making a really big robot that doesn't walk, this is a cheap way to go. If you were going to make one of those couches that you can drive around on the street you would probably use lead acid batteries.

Nickel Cadium

Nickel Cadmium (NiCd) batteries are very common in remote control cars, and potentially pretty cheap depending on which ones you buy. You can also get one charger that you can use for a variety of these batteries. NiCd batteries are pretty safe to charge, but they do get hot sometimes.

NiCd batteries come in 1.2 volt denominations, so that you can get: 4.8v batteries, 6.0v, 7.2v and 9.6v. 4.8 volt batteries are usually small and used to drive radio receivers. But they can also work well to drive a microprocessor. In theory hobby servos are meant to run on 6.0 v but they seem to work fine at 7.2v and a lot of the newer digital servos are wound for 7.2-7.4 volts.

7.2 volt batteries (like 7.4v) can be a problem for some sensor or controller circuits. I know for a fact that it you connect a Ping sensor to 7.2v battery you destroy it. You can use voltage regulators to get the charge you want for your circuit, but the bigger the batteries you are doing this to the more heat they will produce. That heat can be another reason for having two sets of batteries.

Nickel Metal Hydride

Nickel Metal Hydride (NiMH) batteries are very similar to NiCd batteries, except that they tend to cost a little more and store a bit more power. The voltages and form factors are usally the same. You can start with cheap NiCds and then spend a little more for better NiMhs.

You use the same kind of charger for both types of batteries, which keeps it simple. As with NiCd batteries the charging is pretty safe, but the battery can get very hot. NiMH batteries seem to leak power over time, so you want to top them off before each session.

Lithium Ion

For walking robots and small combat robots you want your power as light as you can get it. For this reason I have started using Lithium Ion Polymer (LiPo) batteries, which have better weight to power. The problem is that they are expensive, need a special charger, and you have to be careful about how you drain them and how you charge them. I wouldn't make my first robot with Lithium Ion batteries.

Lithium Ion batteries have 3.7 volt cells, so the voltage of batteries are: 3.7v, 7.4v, 11.1v. Dynamixel servos seem to really be designed around the voltage of LiPo batteries.

LiPo batteries seem to be really good at holding a charge for long periods when they are unused. However, if you over drain a LiPo battery you can damage it. There are little circuits you can buy from robot stores or RC stores that shut the battery off if it becomes to drained. If you charge the battery improperly it can catch fire. You can also ruin LiPo batteries by not balancing the power between the cells. You should always charge LiPo batteries in a safe place, and a fireproof bag for the batteries is recommended. I have seen batteries catch fire at Robogames, it is no joke and not something for kids to play with.

Because they need to balance power, LiPo battery chargers are a good deal more expensive then NiCd/ NiMH chargers. Between the cost to batteries and the cost of the charger, you are going to spend a lot more.

LiPo batteries are rated for their discharge rate in “C”s which tells you how many amps you can safely draw off of that battery. You multiply the storage value times the C number to determine the maximum amperage. A 1800 mAh battery with a 20C rating can deliver up to 36 amps of current (20x1.8 = 36). A higher C number is good, but you may not always need it depending on the application, keeping in mind that stalling motors can draw a lot of current.

Lithimum Iron Phosphate

Lithium Iron Phosphate batteries (LiFe) are very similar to LiPo batteries. The main reason why I use them sometimes is that their cells have 3.25 volt charge, so that you can get 6.6v LiFe batteries. That’s a much nicer voltage to run a RC receiver or microprocessor off of.

LiFe batteries have lower energy density (i.e. less power to weight) then LiPo batteries, but more than NiCd or NiMH batteries. I have read that they are less vulnerable to problems from not having a balanced charge then LiPo batteries, and are therefore a bit safer. But they still require the same kind of expensive charger that has balancing connections.