Secondary Batteries for Robots

Lead Acid Cells

These are one of the oldest types of rechargeable batteries invented 150 years ago. They are bulky flooded cells with low energy-to-weight & volume ratio but still widely used as car batteries and backup batteries due to their high power-to weight ratio. The reaction between Lead and Lead oxide as electrodes, immersed in Sulfuric acid creates voltage. 

Nickel Cadmium Cells (NiCad)

These are popular rechargeable cells which were invented when the only viable option was a lead acid battery and uses nickel oxide hydroxide and metallic cadmium as electrodes. There are two types of nickel cadmium cells; sealed and vented.

Vented cells use water for maintenance and should be properly positioned to avoid leakage. Sealed ones, as the name says are sealed and can operate in any position.

NiCad cells have many distinct advantages which make it an ideal choice for digital cameras and cordless phones. They suffer less damage and tolerate deep discharge compared to other cells. However recent advancement in Nickel-Metal Hydride batteries may make these cells obsolete.  For robots, NiCad is a good choice as they have a high current output, but suffer from memory effect.

Nickel-Metal Hydride (Ni-MH)

Similar to a Nickel Cadmium cell, these cells use hydrogen adsorbing alloy forelectrode instead of cadmium with more energy to size ratio compared to NiCad. However these NiMh cells have shorter shelf life when compared to lithium ion batteries, but can withstand more recharge cycles then the typical NiCad cell. For small sized robots, this is a good choice if you can get them in an online store.

Lithium Ion (Li-on) & Lithium Poly (Li-Po / Li-Poly)

Enter Lithium Ion cells; an ideal choice for your robots. They are lighter than previously discussed batteries with best energy density, zero memory effect, environmentally safe and a long shelf life which makes them popular. In a typical Li-on cell, negative electrode is a metal oxide and positive electrode is made from carbon; electrolyte is a lithium salt in an organic solvent. Lithium ions move from negative electrode to positive electrode during discharge and the other way during recharge.

These cells are expensive compared to Ni-Mh and NiCad cells and can easily catch fire if overheated or over charged. If you think you can safely handle them on your robots, they make sure to get a smile on your face.

Another variant of Lithium-ion cells are Lithium-ion-polymer batteries. Most RC aircrafts and RC cars use Li-poly cells due to their ruggedness, low weight, inexpensive compared to Lithium-ion batteries, increased discharge capability and higher run times. Unlike Li-on batteries, Li-Poly batteries use lithium salt held in a solid polymer composite which makes it easier to design in a wide variety of shapes.

There are many different types of rechargeable batteries and the above lists the most common ones. For robots, I would always suggest you to go for a rechargeable battery; Ni-Mh, Ni-Cad would be good for small robots and Li-on and Li-Poly when your need more energy (bigger robots?). Whatever you use, each of them has its own advantages and disadvantages. Study more about the battery and see if it fits your requirement.

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