Batteries are used for all devices and we take it for granted for robotics as this is something that is at the back of your mind (for some of us). But it is an important consideration when you develop a mobile robot as determines how long your robot will last in the field.
Depending on the scale of the robot that you are developing, you might have to select different types.
- Lead Acid batteries come in different sizes and they are rechargeable. They are widely used in vehicles around the world and hence are the cheapest form of batteries for their size. They are usually heavy but one advantage of using lead acid batteries is the high discharge rates that it can handle. Hence they are still used in vehicles and even some UPS (uninterrupted power supplies) for computer servers.These batteries come in various sizes and can be used for smaller to giant robots.
- Lithium-ion (Li-ion) batteries are the batteries we use for mobile devices like handphones and laptops. The advantage of lithium batteries is that it has a high energy capacity (hence lighter than batteries of the same capacity). It has zero memory effect, which is an improvement from the NiMH batteries. This means we never have to drain the battery before charging it. These batteries are expensive but the price of these batteries are dropping as more devices are using this. Li-ion batteries typically have low current output. If your current output requirement is higher, the lithium polymer (Li-Po) batteries have higher current output capabilities. I would suggest using this batteries if weight it a problem and you require high capacity batteries. These batteries come in various sizes and can be used for smaller to giant robots.
- Nickel Cadmium (NiCad) batteries are between the Lead acid and the Li-ion batteries in terms of energy capacity. They are cheaper than the NiMH and are commonly used for RC planes and cars. We use it for some of our robots as well as given our requirements are similar to RC planes. These batteries have good current output but they must be fully discharged before you can charge the batteries.
- Nickel Metal Hydride (NiMH) are used in older mobile phones. They require long charging time. They can last longer than the NiCad batteries (therefore higher cost) and they have the highest energy capacity.
There are other alternative like fuel cells and alkaline batteries and there will be more innovation in this area given electric cars are on the horizon. The cost of technology always reduces with the volume of use. This will drive innovation in getting the cost down with higher energy capacity batteries. I certainly look forward to hear more.
How do we select batteries?
- Voltage required for devices on the robot and the robot itself.
- Current drawn by devices and the robot. It’s easier to calculate for devices that are always on. The difficult is in estimating the current drawn by the motors on the robot. I would usually estimate the duty cycle of the motors in use and give the system some buffer.
- Determine how long you need your robot to run, and calculate the capacity required. Batteries capacity is defined in Ampere-hour (Ah). Simply put is how many Ampere the battery can provide in an hour. So if your draw current is around 2A and you have a 4Ah batteries, you will be able to run the robot for 2 hours.
- Once you have the capacity requirement, you can then select a battery you want base on volume, weight and of course price of the battery.