Friday, 27 May 2016

Choosing a battery for your robot

One of most difficult things that you may face while building a robot is not just the circuit or the program or the mechanics but the power-source. You would want to invest on a reliable power-source that you can use for a long time in a number of projects. I personally dealt with this difficulty a lot of times and in some cases I had to stop the entire project midway due to the lack of a good power-source.

Always look out for these 2 things while selecting a battery:-
1. The capacity (Usually given in mAh) - If a battery as 2200 mAh capacity it means the battery has the ability to deliver 2200 mA (2.2 A) for an hour. (This figure is not always accurate and is mostly observed only under laboratory conditions.)
2. Voltage - Voltage determines how well the battery can push the robot (symbolically and not to be taken seriously). Higher voltage means the battery will be able to deliver the current more easily against resistance.
Alkaline batteries


Alkaline batteries are the most abundant and easy to find. They are cheap as well which makes them ideal for simple robots and for testing, though for a long period of time these may end up costing a lot
due to the constant need for replacements.


NiMh battery
MiMh batteries are what I prefer as they are fairly cheap compared to LiPo but still provide the ability to recharge along with a high capacity. The only drawback is that they discharge even when not in use thus they cannot hold the charge for long. 

LiPo battery


LiPo batteries are the best if you do not consider their price. They are light weight, have high energy density and can be recharged hundreds of times. These are good for almost all types of robots. These batteries are usually found in mobile phones and laptops.


Li-ion battery
These batteries have a light weight while maintaining the high energy density. These are suitable for robots that have medium to low energy demands. These are comparatively cheaper than LiPo and provide great alternative. 

Lead Acid

Lead acid batteries are unbeatable when it comes to delivering high power at low cost.  These batteries are generally used in robots that have a high energy demand like combat robots. Lead acid batteries are used in vehicles.

There are other types of batteries like NiCd but try to avoid them as NiCd type batteries have something called memory effect i.e. they lose some of their capacity if you charge them before they drain completely.

All robots are not built equally. Some robots are built for combat and high power work like sumo robots while some have lots of sensors and processing equipments while others are simple like line or light followers and their energy requirements vary accordingly. Let's look at an example:-

Let us assume that you have built a robot which has 2 motors, a microcontroller (say an Arduino) and some sensors. The motors require atleast 6V each for efficient performance and draw a current of 200 mA under load. The microcontroller and the sensors together draw about 200 mA (just an assumption). The total current requirement for the entire robot is 200 mA + 200 mA = 400 mA. Now you chose to power the robot using four 1.5V NiMh batteries each having a capacity of about 2000 mAh. Based of the current requirements, your robot will last for about 5 hours (2000 mAh/400 mA)