I have all but a few parts now for my air quality monitoring project so I decided to start working out I’m going to power it. Since I plan for this system to be portable I must choose a battery to power the system, but first I need to work out how much power is required during constant use. For a battery with a given voltage the amount of power is given by the number of milliamp hours (mAh) the battery has. Essentially it’s a value that you can use to determine the amount of time the battery will provide power at a given milliamp load.
For example if i had a battery with a 1000mAh rating it could draw 1000mA for an hour or 500mA for 2 hours or 10 mA for 10 hours etc…
If you know the amount of current you want to draw and the amount of time you want to power your device for you can work out the mAh ration you require. This is what I plan to do. There are other factors to choosing a battery (specific energy, cell voltage, c rating etc..) however I’m not going to get into them right not.
So here are my loads:
- Temperature/Humidity Sensor: 0.12mA
- LCD Display: 20mA
- GPS: 25mA
- General Air Quality Sensor: 120mA
- Dust Sensor: 70mA
- Carbon Dioxide Sensor: (has not arrived)
- Ozone Sensor: (has not arrived)
Adding up all the loads together and taking a guess (120mA each) for the two sensors that haven’t yet arrives it looks like I’m going to need about 500mA or 0.5A to run the air quality monitor. One of the specifications it to be able to have operation of the device for around 8 hours therefor it looks like I’ll need a battery with a capacity of 4000mAh or more. There are other factors to choosing a battery (specific energy, cell voltage, c rating etc..) which I’ll need to consider before choosing a battery but for a initial look i’m thinking a battery design to recharge tablets may work quite well, this option would also mean I wouldn’t have to design any charging and discharging control gear. I’ll probably hold off on buying the battery for a while though until all the parts have come in and I have a prototype running, until them I can power everything from my benchtop power supply.