Know your battery

When you're the chief technology officer of a device-networking company, you spend time talking to customers about their respective connectivity challenges and technology options. Over the past few years, a shift has occurred as a result of two things: the expanded use of wireless technologies and the resulting desire to connect things that have never really been connected before.

The blessing of the good old wired connectivity days was that you generally always had a way to get power as well. Creative minds have come up with ways of sharing wired media be it the legacy 4-20mA current-loop system or the more modern power-over-Ethernet standard.

Cutting the data line, or making it wireless, means that we need to cut the power line as well. It means that we are left with harvesting power from our environment or accessing power stored in batteries. The main questions I get asked when speaking with customers about wireless sensor networking are "which wireless technology should I use?" and "what kind of range will I get?". These invariably come up, but the number one question is "how long will my batteries last?".

The bad news is that the initial answer is "well, it depends". But it's possible to get a more accurate answer if you start to look at how the system interacts with its power source.

Just like a tank full of water, the energy in a battery is greatest when the tank is full and warm. So, also, is the voltage. As the battery empties, the voltage tends to decrease. The reality of battery usage is that it never really empties: the voltage drops until the cell becomes unusable. From a physics perspective, the output voltage decreases because the internal resistance inside the battery increases. Typically, for a 1.5V AA-size cell, we consider the battery empty when the voltage drops to around 0.9V.

Playing by the rules

It is time to look at two fundamental battery rules.

The first rule relates to temperature. Cold batteries store electricity well but, unfortunately, they don't supply power well. Some technologies do better than others but, in general, it is sensible to store most batteries in cold places - just make sure to warm them up before use.

The second rule, which has a bigger impact on design, relates to the non-linearity of batteries. This is where our storage tank metaphor breaks down. The capacity of a battery is related to the current it is being asked to provide, and this is not at all linear. High current draw makes battery capacity smaller. For example, a battery might have a capacity of 1,000mAh for current draw of 5mA. However, the same battery might have capacity of 500mAh for current draw of 200mA.

This is very important to understand because most people look at the capacity on a battery spec sheet and assume that the calculation will be linear across current draws and temperature. This is a very dangerous practice because it only works when the change in temperature and current is very small. Not only that, different battery types have their own characteristics.

Taking usage and battery type into account, let's take a look at calculating the battery life for a simple example. In a setup where a temperature sensor is connected to a Zigbee-based network that relays data to a display, the goal is to read the temperature out on the display every minute. Let's assume that each read transfers 128B of data and that the time to transfer the data will be about 200ms.

In order to preserve battery life, the radio must sleep until needed for transmitting and receiving data. For the sake of discussion, we'll assume that the data transmit and receive time is 200ms, and wake-up transmit and receive time is 50ms. The sleep time is whatever is left.

As the device has to wake up and transmit 1,440 times in one day, its total waking time is 360 seconds. Next we need to get the device current consumption. For the sake of simplicity, we assume that the sleep current, for the 23.9 hours the device is snoozing, is 11µA, while the wake current is 14mA. Using these numbers, we can calculate our daily energy consumption as 1.66mAh. The average current will be 69µA.

Now we get to the tricky part. For an alkaline battery, we use the AA alkaline model for an E91 battery. Using these values and plugging into the current consumption and daily current consumption, we get a capacity of 2217mAh at 0°C and 2839mAh at 20°C. Dividing that into the daily demand, we end up with lifetimes of 1,335 and 1,710 days, respectively.

Doing the same for the lithium battery, the model gives lifetimes of 1,807 and 1,946 days respectively, suggesting that the coin cell is a better option.

The key thing is to understand your objectives before choosing a battery and to take into account current draw and temperature. Remember, you only get to lose capacity once - it doesn't come back.

Joel Young is CTO of Digi International