Battery technologies for wireless sensor networking

Joel Young
12 Nov 2008
00:00

When you're the CTO of a device networking solution company you spend a lot of time speaking to customers about their respective connectivity challenges and different 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 subsequent desire to connect equipment that has never really been connected before. The blessing of the old wired connectivity days was that you generally always had a way to get power. Sure there might not have been a handy receptacle, but creative minds have come up with ways of sharing wired media such as the legacy 4-20 mA current loop or the more modern power over Ethernet with 802.3af.

Alas, switching to a wireless data line means that we need to cut the power line as well. This means that we are left with harvesting power from our environment and/or accessing power stored in batteries. As it turns out, the number one question I get asked when speaking with customers about wireless sensor networking is not 'Which wireless technology should I use‾' or 'What kind of range will I get‾' (although these questions invariably come up), it's 'How long will my batteries last‾'

The initial answer - 'Well, it depends' - is accurate but not comprehensive enough. So this article is intended to start users down the path toward a better answer. It will cover battery vocabulary and two simple but fundamental battery rules. It will also break down different battery technologies and provide a formula for calculating battery life. For my examples, I'll be referring to AA-sized batteries. While almost any size will do, AA batteries are the most common battery size and, because they are readily available in most battery chemistries, they are easy to compare.

The fundamentals

A battery acts like a storage tank. Think of current as the flow out of the tank and voltage as the energy or force behind the flowing liquid. Just like a tank full of water, the energy is greatest when the tank is full and the tank is warm. In batteries, we describe the potential in volts and this voltage tends to decrease as the battery empties.

Overall battery capacity is measured in milli-amp-hours (mAh) or amp-hours (Ah). A rating of 3 Ah means the battery can supply 3 A for 1 hour or 1 A for 3 hours. Sometimes the tank is not quite empty, but there is insufficient potential energy in the battery to get the remaining charge out. We tend to say that a battery is empty when the potential or voltage drops below a certain level. Typically, for a 1.5 V cell such as an AA-sized cell, we consider the battery empty when the voltage drops to around 0.9 V.

Now that we are armed with some terminology, it is time to look at two fundamental battery rules.

Rule 1: This first rule relates to temperature. Cold batteries store electricity well, but cold batteries do not supply power well, although some technologies do better than others. With that in mind, it is a good idea to store most batteries in cold places - just make sure to warm them up before use. If you don't warm them up before use, you don't get any of the benefits of cold storage. To extend the storage tank metaphor, it requires more effort to get a cold liquid to flow through a pipe and the same holds true of batteries. Once you use up that energy, you don't get it back.

Rule 2: The second rule relates to the nonlinearity of batteries.

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