SCIENCE MADE EASY
We are a society always seeking. No, I’m not referring to our search for purpose, a career or love. I’m talking about the constant search for outlets to charge our electronic devices. We charge our phones, tablets, etc. hoping the battery lasts the same amount of time as the day of purchase, only to be sent off running for the nearest outlet to charge yet again much too soon.
So what happens? Why does battery life, or the amount of charge a battery can hold, decrease when a battery is basically a storage device? My plastic storage containers don’t hold less stuff over time, so why do my batteries seem to hold less charge? To understand the answer to this question, we first must understand the battery itself.
Just about every battery works in the same general way. Each battery has three parts: two metal electrodes – one is negatively charged (the anode or the side of a standard battery with a minus symbol) and the other positively charged (the cathode or the side of a standard battery with a plus symbol) – that are separated by an electrolyte (a solution containing charged particles called ions).
When the battery is supplying power, the ions interact with both electrodes causing a chemical reaction to occur. At the anode, metal atoms separate into positive and negative particles. The positive particles can travel through the electrolyte to the cathode, giving the cathode its positive charge. However, the negative particles, called electrons, are left stuck at the anode, giving the anode its negative charge.
The negative charges don’t like being squished in together. When they are given a path to get away from one another, say when the battery is connected in your cellphone, they take that path and head toward something positive – the cathode – where they rejoin the ions they left behind in the electrolyte. Along the way, the electrons do the work to light up your screen or post that photo to Instagram. Your battery is dead when there are no more electrons left at the anode.
In the case of rechargeable batteries, like your cellphone or tablet, plugging your device into an outlet reverses the process. The electrons separate from the ions in the cathode, with the electrons traveling in the reverse direction through the circuit and the ions traveling back through the electrolyte where they recombine together at the anode. With the electrons back at the anode, your battery is charged, and the whole process can begin again.
Does a battery die because the electrons get tired after so many times of flowing back and forth and some of them just don’t want to repeat the process?
Well not quite.
As researchers with the U.S. Department of Energy found out in 2014, the answer lies with the ions. As the ions travel back and forth through the electrolyte, some of them react with the electrode materials. These reactions decrease the total number of ions available to flow back and forth and form barriers which build up over time. The barriers slow the movement of the few remaining ions.
Fewer ions means fewer electrons can recombine when they reach an electrode, and therefore the unpaired electrons cannot migrate back in the other direction. Think of the electrons as playing musical chairs where the ions are the chairs. With each charge and discharge cycle, more ions are removed. If an electron doesn’t find a chair, then it’s out of the game.
If you still follow me, let’s take things up a level. Most rechargeable electronic device batteries are lithium ion batteries, meaning the ion that travels back and forth is a positively charged lithium ion. At teeny-tiny imperfections on the electrodes, these lithium ions react with the electrode materials to form salt crystals (not quite table salt) which are the barriers slowing the flow of ions. These crystals create new imperfections for more crystals to grow, so lithium ions are used up even faster and the remaining ions are slowed down even more.
What can you do about battery capacity loss?
The DOE researchers realized extreme heat (i.e., leaving your phone in your car during the summer or taking it into a hot yoga studio) and fully discharging and fully recharging tends to speed up this process. Many people believe a complete discharge is better for battery life, but this idea is based on previous battery technology which would “remember” the amount it was charged to and treat that “memory” as the full battery capacity.
Extreme cold is also not good for battery capacity, so don’t think putting your phone in the fridge will undo a few hours in a hot car. In other words, don’t wait until your battery is down at 10 percent to charge (a study reported by Battery University found the sweet spot to be charging to 85 percent and discharging to 25 percent) and don’t leave your electronics in extreme temperatures.
Are you curious about how the world around you works? Ask me, and we can learn together. Send me an email at firstname.lastname@example.org, and I could answer your question in next month’s column.