The lowdown on batteries: Zinc-Air

Here's another instalment of this series of articles that focuses on the various battery technologies available to us. Along the way, in addition to the nitty-gritty technology details, I'm including tips and tricks on selecting the most appropriate battery technology for your application, along with tidbits of trivia and nuggets of knowledge.

Gathering all this information has turned into a pretty large project that is taking some time. I applaud Max Maxfield for hanging in here with me, but I have to say that he does seem genuinely happy to get the information. Also, please pardon the long delay from my previous post in this series. I finally sold my house and moved across town. This has consumed all my waking hours for the past few weeks.

In this article, we consider zinc-air batteries, but first...

Tip No. 6: Building and using battery packs
When designing a battery pack, do everything possible to use exactly the same types of cells in the pack. If at all possible, acquire them from the same manufacturing lot. Also, consider if the pack will need protection circuits or environmental measures—over-current, over-temperature, under-voltage lockout, overcharging, mechanical abuse, pressure seal, moisture, etc.

Another overlooked design issue is the assembly procedure, including the state of charge of the cells as they are connected into the pack. This is particularly important for batteries connected in parallel. If two cells are connected in parallel, and one is discharged while the other is fully charged, the charged battery could damage an interconnect pin and dump high currents into the other cell, thereby shortening the service life of both cells. Finally, consider the state of charge and connection of the pack into the final product. Many devices have substantial decoupling capacitors across the connection to the pack, for example. In this case, a fully charged pack will produce a large current while the input capacitors charge up to the pack's voltage, and this must be taken into account.

It's important to note that getting any of these things wrong can lead to catastrophic, premature failure. I once saw a prototype pack comprising a stack of NiCd batteries in series, which was connected in parallel with a 9V battery. This situation was very bad all the way around, since there was a primary battery in parallel with secondary types of different internal resistances and capacities. Needless to say, the pack didn't work as intended. The NiCd batteries did all the work and performed nearly the same job with or without the 9V battery being connected. However, on one occasion as a pack was being assembled, the NiCd cells were well above the 9V cell's voltage and almost fried it. (The smoke it made might be considered frying.) This wasn't my design, I hasten to say, but it certainly made for a nice learning situation.

A little history
A porous platinum/carbon-air combination was found to work as a replacement for MnO2 in the Leclanché cell in 1878. George Heise and Erwin Schumacher of the National Carbon Company came out with products that began to take advantage of this exchange in 1932. Their cell used wax in the electrodes to keep down the flooding problem. Cells built this way are large and have a lower capacity (low discharge rate), but they can still be found in the railway industry.

In the 1970s, electrode improvements allowed the development of button cells for medical devices, where hearing aids are king. In 1996, the Slovenian innovator Miro Zoric created the first rechargeable zinc-air battery, which began their commercial life in vehicles in 1997.

Companies are now evaluating the battery for use in grid storage, since it has one renewable component (air), and renewable energy is a hot topic these days.

The zinc-air battery
This type of battery has a very high energy density and is very light compared to sealed types. Since oxygen is a reaction consumable, the battery isn't usually sealed, or it would run out of air and quit prematurely. The battery has a high internal resistance, so it is suited to low-rate discharge, except for large-vehicle packs, where forced air improves the overall performance.