Some time ago, Max Maxfield roped me into his ongoing robot project. I must say that Max is one busy techno-geek -- he has got more things going on at once than you can shake a stick at. (I bet he's probably done that stick-shaking thing somewhere along the line, as well.)
Be that as it may, this led to my writing this series of articles 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 with regard to selecting the most appropriate battery technology for your application, along with tidbits of trivia and nuggets of knowledge, as Max would say. In this article, we consider lithium sulfur dioxide (LiSO2) batteries, but first...
Tip No. 9: What size are you?
When designing a product, from time to time we all struggle with the question: "What will fit inside?" Who gets to decide how a number is assigned to a given shape/size and what it is called? Here are two of the organizations I use:
Just in case you were wondering, the IEC and ANSI standards do not always agree, but they are mostly the same, and things have been improving over time. Everything we need to know about the characteristics of batteries for fitting and interchangeability can be purchased at the above websites.
I can't include everything here, since the list is so large, but for an example of what can be found, consider the ubiquitous AAA battery, with a diameter of 10.5 mm (0.41 inch) and a length of 44.5 mm (1.75 inch).
The LiSO2 battery
Constructed with sulfur dioxide on carbon bonded with Teflon, this battery technology is known for its wide operating temperature range, flat voltage-discharge curve, and high energy density. However, these benefits come with disadvantages. Because the SO2 is at high pressure, which requires a safety vent, there is an explosion hazard, and this is a high-cost battery. In addition, the forms of this battery that contain acetonitrile can produce small amounts of very poisonous hydrogen cyanide when subjected to extreme temperatures.
Another consideration: Especially after long storage and/or being subjected to low temperatures, the terminal voltage may dip suddenly at the first application of a load and then recover. The effect is similar to lithium-thionyl-chloride chemistries. (See All About Batteries, Part 7: Lithium Thionyl Chloride.)
- Specific energy: approximately 250 Wh/kg
- Energy density: approx. 400 Wh/L
- Specific power: 15 W/kg (light loads)
- Discharge efficiency: approx. 50-72%
- Energy/consumer-price: 0.12 Wh/dollar
- Service or shelf life: 10 years
- Self-discharge: 0.25%/month
- Cycle durability: primary battery/NA
- Nominal cell voltage: 2.85 V (3.9 V with bromine monochloride added)
- Open-circuit voltage: 3.0 V (varies by manufacturer)
- Cut-off voltage: 2.0 V (varies by manufacturer and use)
- Temperature: -55 to +70°C (selected versions to -60°C)
Anode: Li → Li+ + e-
Cathode: 2SO2 + e → S2O4
Overall: 2Li + 2SO2 → Li2S2O4
Though the military has typically been one of the larger users of this battery type, I think one of the more interesting applications is inside a spacecraft named Huygens that landed on a moon of Saturn called Titan.
The representative illustrations below are for the Saft G 06/2 (0.95AH) battery.
Voltage at mid-discharge versus current and temperature (2.0V cutoff).
Typical discharge profiles at +20°C.
Capacity versus current and temperature (continuous
discharges, 2.0V cutoff).
In my next column, we'll look at some more tips and tricks, and we will consider another battery technology. In the meantime, as always, I welcome any questions or comments.