Some time ago, Max Maxfield roped me into his ongoing robot project. 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 on selecting the most appropriate battery technology for your application, along with tidbits of trivia and nuggets of knowledge, as Max would say.
Gathering all of this information has turned into a pretty large project that is consuming a lot of time. I'm not sure how I let Max talk me into this. Perhaps it's the fact that he keeps putting up with my bad jokes. For example, here's a real-world exchange from just the other day, when I stuck my head into Max's office:
What's the difference between a rabbit?
Between a rabbit and what?
It's not the difference
between them that counts,
it's how they multiply
. You see, it's a math
What's the difference between a frog
I don't know.
I don't know either, but whatever it is, I'd better hop
Tip No. 7: Designing enclosures
When designing an enclosure for batteries, do not overlook the following:
- The effects of wire inductance and resistance
- The effects of contact resistance and corrosion over time
- Mechanical protection from vibration, punctures, etc.
- Minimum spacing requirements for safety (IEC 60950 and others), creepage, and clearance
- Thermal management
- Ease of replacement/serviceability
- For shipping, protection from shorts or accidental device activation (providing disconnection for example)
- EMI (radio emissions) and susceptibility (withstands high-power RF). This is especially true for smart batteries
- Environmental concerns, such as outgassing, leaking, air breathing, radiation, explosion, pressure, etc.
- Design for manufacturing and avoidance of high currents when connecting the battery for the first time
The lithium thionyl chloride (Li-SOCl2) battery
This specific type may have a liquid cathode, suitable for temperatures as low as -55°C, which puts it firmly in the category suitable for low-temperature performance. Unlike other battery technologies using liquids that produce a gas by-product, this technology is very good, with limited emissions even under abusive conditions. Unfortunately, the electrolyte is toxic and reacts with water.
The battery has a high specific energy and low weight, but makes sacrifices for a very high internal resistance, and therefore has a low-rate-only discharge with limited short-circuit current. An additional concern is that, following long-term storage, the battery exhibits a delay in producing a good terminal voltage when finally put into service. At least one company has compensated for some of these shortcomings by including a capacitor inside the package. (See TadiranBat.com.)
Very low-current versions of this technology find use in battery backup for memories and remote monitoring-metering, while higher-current versions are used in some military and automotive applications. Safety concerns (such as explosion and its Class 9 Hazmat shipping classification) and high cost have prevented more widespread use of this technology. Click here for information on handling and safety.
- Specific energy: approximately 500 Wh/kg
- Energy density: approximately 1200 Wh/L
- Specific power: approximately 18 W/kg
- Discharge efficiency: 6 to 94 % (highly dependent on load)
- Energy/consumer-price: 5.1 Wh/dollar
- Self-discharge rate: 0.08 %/month
- Cycle durability: (primary battery)
- Nominal cell voltage: 3.5V typical (3.65V new, open circuit)
- Cut-off voltage: 3 volts per cell, loaded
- Temperature range: -55 to +85°C typical (some specialty types up to +130°C)
4Li + 2SOCl2 → 4LiCl + S + SO2
The following images illustrate some interesting characteristics associated with lithium thionyl chloride batteries:
Figure 1. Half AA size, Blue curve=180K Ohms for four years. Grey curve=180 Ohms for 30 hours. Zoom circles are closeup of voltage recovery whenever the discharge is interrupted.
Figure 2. Voltage delay effect, terminal voltage verses time.
Figure 3. Dependence of capacity on current -- self-discharge increases with operation life.
Figure 4. Terminal voltage and internal resistance with depth of discharge for two load types.
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.