Thursday, March 29, 2012

Lithium Power Primer

We hear a lot more about Lithium batteries in the press lately. The articles often only scratch the surface without getting into any particulars.
Since the battery technology is what I have been working with for the past couple of years, I would like to share with you a few more details about the larger Lithium batteries or rather battery packs used in transportation, renewable energy storage, backup power and many other applications. This is intended for more technical audience curious about what such system is made out of or how to talk about it.
Of course, as this is subject relevant, I have to mention my perimeter security robotics exploits that can be labeled as a form of weird transportation for sensors and computing – see my successes page on lubomorhac.com. These unmanned ground vehicles are indeed using Lithium power and electric propulsion.

OK, back to battery packs. There are many different types of Lithium Ion chemistries, each offering slightly different properties, making the batteries suitable for one or other application. When you discuss Lithium batteries you want to ask the right questions, often related to key specifications listed here in no particular order:
  • Cell type (prismatic, pouch, cylindrical)
  • Cost
  • Cycle life (n cycles at n% DOD)
  • Individual cell Voltages
  • Power density (Wh/Kg)
  • Charge and discharge currents
  • Operating temperature ranges
  • Heat dissipation requirements at recommended and max. charge / discharge currents
  • Safety features
There are many other specs and best practices you need to factor in when you design power systems using Lithium cells, but this is out of scope of this blog.At least for now.

A large Lithium packs (array of Lithium cells) are not complete without the essential component called BMS – Battery Management System. Here are some functions of a good BMS:
  • Monitors packs temperature and each cell individually for various parameters
  • Protects battery pack from over charging, over discharging and overheating
  • Evaluates and computes State of Charge (your energy gauge), State of Health and other important operational and safety parameters
  • Balances cells to maintain the pack’s maximum capacity and health
  • Monitoring parameters locally via gauge, computer or remotely over the Internet

The following weekend testing picture gives you an idea what a battery pack with all the goods attached looks like in action. The netbook computer is connected to the BMS for configuration and pack / cell monitoring. The small, black box with wires attached to the front of the pack is the BMS. The battery pack and the plywood test jig featured here is the most expensive electric heater ever! You can see the red glow coming from the electric heater elements - a black box with the duct attached.
The test jig is really used for charge / discharge testing, pack protection and current sensing. 
Under normal circumstances the test jig components are in much more elegant configuration, usually enclosed in a metal box or in the belly of some vehicle. There are many different ways to package the components inside the battery box or keep it externally.

For more benefits of Lithium power in large battery packs visit our X4i-LB product page, which is not intended as a shameless self promotion, but to give you more broad idea about the use of lithium power and show you more design pictures.

Stay tuned for more about the Lithium power...

Lithium Ion battery density - just how far along is the technology?

A while ago I was a bit baffled by a barrage of media releases pertaining to one of the developers of Lithium-ion battery materials touting its breakthrough in the energy density.
While I applaud  Envia Systems for reaching a very significant milestone, I also got thinking about the practicality of the new cell technology due to lack of other supporting data. I love the 400Wh/kg energy density, but in this particular case the claim that got me going was related to a long cycle life and unknown thermal charge / discharge performance. The data posted on the company web site indicates the new technology barely reaches the cycle life of a lead acid battery (not so great). This means to me that more work is to be done in the lab and more so getting it into a production.
I would like more context here: Just how far is the product from a commercial readiness standpoint? I had no response from the company to my inquiry.
I'm hopeful that Envia will soon elaborate as to when the technology will be ready to ship and how good it really is. I'm hopeful, so perhaps will see this technology in one of the future releases of Chevy Volt...