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Issue #228 July 2009
LiOn King
A Look at “Battery-in-a-Chip” Technology
by Tom Cantrell
Start | Energy In A Chip | Charge It | UPS-cLite | Dust Storm | Tips & Tricks | Harvest Time | Sources & PDF
DUST STORM
When comparing Cymbet’s EnerChips to existing solutions such as coin cells and SuperCaps, it’s all too easy to fall into the “us vs. them” trap. There are no doubt applications where an EnerChip is the clear-cut winner and should replace the earlier devices. But “us vs. them” overlooks the fact that there are a lot of situations where “us and them” can work well together. Let’s take a look at some EnerChip-based gadgets and you’ll see what I mean.
Photo 2 shows what might seem an unlikely pairing of a coin cell battery and an EnerChip, but it’s actually a combination that makes a lot of sense. The application would generally draw from the coin cell, calling on the EnerChip to “bridge” the power-gap when it’s time to replace the coin-cell. The EnerChip would allow “in-flight refueling” (i.e., application continues to run) and preserve critical data across battery swaps. For instance, keeping a real-time clock alive with an EnerChip would put an end to the embedded equivalent of the flashing “12:12:12” problem (i.e., devices that lose their minds and need to be re-initialized when you change the battery).
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| Photo 2—This design shows how EnerChips and coin cell batteries can work together.[4] The EnerChip keeps the system alive when the coin cell needs to be replaced. |
Look no further than Cymbet’s “Solar Energy Harvesting” demo kit (CBC-EVAL-08) to see how EnerChips and capacitors can be best buddies too. The kit utilizes a three-tier hierarchy of power generation starting with a solar panel that picks up what energy it can, when it can, from ambient light. The solar panel output feeds a boost converter that steps up the voltage to a useful level (3.5 V). When solar energy is sufficient, it drives the load and charges a pair of CBC050 batteries. If the light fades, the EnerChips take over supplying the load.
So far, so good. The only gotcha being said load had better be pretty small. `Whether powered by the solar panel in bright sun, or running off the EnerChips, we’re talking about only tens to hundreds of microamps on tap.
Here’s where our little friend the capacitor comes in. Capacitors may be leaky, but they’re also more than willing to give it all they’ve got in a big bang (i.e., high discharge current). That brings us to the third tier in the power-generation pyramid: a 1,000-µF capacitor. Although hardly a “SuperCap” (real ones are measured in Farads), it can nevertheless deliver a whopping 30-mA discharge for 20 ms, fully 50 times the 600-µA surge the pair of EnerChip batteries can provide. Of course, there’s no free lunch. The battery resistance and the capacitor form an RC network that takes a few seconds to recharge.
Is that enough energy to do anything useful? Texas Instruments says so, and to prove it, they’ve come up with the eZ430-RF2500 Solar Energy Harvesting kit (see Photo 3) that uses the Cymbet Solar Energy Harvester to power a wireless sensor solution based on their MSP430 flash memory MCU and CC2500 802.15.4 radio chips.
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| Photo 3—The Texas Instruments eZ430-RF2500 Solar Energy Harvesting kit puts Cymbet EnerChips to work with TI silicon in a “zero-power” wireless sensor application. |
The kit comes with the Solar Energy Harvester, a USB adapter that connects to your PC, and a plug-in MCU-plus-radio module for each. The software comprises a simple temperature-sensing application with TI’s home-grown SimpliciTI network stack running on the nodes, and a PC-monitoring program that displays the network in action (see Photo 4).
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| Photo 4—The PC software that comes with the TI kit shows the network in action. Note how the node counts down the number of packets it will be able to send on EnerChip power alone (i.e., in the dark). |
Yes, the application is trivial, but the design implications aren’t. This stuff really works! The solar panel was able to power the load and keep the EnerChip charged in moderate lighting conditions, even indoors. But when there wasn’t enough light, the EnerChip seamlessly kicked in, able to keep the node on the air for up to 400 additional packets on battery power alone.
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