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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-Lite | Dust Storm | Tips & Tricks | Harvest Time | Sources & PDF
ENERGY IN A CHIP
Battery technology will play a pivotal role in the Green revolution. For instance, if the question is about the widespread move to electric vehicles, the answer is battery technology that can deliver the range and performance fossil-fuelers are used to.
At the other extreme, Cymbet is taking a “small is beautiful” tack with their “EnerChip” thin-film rechargeable lithium battery technology. In short, as the name implies, it’s a battery-in-a-chip. Combined with the latest in nanopower silicon, the EnerChip offers an intriguing option for designers to consider.
Just keep in mind we’re not talking about a lot of energy here. So far, the Cymbet batteries top out at 50-µAh capacity (CBC050) with a lesser 12-µAh model (CBC012) also offered, although this is on the order of a thousand times less than the familiar lithium “coin cell.” At least the output is a healthy 3.8 V. That’s suitable for use in typical (e.g., 3.3-V) designs. Of course, as with any battery technology, you can always lash them together to increase voltage, current, and capacity.
It’s a fact of life for all batteries that specs like “50 µAH” and “3.8 V” are overly simplistic and tend to obscure the fact that a myriad of other application factors come into play. The Cymbet batteries are no exception. Let’s take a closer look to get a better understanding of how they can serve existing designs or, better yet, enable exciting new ones.
Although on a tinier scale, the Cymbet batteries exhibit the same desirable performance characteristics that have made their larger lithium coin cell cousins so popular. For instance, the voltage discharge curve is as flat as a board, which guarantees virtually full output until the bitter end (see Figure 1). If there’s a downside, this means you can’t really expect to use the voltage output level as a foolproof indicator of remaining battery life. Power management will have to be smarter than that, which is all the more reason to better understand the specs.
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| Figure 1—One advantage for lithium batteries, including EnerChips, is a flat discharge curve. Just watch out as you approach the “cliff” since deep discharge isn’t good for the battery. |
With a throwaway coin cell, using it until it’s dead (and then replaced) is standard procedure. By contrast, with the Cymbet rechargeable, you need to be careful not to run it off the cliff shown in Figure 1. There’s no simpler way to say it than the CBC050 datasheet does: “Failure to cutoff the discharge voltage at 3.0 V will result in battery performance degradation.”
Another benefit of lithium cells is the ability to deliver surprisingly high surge currents—in the case of the CBC050, up to 300 µA. But watch out, because the capacity declines almost linearly with the load as shown in Figure 2. If you think a “50-µAh” lithium cell should be able to run a 300-µA load for 10 minutes (i.e., 50/300, or 1/6 of an hour), you’ve got quite a surprise coming. Extrapolate the line in Figure 1 to the right and you’ll see what I mean. Using a car analogy, drive with a heavy foot on the throttle and your mileage will suffer, so a full tank of gas won’t take you as far as if you drive more sedately.
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| Figure 2—Although capable of relatively high discharge (e.g., 300 µA for 20 ms), note that effective capacity decreases with increasing loads. |
In the old days, NiCad batteries were the all the rage for rechargeable applications. Do you remember the infamous “memory effect”? That referred to the propensity for NiCad batteries to “remember” repeated partial discharge levels and get stuck at a less-than-rated capacity. To counter, users in the know would be careful to “deep discharge” their NiCads to wipe the “memory” clean.
The Cymbet rechargeable lithium technology is somewhat the opposite. It’s more like a car (i.e., lead acid) battery in that deep discharge is something you want to avoid because it reduces the number of potential recharge cycles. Ponder the specs and you’ll see the effect is by no means trivial. For instance, keep the CBC050 “topped off” by limiting discharge to 10% and you can expect to get a full 5,000 discharge/recharge cycles out of it. But if you routinely run it down to half full (i.e., 50% discharge), that spec drops by a factor of five to 1,000 cycles.
Temperature also plays a role. The aforementioned specs are for 25°C operation. Boost the temperature to 40° and cycle counts are cut in half (i.e., 2,500 and 500 cycles at 10% and 50% discharge, respectively). Also take note of the operating temperature range of –20° to 70°C, a possibly limiting spec in “harsh-environment” applications.
Self-discharge can be a problem for lesser battery technologies. For instance, I have an older digital camera with rechargeable NiMH batteries that I might use once a month to take a picture for this column. Despite being nearly fully charged when I last used it, it is invariably drained when I fire it up a month later.
By contrast, the CBC050 is quite happy to sit idly by for many months or even years. The self-discharge spec comprises two parts. The first is “recoverable” self-discharge. Yes, the capacity will decline over time, but the next recharge will make everything right again with no permanent damage. There’s also a “non-recoverable” self-discharge, or “aging,” that’s more serious in that it represents a permanent loss of capacity.
That all sounds scary until you look at the CBC050’s specs. The “recoverable” self-discharge rate is 8% and the “non-recoverable” rate is 2.5%. But that’s per year! At a total of 10.5% per year, that means the CBC050 could come to life after almost 10 years in storage, and it would still be quite serviceable (i.e., able to charge back up to 75% of the original rated capacity).
Put all the specs together and you start to get a realistic picture of what a CBC050 can deliver in a particular application. Consider these different application scenarios.
The first has the CBC050 fulfilling the typical role of “battery-backup” for a low-power CMOS chip (i.e., MCU, SRAM, RTC). It’s quite well-suited to the task, but faces notable competition from an unlikely source: not another battery, but the so-called “SuperCap” high-value capacitors. But put on your “Green” eyeshade to look closely at a SuperCap datasheet and notice the very high self-discharge spec. In essence, SuperCaps need to be kept on the charger at all times in order to be ready for a call to action. It also means there’s energy wasted keeping them topped-up. The difference may not seem like a big deal, but from a holistic “energy consciousness” point of view, the potential self-discharge energy advantage is significant. According to Cymbet, up to one-third the energy spent charging a SuperCap (e.g., 0.2 F) is lost to self-discharge versus a tiny fraction of a percent for an EnerChip.[1]
Safety is something easy to overlook, at least until it jumps up and bites you. Everyone has seen the headlines about exploding laptops and such. Fortunately, Cymbet says that even a dead-short won’t lead to unwanted pyrotechnics.[2]
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