Batteries eventually die, whether due to excessive recharge cycles, deep-discharge, or other factors. Capacitors, in contrast, often don’t hold charge long enough. What’s an engineer to do?
When the Pentax Q compact digital camera that I told you about last month:
showed up at my front door, I excitedly opened the packaging, tossed the battery on the charger to rejuvenate it, then slotted the battery inside the camera, bayonet-mounted a lens to the body, pressed the power button to turn the Q on, and…was immediately prompted to enter the current time and date, along with the desired format for the latter:
Not a huge surprise, at least at first. The Pentax Q is nearly a decade-and-a-half old at this point, after all. I figured that the camera had been sitting around without a primary battery in it—or maybe that primary battery had just drained—with draining of the backup battery (commonly referred to as the CMOS battery in functionally-equivalent PC settings storage terminology) following in short order.
So, after thoroughly testing the camera to make sure it was otherwise operating properly, I popped the primary battery back out to top it off again, then slotted it back in the camera body and let everything sit overnight to recharge the backup battery.
Drained brains
The next day, I turned the Pentax Q back on and…once again was immediately prompted to enter the current time and date, along with the desired format for the latter. What the heck? I hit Google and learned that mine wasn’t remotely a unique issue. Unsurprisingly, in retrospect, the embedded battery had exceeded its maximum recharge cycle count and/or had experienced deep discharge degradation from which it was unable to recover. CR2032 cells on PCBs are admittedly prone to suffer similar fates:
with one key difference; they’re much easier to access and replace. I trust you’ll resonate with my reluctance to disassemble my photographic antique and attempt similar surgery on it. Yes?
At the end of the day, of course, this is a First World problem, the latest in an admitted long list that I’ve shared with all of you over the years. Time, date, location and directly related settings are the only ones that don’t survive primary-cell separations and drains; all the others (including the all-important user interface language setting, critical for someone who’s fond of Asian-sourced electronics but can’t visually-or-otherwise understand any Asian dialects) are generally stored in nonvolatile memory instead of battery-backed SRAM (because, speaking of cycles, these other settings change comparatively infrequently and are therefore unlikely to hit the max-rewrite cycle count of the EEPROM, flash memory or other technology housing them).
Imperfect workarounds and alternatives
If I could just remember to plug my primary battery-housing camera in to recharge every once in a while, I might also dodge the flaky-backup-battery issue that way…except that the Pentax Q can’t be recharged over its proprietary USB-derived connector. And anyway, I avoid recharging primary batteries in situ whenever possible, in case the cells were to swell and permanently embed themselves in the battery compartment. And speaking of ejecting batteries, I’ve found at least two other hacks:
- Have a spare fully-charged battery sitting nearby, and pop out and replace the drained battery with it really quickly (the backup battery’s charge storage capability apparently isn’t completely neutered, only severely compromised)
- Or just hit “cancel” after seeing the initial-settings screen to skip past it…with the obvious downside that subsequently logged date and time info will be (quite) incorrect!
What about so-called “supercapacitors” (aka, ultracapacitors) as an implementation alternative to conventional backup batteries?
The obvious key advantage here is that they support near-infinite recharge cycle counts. They also have comparatively high output power density (translation: high output current, although this attribute isn’t necessary in the application we’re discussing today) and there’s also no worry about the overheating-induced thermal runaway (translation: heat, smoke, flame) to which batteries are prone to varying degrees depending on implementation-technology specifics.
Nothing’s perfect in real life
Alas, there are also downsides. Although you can, to my previous high output power density comment, drain ‘em really fast, they also drain comparatively fast all by themselves (in weeks, versus months or even years for batteries), even in the absence of a load…which kind of defeats the purpose of using them for long-shelf-life settings-backup purposes, yes? Plus, as the above image suggests, they tend to have comparatively poor storage density. Translation: they’re huge in both linear size and volume in comparison to a comparable battery alternative.
Comparative size isn’t so much of a problem with an available volume-rich application such as a desktop computer or server. In a camera, or even a laptop computer, or any other diminutive device for that matter, it’s more likely to make a supercapacitor a non-starter. Alas, as I alluded to earlier, those same compact form factors are also more likely to be difficult-to-impossible to disassemble in order to do a backup battery swap, so…
In closing, I freely admit that I’m not a power electronics expert. That’d be my predecessor. So, I’ll stop pontificating at this point and pass the keyboard over to you.
What characteristics make a backup battery the obvious choice for a design, and conversely lead you to definitively select a supercapacitor instead? How do you decide between the two when the differentiation is more muted? How have any recent implementation innovations in either/both product categories evolved your thinking in this regard? And are there other technologies besides these two that your readers and I should also consider?
Let me know your thoughts in the comments, please!
—Brian Dipert is the associate editor, as well as a contributing editor, at EDN.
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