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The lowdown on batteries: Zinc/silver-oxide

Posted: 23 Jun 2014 ?? ?Print Version ?Bookmark and Share

Keywords:battery? IEC 60950? EMI? lithium thionyl chloride? voltage?

Here's another instalment in this series of articles on the various battery technologies available to us. 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 Maxfield would say.

Tip No. 8: Calibrating the battery
What the heck does that mean, anyway? Sometimes, a so-called smart battery isn't as smart as it needs to be with regard to telling how much juice remains on a given run over its service life, even due to simple factors such as usage history, heat, and overall age. After a while, the run-time or charge-remaining "meter" gets to be just plain wrong. Therefore, we (the users) have to send the thing back to school. Laptops and their batteries seem to be some of the worst offendersor could it be that it only seems that way because there are so many of them? How does one know if one needs to recalibrate? Well, we should consider recalibrating if:
???Immediately after a full charge, the "per cent remaining" display indicates 92% or less.
???It's been more than three months since the previous calibration.
???You realise that the meter is wrong (it loses a huge percentage in only a second and then settles, for example).
???Some software application pops up and tells you to consider recalibrating.
???The system dies suddenly, even though the meter shows plenty of run-time remaining.

What does a typical recalibration involve? First, check your system's manual to see if recalibrating is required and how to do it. Some systems are very good and never need it. However, even if there is no mention of it, if there are symptoms to correct, then you should give it a go as follows:

1. Charge the system to full.
2. Let the system rest for two hours (to cool down). If the system is a computer that normally gets hot while running, shut it down during this time, but leave it plugged in.
3. Open the power management settings, and set the system to sleep or hibernate automatically at 5%. It will be best for you to be around as this 5% approaches, so you can determine how quickly it reaches this limit. Make sure other settings don't prevent the system from getting there in a reasonable time (such as turning off the display or disks to conserve power). We want to force the system to get fully or nearly fully discharged.
4. If the system falls to 5% way too fast, power up again, and force the system to stay up with the settings on 0% until it actually dies.
5. Let the system sit unplugged for five hours or so.
6. Plug the system back in, and charge it to 100%.

If the above sequence solves the problem, or if this was a scheduled maintenance, that's great. If there is a problem that still needs solving, try the procedure again, except keep turning the system back on with the settings at 0% until it won't power up at all. Next, remove the battery for the five-hour rest period. Lastly, recharge to 100% when the system is cool. Sometimes, when performing this force-it-discharged step, the system will stay up quite a long time. The 5% detection was probably incorrect, and the system was shutting down too soon.

If none of these things work to correct the problem, consider replacing the battery, but keep it around, in case the problem ends up being in the charging circuit.

The zinc/silver-oxide (Zn/Ag2O) battery
The battery is known for its high energy density and flat discharge curve. It has found use as a backup power source in the space industry, as well as for the military in some speciality areas. In the consumer arena, it finds itself in hearing aids, watches, calculators, photoelectric exposure meters, instruments, and pagers. Unfortunately, because of the silver content, it is somewhat more expensive than other batteries in its category. Two electrolytes are common: KOH is used where pulse-loads are required, whereas NaOH is employed when longer life is desired.

???Specific energy: approximately 132 Wh/kg
???Energy density: approximately 505 Wh/L
???Specific power: high W/kg, but widely variable
???Discharge efficiency: 40-90% (highly dependent on load)
???Energy/consumer-price: 0.1 Wh/dollar
???Self-discharge rate: 0.7%/month (average)
???Cycle durability: (primary battery)
???Nominal cell voltage: 1.55 V typical (1.6 V no-load, fresh)
???Cut-Off voltage: 1.2 V
???Temperature range: 0 to +60C typical (storage to -40C)
???Zn + Ag2O ZnO + 2Ag (for KOH or NaOH electrolyte)

The following images illustrate some interesting characteristics associated with zinc/silver-oxide batteries.

Figure 1: Voltage verses time (Duracell D389/D390).

Figure 2: Discharge for the Energizer 389; continuous discharge for 548 hours to 1.2 V, 10K load, at 21C.

Figure 3: Energizer 389 and the effect of temperature on life.

Figure 4: Effect of temperature on shelf life.

In my next article, 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.

About the author
Ivan Cowie is the Chief Engineer at MaxVision.

To download the PDF version of this article, click here.

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