Zen and the Art of Battery Maintenance: Part Two
(with apologies to Robert M. Pirsig)
Notice: In keeping with the new Americans With Disabilities (ADA) act, we are required to make the following statement: If you are a certified moron with an IQ lower than a sergeant's, this article will be made available in comic book form.
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This series of articles (looks like it will be at least 3 parts at this point), is discussing the various types of batteries available for the different types of electrical and electronic equipment we use in our profession. In part 1, (July/August 1993 issue), we learned that a "battery" is really a combination of individual "cells" (and I will use the terms more or less interchangeably even though they're not, because I do not want to sound like a semantic snob), and that there are two basic kinds of batteries: throwaway (or "primary"), and rechargeable (or "secondary"). We examined some of the pros and cons of inexpensive carbon-zinc, alkaline and lithium batteries, and when (and when not) to use each. We discussed the possibility of charging batteries intended to be thrown away. And much more -so you might want to have a copy of part one to refer back to as we explore further.
This time we will concentrate on how to figure battery life and compare different types of batteries. With each manufacturer picking a rating system of his own, it's hard to compare different brands and types of batteries across the board. But this information is necessary to answer the universal question, "How long will my battery last?"
Last issue we sort of ran out of space (plus I had to go to the bathroom...), so I didn't get to cover quite everything that I wanted to. One more comment about lithium batteries - there are some available on the general consumer market, and they aren't bad. But I wanted you to be aware that the consumer lithiums are not the best that technology has to offer. There is a considerably more potent battery (at least in the 9 volt version) that is available only to OEM's (Original Equipment Manufacturers). We have to jump through a lot of hoops to get the potent ones, such a signing a liability release and placing minimum opening and follow on orders. These OEM-only lithiums are not sold to the public, but are available from a certain few sources. Unless otherwise mentioned, the current ratings and other specs I mentioned in part one and here in part two refer to the consumer battery that anyone can buy. Lithium batteries can be dangerous in untrained or careless hands, which is why only a watered down version is sold to the public. But even the watered down ones still are excellent.
An interesting note - along with electronics, batteries also are getting smaller and smaller. In many cases, the limiting factor in how small a device can be made is the size of the battery that powers it. I happened to notice a laser pointer recently that was hardly any thicker than a soda straw. On opening it up, the batteries in it were AAAA (Yep 4 - count 'em - 4 A's). We all are familiar with the standard "penlite" AA battery, and probably most of us with the AAA size that is smaller than the AA. Now the AAAA comes along, and it is the smallest yet. The ones I noticed were made by Eveready. Technical literature says they have a capacity of « of that of the AAA cell, and 20% of that of the AA. Soon we may be seeing AAAAAAAAAA cells (and will refer to them, for sake of convenience, as A10 (A to the tenth) cells. Why weren't battery sizes reversed - to go higher in letters rather than lower, with decreasing size? I guess back in the early days they were thinking more power rather than smaller size. Of course, with all the battery types in use nowadays, we'd probably have ZZZ batteries... And now there are in between sizes appearing, such as C. And how many of you old buzzards remember the F cell? A prize to the first person who calls or writes with the answer - where would (where can) you find an F cell? Hint: an F cell is larger than a D cell.
You probably know that batteries originally were sized by assigning them letters, with lower letters indicating a smaller battery (actually, a cell). A "C" cell is smaller than a "D". Once we ran out of letters on the lower end, "AA" was used to indicate a cell smaller than an "A", and of course "AAA" is smaller than an "AA". I was very young at the time, but I suspect all this dates back to when battery operated radios and such had tubes in them, and several different batteries were needed to operate the different parts of the radio. a "B" battery provided the high voltage for the plates of the tubes, and we old timers still refer to the "B+" when discussing the primary power in a circuit, even if a solid state circuit with only a few volts applied. There also were separate batteries to power the filaments. The B batteries generally were high voltage, with 45 and 90 volt batteries not uncommon. In fact, an early telephone analyzer used, I believe, 500 volt batteries, and there were two of them in the equipment (and in series for 1000 volts...). I know there were (or maybe still are) 90 volt batteries around, because as a kid I used them to power neon light circuits and other dastardly things a teenager can do with a pocket sized high voltage source... The physical size of a battery doesn't have much to do with the voltage - a nine volt battery (and in this case, it actually is a battery, because it is composed of six individual cells of 1 « volts each) can have lots of voltage, but a low current capacity. A car battery is only 12 volts - but can provide enough current to weld with. And if you're old enough to remember the #6 dry cells (the big round batteries with screw terminals found in electric fence chargers and old time relay operated burglar alarm control panels, about 8 inches tall and 3 inches in diameter), you'll remember that they were only 1 « volts, but could provide enough current to melt a nail laid across the terminals.
Which leads us into the following - There is a term used in reference to batteries, both primary and secondary, called the "amp hour" rating. This, basically, is a measurement of how much work the battery can do, and for how long. Any battery has a number of ratings, but the two we will be most interested in are the voltage and the current capacities. Raw power, in general terms, is equal to the voltage multiplied times the current (and is called watts in the industry). This is not a basic electronics class, but you can think of voltage (volts) as the "push", and current (amps) as the actual amount of electricity.
We need to define a few terms here. An amp (short for ampere) is a unit of current, and is measured in amps or milliamps. A milliamp is 1/1000 of an amp, or an amp is 1000 milliamps, to say it the other way. Volts and amps are the same in both English and Metric measurements, so we are politically correct automatically.
Speaking of volts, here's a piece of trivia for you to throw at your technical friends: did you know that the volt was redefined a few years ago? True - with the newer, more accurate methods of measurement, it was discovered that the old reference volt maintained at the National Bureau of Standards (previously called NBS, now called National Institute of Standards and Technology, or NIST) was not precisely accurate. So, at a given changeover day, the new reference for the volt was established. It was only a very small difference, but very real. For a short while around the time of the changeover, newly manufactured precision test equipment was labeled as to whether it was calibrated for the old volt or the new volt. The difference was so small that only high precision test equipment was affected. This is not the April issue, and this is not a joke. There is a new volt and an old volt. Maybe you can win a beer with this useless fact. Of course, you'd probably have to call NIST or Fluke or somebody to prove your position.
The amp hour rating (abbreviated aH. or mAh for milliamp hour) indicates how many amps (how much current) a battery can supply over a given period of time. More is better. For an amphour rating to mean anything, you must specify over what period of time the current is drawn in making the measurement for the rating. The old standard used to be 20 hours, and was called the 20 hour rate. In reviewing the technical literature in my files from various manufacturers, I find that there actually is no standard anymore, because one manufacturer rates their cells at the one hour rate (a very fair method), while others use the 5 hour rate, others the 10 hour rate and still others the 20 hour rate. The longer the period used, the more potent a given cell will seem, although there is only about a 10% difference between the two extremes. So if you are a purchasing beancounter or otherwise are responsible for writing or interpreting specifications, be sure to specify over what period of time an amphour rating was derived, to be sure you are comparing apples and apples. The standard for as far back as I can remember was 20 hours, but then again I also remember when gold was $35 an ounce...
There is no such thing as a free lunch - if one manufacturer's battery seems to be far superior to another, you are not being told the whole story. Make sure you are comparing similar ratings, and similar duty cycles. Generally, physical size will determine how much power a battery will deliver over a given period of time. Regardless of the apparent specifications, for example, a handheld radio that gives 5 watts of power for 8 hours needs a big battery. If the specs say 5 watts, 8 hours, small size and not very heavy, you need to read the specs carefully (what's the duty cycle?) with your hand on your wallet.
If a battery could supply one amp for one hour, that theoretically would be one amp hour. But using the 20 hour reference, a battery that could supply 50 milliamps (or 50/1000 of an amp) for 20 hours would be a one amphour battery. If you drew one amp from it, it would last less than one hour. Or, to take a real life figure, an alkaline D size cell is rated at 14,250 mAh (or 14 ¬ amphours). So we should be able to draw 1 milliamp from it for 14,250 hours, 10 milliamps for 1,425 hours, 100 milliamps for 142 « hours, or 1 amp for 14 ¬ hours. Now, if the ratings refer to the 20 hour rate, draining the battery quicker than 20 hours will result in a lower amphour rating, and over longer periods than 20 hours will give more than the rating.
Make sense? I know it sounds complicated, and looks scary when we start getting into weird numbers and abbreviations. Understanding the things that matter about batteries depends on understanding the amphour ratings, so stick with us here - it's important. If it's not perfectly clear, try this: You can have a 10 gallon bucket of water (10 gallon hours?) which could represent a battery with a 10 amphour rating. Now hang the bucket up in the air and punch a small hole in it. The water dripping out of the hole will represent the electrical drain on the battery.
If we drain 1 gallon of water each hour out of the bucket, in 10 hours the bucket will be empty (or the battery will be dead, to stick with our analogy). If we draw twice this amount from the bucket, say 2 gallons per hour, it will get empty twice as fast - or only last half as long. Conversely, if we take out only « gallon per hour, the water in the bucket will last 20 hours instead of 10 hours.
The quicker we pull current out of the battery, the shorter it will last. So a higher amphour rating means the battery will last longer, assuming it is doing the same amount of work. Ignore the bucket analogy for this statement.
The above analogy comparing a battery to a bucket of water is very rough, but it should help you understand the amphour concept.
In comparing batteries, you need to compare the amphour rating along with cost to see which is the better buy. Of course, the voltages of the two batteries under consideration must be the same for any other comparison to be accurate. If you understand the amphour rating, you can mentally determine about how long a battery should last in your application (relatively). This could be very useful if, for example, you need to know how big a rechargeable battery to buy to power, say, a camcorder for a certain number of hours.
As one use of amphour ratings, below is a chart of capacities of typical small alkaline batteries:
And here's some ratings for lithiums:
NOTE: Ratings are in milliamp hours, 1000 milliamp hours equals 1 amphour
We can't do a direct, real world comparison of lithiums to ordinary dry cells because lithium cells are 3 volts and others are 1.5 volts. But if we compared one lithium AA cell (3 volts) to two AA alkaline cells (1.5 volts times 2 equals 3 volts), we would get the above ratings.
Interpreting the above, we can determine, very roughly, the operating time of a particular piece of equipment by applying the following:
The amphour rating of battery pack divided by the current drain of equipment in amps equals the operating life in hours
So a flashlight drawing one amp operating from alkaline C cells should run approximately 7 hours.
I say approximately because, as we learned earlier, the amphour ratings are accurate only at a certain discharge period - draining them quicker than the certain period will give less than the capacity indicated (and vice versa).
We live in a complicated world full of exceptions. To further confuse the issue, the amphour ratings also depend on just when you consider a battery dead. Dead is not zero volts. Dead is considered generally to be the point below which, if you continue to drain the battery, it will be permanently damaged. With throwaway batteries a realistic "dead" is when the thing they are operating stops operating well enough to suit you. For example, many surveillance transmitters are rated for a battery life of X hours. This usually refers (with an honest and competent manufacturer, which fortunately is most of us) to the point at which the power output of the unit is down to half of where it started. Half power may be referred to as the "3 dB point". 3 dB is relative measurement meaning half the power (or twice if you're going the other way). We discussed this in detail a few issues ago in the antenna article, so I'm not going to repeat it here. A one watt transmitter will be considered to have dead batteries when its power is down to « watt. Some manufacturers consider dead as 6 dB down, which is ¬ of the original power (« of «). Any method is acceptable as long as the manufacturer isn't trying to hide anything from you and you understand how to make accurate comparisons between the two. Give me a call if all this still isn't clear and I'll explain it further.
Each type of battery has its own definition of dead. So it's technically incomplete to say a battery lasts X hours without saying what performance it will have along the way and what criteria are used to determine when it is dead.
With nicads, (a type of rechargeable battery we haven't covered yet) dead is considered 1 volt per cell. If you discharge the nicad below that voltage you might ruin it. Alkaline batteries are considered dead at 0.8 volts per cell (starting from 1.5 volts per cell). Lithium batteries are dead when their voltage drops to 5.4 volts (60% if a multi cell battery) of where it started (3 volts per cell).
To word the previous paragraph differently, but to same the same thing, we'll take a 9 volt lithium battery as an example. Ultralife's spec sheet says their part number U9VL battery has a rated capacity of 1200 mAh (fall 1993 figures). This rating means that the battery will provide a certain amount of current (in this case, 10 milliamps) until its voltage drops to 5.4 volts (60% of the 9 volts where it started). And it will do that for 120 hours (1200 milliamp hours divided by rated current of 10 milliamps equals 120 hours). Note that this does not mean that you cannot draw more than 10 milliamps out of this battery - they just picked that current to use in their ratings, and possibly because it makes all numbers come out even. Theoretically, you could draw 100 milliamps from this same battery and it would last for 12 hours. Or 1 milliamp for 1200 hours, etc. Note that the maximum continuous discharge current for this battery is listed as 120 mA. This probably is the point at which internal heat buildup is at the maximum that the physical package of the battery can dissipate safely. Drawing more continuous current would build up heat quicker than it could be dissipated, with some sort of results, most of which are not nice. But, you could draw more current that the maximum if it was in pulses, allowing the battery time to cool between pulses. This is starting to get into engineering which is beyond the scope of this article. Call me if you would like further information.
A rating for a battery also will specify the temperature at which the ratings were determined. Because batteries use chemical reactions (and in fact, a definition of a battery is a device that converts chemical energy into electrical energy), the temperature is important. Higher temperatures mean quicker chemical reactions. Some batteries are more temperature sensitive than others.
We're going to knock off here. Next issue we'll pick up with rechargeable batteries. Be patient, and don't get your liver in a quiver. The information in this part is necessary to understanding much of what we will cover on rechargeable batteries. And we'll give sources on where to buy the different types of power sources we've been discussing.
We'll wrap it up for this time with another bit of trivia about an interesting battery. Last time I told you about short lived batteries used on some military missiles I worked with once. Here's another one. This information was provided by my father regarding a special battery he had designed for a military rocket. The rocket was named Track Break (presumably declassified by now). It used a one shot thermal battery. Thiokol in Maryland designed the battery to fit in the rocket. On receiving a 28 volt pulse, some pyrotechnic material was ignited which burned rapidly (of course) such that the battery's normally inert electrolyte would melt and become active within one second. The life of the battery was one minute.
As always, please feel free to call or write my office if you have specific questions on batteries you would like answered in a future issue, or to comment on anything we've covered so far. All your inputs are taken seriously. And if you'd like to see more of this type of material (or less...), call Al Menear at Police & Security News, 215-538-1240, and let him know. Call him also if you need a copy of part one of this article from last time, or if you are not a subscriber to this magazine but would like to be.
I appreciate your calls and meeting you on the phone or in person. Thanks to everybody who stopped by our exhibit at NATIA and commented on these articles. Great time with great people. See you next time.
Copyright August 1993 by Steve Uhrig, SWS Security