Tech-Talk
Part 21
Welcome to part 21 of our series.
We've been talking about DC Power. When we're home, we
generally use power supplies that convert house current (110V
AC in the US) to the nominal 12V DC that our radios require. But when
we're operating portable, whether for fun, emergency and public service
communications, or even "at-home portable" when the lights go out, we
turn to battery power. Let's find out a little more about batteries....
Safety First, Second, and Third!
Batteries can deliver a tremendous amount of current in a
very brief time. Never short the battery terminals. Never place the
battery where metal can accidentally contact the terminals. Always fuse
the positive wire as close as possible to the battery.
Batteries fall into two broad categories. "Primary" cells
are designed for one-time use. When they can no longer deliver
sufficient current -- that is, the voltage is too low to power the load
-- we consider them dead and must replace them. Typical examples are
the D, C, AA, AAA, and 9-volt sizes that we're all familiar with.
They're handy for running small devices like flashlights, but not too
practical for most Ham Radio applications.
So we turn to the other choice -- "secondary" or
rechargeable batteries. They start our cars, and power our cell phones,
tablets, and laptops. When they go dead, we plug them in, charge them
up, and we're back in business.
Rechargeables come in a wide variety of types,
differentiated by their "chemistry" -- the types of material that make
them up. All of them rely on some type of chemical reaction to generate
useful electricity. The simplest, and oldest, type is the lead-acid
battery. Car batteries and so-called gel-cells are typical examples.
Early battery packs for hand held devices were Nickel-Cadmium (NiCad)
cells. Later, Nickel-Metal Hydride largely replaced NiCads, and in turn
were supplanted by even better refinements like Lithium-Ion and
Lithium-Polymer. Even more exotic technologies exist and will almost
certainly work their way into mainstream products in the coming years.
So what makes each advance in battery technology "better"?
To oversimplify, it's an increase in Power Density. Let's take a look
at that concept.
A common way to measure battery capacity is in Amp-Hours,
abbreviated as AH. Smaller batteries are often rated in Milliamp-Hours,
or mAH. In essence, this means the battery's ability to deliver a
specified level of current for a specified amount of time before its
voltage drops to a specified level. For example, in theory a common 7AH
battery should be able to power a load of 1 amp for 7 hours; a load of
500 milliamps for 14 hours, or a load of 2 amps for 3.5 hours. In
practice, most batteries will exhibit higher capacity when powering
smaller loads, and lower capacity at higher loads -- in our example, the
500 mA load is likely to run for longer than 14 hours, but the 2 amp
load will stop at something less than 3.5 hours.
Power Density is -- again, oversimplifying -- a measure of
Amp-Hours per pound, or Amp-Hours per cubic inch. At left are three
common sizes of sealed lead acid gel cells. In front is a 7AH battery.
It weighs about 5 lbs. These are often used in alarm systems and
emergency lighting units. In the middle is a 35AH version. This one is
referred to as a "Group U1" size, and weighs about 22 lbs. And at back
is a "Group 24" size -- similar to many car batteries. It's rated at
79 AH and weighs a hefty 50 lbs or so. In contrast, a modern
Lithium-Ion battery of similar capacity would weigh about 1/3 as much as
the lead acid cells. But as I'm sure you've guessed, there are
trade-offs. First off, Lithium batteries are much, MUCH! more expensive
per Amp Hour of capacity. In addition, they require specialized
charging circuits, which add to the cost and complexity of use. Due to
both factors, they are not generally available in standard sizes like
the ones at left.
Now, battery capacity may seem like a pretty straightforward
idea. But things get slippery quickly. As a fully charged battery
delivers power to its load, its voltage drops. A "dead" battery is not
necessarily (in fact is probably not) at zero volts. It simply can no
longer supply sufficient power to the load. It follows, then, that to
accurately specify a battery's capacity in Amp-Hours, a manufacturer
must also specify a lower limit to the voltage. Specifying a lower
voltage will make one battery seem to have a higher capacity than
another. Surprisingly <g> some manufacturers use this trick to
hype their product over their competitors' products. And here's
"surprising" trick #2. As we noted above, batteries will exhibit
somewhat higher capacity with smaller loads. In our example, a 7AH
battery with a 500 milliamp load lasts longer than 14 hours. If, say,
it powers the load for 18 hours, then some vendors will claim it's a 9AH
battery!
A related issue is recharging cycles. Let's take a brand
new 7AH battery. Well, after it's discharged and recharged, it may be a
6.999 AH battery (note that I'm just making up numbers to illustrate
the point). After the second discharge/charge cycle, it may be 6.998
AH, and so on. When we say a battery doesn't hold a charge, we mean it
has effectively become a zero AH battery. You've no doubt seen this
yourself as your gadgets gradually operate for less time between
charges.
Now by altering the physical construction of the battery,
it's possible to make it with a higher initial capacity. But here's the
catch -- what they don't tell you is that the capacity drops off at a
steeper rate, so you get fewer charge cycles. Therefore, you'll need to
shell out for a new battery sooner and more often. Caveat Emptor! As
always, your best bet is to purchase your battery from a reputable
vendor. Just remember -- if it seems too good to be true, it probably
is.
That's it for this one, gang. Next time, we'll continue
looking at portable power, and try to answer the #1 question -- what
size battery do you need?
73 for now
John Bee, N1GNV
Quicksilver Radio Products
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