|
Starting power: You can't leave home without it. Ever since the 1912 Cadillac
appeared with the first electrical cranking system, it's been provided by lead
and acid, but if you think that means not much technological development has
gone into batteries, you'd better keep reading.
Chemistry, anatomy
A quick recap of battery theory and anatomy will help you understand the
changes that are going on. If two dissimilar metals are placed in an electrolyte
that can attack them, voltage potential is created. Electrons will flow if a
connection is made between the metals, and that's what electricity is.
In a wet cell, the metals are sponge lead (Pb) and lead peroxide (PbO2), and
the electrolyte is dilute sulfuric acid (H2SO4). The reaction begins as sulfate
(SO4) breaks away from the acid and unites with the lead of both the positive
and negative plates to form lead sulfate (PbSO4). The oxygen (O2) is thereby
liberated from the lead peroxide and joins with the hydrogen (H2 -- what's left
over after the sulfate left the acid) to produce ordinary water (H2O), which
dilutes the electrolyte.
Eventually, both the plates turn into lead sulfate, the electrolyte becomes
very weak, and current stops flowing.
But reversibility is the wet cell's most important characteristic. When an
outside power source pushes electrons through the cell in the opposite direction
to that of discharge, sulfate separates from both plates to rejoin the hydrogen
in the water, forming a new batch of sulfuric acid. The oxygen goes back to the
positive plate to recreate lead peroxide, and the electrical potential is
restored. If charging continues after all the sulfate has gone into the
electrolyte, the water starts to decompose, releasing free hydrogen and oxygen,
an explosive couple.
The traditional automotive battery has plates made of a combination of lead
and antimony impregnated with the metals involved in the reaction. The positive
plates are separated from the negatives by sheets of porous material that
insulates them electrically from each other, but allows the electrolyte to pass
(although such things as balsa wood have been used, sealing the positive plates
in plastic envelopes is commonly done today to keep the active material in place
that had previously been allowed to drop piece by piece into the space under the
elements, lowering the cell's capacity and sometimes shorting out the plates).
Numerous plates of each metal are interlaced within one cell, but whether two or
a dozen are used the cell produces a "pressure" of 2.1 volts. Six
cells are connected in series to give the 12.6 volts almost all cars have needed
since the fifties.
Strength specs
You've got to understand performance ratings to be able to choose exactly
what you need. The old ampere hour standard isn't used much anymore. In its
place, we have the Battery Council International rating, which tells you how
many amperes a full-charged battery can deliver a 0 deg. F. for 30 seconds
without going below 7.2 volts. This is called "CCA" (Cold Cranking
Amps) and should be at least as high a number as the cubic inch displacement
figure of the engine.
Then there's the Reserve Capacity Rating -- the number of minutes a new,
juiced-up battery at 80 deg. F. can sustain a 25 amp drain before dropping to
10.5 volts.
There's a good chance that you're familiar with these two measurements, but
most people don't realize that there's a trade-off relationship between them. If
the first is pushed up in a particular design, the second has to go down, and
vice versa. In other words, you might find a battery with a macho CCA number,
but a wimpy RC. Compare them before buying.
No antimony
One of the biggest departures occurred in 1972 when the maintenance-free
battery appeared. Its plate grids are made of calcium lead alloy with no
antimony, which reduces gassing by up to 97%, and cuts water loss and terminal
corrosion. It also prevents thermal runaway, a condition wherein conventional
plates destroy themselves by overheating when fed too much power. Also, there's
very little self-discharge, so an M-F unit can retain enough wallop to start a
car after sitting for up to a year (that is, if it's disconnected -- the
parasitic drain of the vehicle's electronic systems will run any battery down in
a matter of months).
Sounds great, right? But there's more to the story. As one expert told me,
"There's really no such thing as maintenance-free. Every battery will
evaporate some water from heat, and gas a little from electrolysis." In the
battery business, M-F's with non-removable cell caps are frequently referred to
as "maintenance impossible."
L-M and hybrid
Which brings us to the low-maintenance type. It has a little antimony in both
its negative and positive plates, which results in slightly more gassing and
self-discharge than with M-F. But an L-M unit is better at withstanding deep
discharge, its cell caps are removable, and it requires lower voltage during
charge, which reduces the potential for heat damage. It's a very forgiving
battery.
Between the two, there's the hybrid or dual-alloy. It uses calcium for the
negative grid, but retains a small amount of antimony in the positive. It gasses
but very little, has better "bounce back" than the calcium/calcium
type, and you can add water.
Pulsar
I should mention a unique design that looked for a while like it was going to
take over: Pulsar. Instead of using positive and negative plates and insulators
stacked like a deck of cards and welded together at the top to form separate
two-volt cells, it has modular injection-molded "power panels." Each
panel has six vertical sectors, three negatives alternating with three
positives. Two panels are mated with an insulating layer between, which creates
a complete 12-volt battery half an inch thick. Any number of modules can be
joined to produce the desired amperage capacity -- one example has 18 pairs.
Along each side of the stacked panels is a brass bus bar, one positive and one
negative, which shortens the power path. Also, internal copper bus strips are
used because that metal has less resistance than lead.
Great claims were made for this innovation in construction, which was
developed by Australian Bill McDowall of Pacific Dunlop in 1972. Exceptional
vibration resistance, for example. In lab tests at three G's/50Hz, regular
batteries failed in 20 minutes, while Pulsar units were doing fine after 40
hours. Light weight is another advantage -- 25% less for the same capacity. Then
there's design freedom. Separate small, thin batteries could easily be
manufactured to handle various power consumers (starter,
computer, stereo, etc.), and be mounted wherever practical, which would leave
more room in that cramped engine compartment. Also, if the car makers move to
higher voltage systems, Pulsars could be hooked in series more conveniently than
traditional units. Manufacturing is highly automated, too.
But there's never a rose without a thorn, and this new approach has a couple
of sharp ones. Relatively low reserve capacity, for example -- 108 minutes for a
640 CCA specimen. But that's not as troubling as the fact that it's difficult to
recycle a Pulsar. While nobody would've cared much about that in the past,
recycling is one of the major issues in the battery industry today because of
existing and pending regulations. There's no problem recovering almost 100% of
the materials in conventional units, so they'll continue to dominate.
More can be less
The equivalent of a horsepower
race is going on among battery manufacturers where CCA is concerned. Passenger
car units with ratings of up to 1,000 are on the market, and that's enough to
crank a semi on a frigid morning.
But, as I said above, there's a trade-off between CCA and RC ratings. This is
because the more numerous plates used to increase cold cranking are by necessity
thinner, so the overall proportion of reactive material is smaller and reserve
capacity goes down. Another problem with thin plates is that they're more apt to
buckle at high temperatures.
Sauna survival
Speaking of heat, some manufacturers are addressing the fact that it's way up
in today's cars, which causes all kinds of problems. Besides the buckled plates
just mentioned, there's the chronic undercharge condition that occurs because a
hot battery will produce higher voltage than a cool one at the same state of
charge. The voltage regulator doesn't know the temperature, so it assumes the
cells have a full dose of juice when in fact they may be down 25%.
High heat and a continual low state of charge makes hard sulfation build up,
and crystallization starts breaking the bond between the active material and the
grid. The lead becomes so soluble it attaches to the separators forming a "tree"
that can short out the plates right through the pores in the plastic envelope.
Specific gravity rises too far because the water evaporates while the acid stays,
and separators can become "charred."
All this adds up to a surprising statistic: Average battery life is actually
shorter in the south (30 months) than in the north (38 months). So, there are
batteries on the market designed specifically to combat heat, and they employ
both new and old technology to do this -- efficient radial grids, but thicker
plates. This lowers CCA, but who needs all that zero weather kick in the sunbelt?
What's really necessary where warm weather prevails is plenty of HCA (Hot
Cranking Amps -- the discharge load in amps a new, fully- charged battery at 32
deg. F. can deliver for 30 seconds while maintaining 1.2 volts per cell) and RC
(how about 130 minutes?).
Another feature is a thermal insulation blanket made of a plastic foam that
floats on top of the electrolyte, displacing the air so there's less gassing and
evaporation.
Invisible spare
Computer-designed radial grids, thinner plates, and other refinements greatly
increase the energy density of modern batteries. To illustrate, a typical
old-fashioned unit could deliver maybe ten cold cranking amps per pound, whereas
a more highly-evolved specimen might produce 18.
This has made a neat new feature possible: a back-up reservoir of starting
power right inside a normal size-battery. If you ever come out in the morning to
find that your energizer is just too run down to crank the engine, just pop the
hood, flip a switch and try again. The back-up (275 CCA in one specimen) is
separated from the rest of the battery by a diode (a one-way valve
for electrons), which allows current to flow in only until the switch is thrown.
That means sufficient juice will be available for starting even if you've left
the lights or that megawatt ear-blaster on all night.
Making water
Ever hear of the recombinant battery? The idea here is to immobilize the
electrolyte and keep the moisture level stable by forcing the hydrogen and
oxygen to recombine into water. This is accomplished by using a different type
of insulator and by keeping the interior of the unit under pressure. Because
this type of battery is insensitive to severe angles, it's particularly
applicable to marine use.
The concept's biggest problem is the relief valve that maintains pressure.
It's hard to keep its calibration accurate in mass production, so you might end
up with a battery that looks like a bowling ball. Also, it takes twice as long
to recharge as a regular "flooded" unit. The first recombinant to hit
the market in the U.S. promptly failed, but others are available now, and the
European aftermarket is showing considerable interest in the principle. I'm not
going to hold my breath waiting for it to take over, though.
New standard, new couples
One big change we'll probably be seeing in the not-too-distant future is a
switch to higher voltage systems -- most likely 24, but maybe 36 or even 48. Why
tamper with a standard that's been around for 40 years? Because using electric
motors to drive such things as A/C compressors and rack-and-pinion
steering gears is becoming a fashionable idea among car designers, and that'll
draw plenty of amps. Instead of resorting to gigantic cables to deliver the
juice, increased voltage can be enlisted.
Finally, are there any new combinations of active materials (called "couples"
in the industry) that show promise as a practical replacement for lead-acid? In
a word, no. As one of the battery engineers I interviewed for this section put
it, "I don't think we'll see a switch from lead-acid. Its cost is low
relative to any other couple, nicad for instance. And lead-acid is eminently
recyclable."
|
