basics of battery
TRANSCRIPT
-
7/29/2019 Basics of Battery
1/6
Aircraft starting batteries have a tough life; much more so than their automotive counterparts.They are left idle for days or even weeks at a time, then asked to start a 500 cubic-inch engine in
cold weather with three gallons of oil the consistency of molasses in the sump. To make matters
worse, aviation batteries have significantly lower capacity than an auto battery, and multi-weight oil can help only to
a degree.
Twenty-four volt batteries provide some help in high-current-demand situations such as speeding up electric
retractable landing gear, but in terms of battery cost compared to 12 volts, a 24-volt system leaves a lot to be
desired on cost vs. benefits.
When left idle, batteries self-discharge about one percent or so a day due to the side effects of components
(antimony) used to keep battery-grid structures from shedding and damage during use. This self-discharge process
has a long-term, deleterious effect on the battery if not corrected by frequent recharging. The term is sulfation, and
when left unchecked it means the gradual self-destruction of the battery. Recharging and the replacement of lost
electrolyte, a natural process in wet-cell batteries, staves off permanent sulfation.
When a battery is allowed to remain in a state of sulfation, either from lack of use or from lack of recharge, it tends
to lose both capacity and life. Initially reversible sulfation hardens with time and it becomes less and less reversible.
As sulfation flakes off and falls to the base of the case, it can short out the cells.
By Definition
First, let's look at volts and amps, two often-confused terms. We will use the common (but not perfect) water
analogy.
The rate of flow through a pipe (amps) is governed by two things: the pressure (voltage) and the size of the pipe
(wire diameter). A third factor also has influence -- pipe length. The longer the pipe the more resistance (ohms) and
pressure (voltage) drop.
Increasing pressure (pipe size) increases the potential flow. More flow, more work. Greater pressure or voltage
allows delivery of more fluid (amps) in a given diameter pipe (wire).
Excessive pressure and flow can cause a pipe too small for the application to burst (a wire would overheat and melt).
Thus, there must be a balance of pipe (wire) size for a given flow (amps) and length combination. That's why
starting batteries need such large wires leading to the starting motor. There are hundreds of amps required at 12 or
even 24 volts (not much pressure) to deliver the large quantity of amps to spin the starter. (Volts times amps equals
watts, or power, in a properly sized circuit for DC electricity. AC can get a little more complicated.)
You can have more volts and fewer amps or fewer amps and more volts to equal the same power, given the wire
size is appropriate for the application. We are over-simplifying here, but our goal is to give the basic concepts for
understanding a battery, not a course in DC electronics.
A Basic Wet Cell
Maintenance
-
7/29/2019 Basics of Battery
2/6
The battery is a simple device, allowing the chemical
storage of electricity. In some batteries, the chemical fuel -
- once exhausted -- means a throwaway.
In batteries such as a wet cell (liquid electrolyte), the
process is reversible, and you can restore an exhausted
battery by adding electrical energy and restoring thechemical potential.
Batteries like this have been around for well over a
hundred years, and the classic model is still the basis of
operation of the majority of today's starting batteries, but
the times are slowly changing.
If you wanted to make a battery, all it would take is
suspending a few strips of lead in a solution of sulfuric acid
(see drawing at right). To charge the battery, a piece of
lead is connected to become the positive terminal and the
other the negative terminal. Connect an outside source of voltage, say 3 volts. Bubbles will form and rise in the acid,
coating the positive terminal with a brown coating called lead dioxide, yielding a positive potential. Your cell has a
voltage of about 2 volts DC. Connect a small bulb to the terminals and -- voila! -- light.
The discharge process converts the lead dioxide of the positive terminal and the lead strip of the negative terminal to
lead sulfate. Water is also released, diluting the acid. Connect the outside charging source and charge again. Connect
six 2-volt cells in series and you have a 12-volt battery.
Capacity is roughly a function of the amount of lead. Typically, the lead plates are alloyed with antimony for plate
ruggedness and resistance to vibration. The down side is antimony promotes shedding of the active plate material.
As a battery begins to discharge at peak rates for more than a few seconds, significant amounts of lead sulfate
immediately begin to form, slowing the reaction as you crank.
Starting batteries have thin plates with fiberglass separators to provide the maximum active surface area for the
greatest burst of current. It's also why a short rest (20 minutes is better) will allow an apparently dead battery tospontaneously recover a bit, sometimes for one last shot to start. The remaining unsulfated lead is being contacted
by the electrolyte through diffusion. Wet-cell batteries are least efficient when you demand maximum discharge.
When a battery is charged, and especially when excess voltage is applied, some of the water in the electrolyte is
converted to gaseous hydrogen and oxygen and then vents. As this condition continues, electrolyte is lost to the
atmosphere, thus the normal requirement to replenish the water in the cells ... preferably with distilled water, please.
Aircraft battery caps also have internal stoppers to help prevent loss of electrolyte in unusual attitudes.
Maintenance-Free
This is a diagram of the basic chemical process.
(Clickherefor larger version -- 40 KB.) One of the
problems is that the plates tend to age with use and
time. The owner can significantly impact the rate of
plate aging by proper charging.
http://www.avweb.com/newspics/battery_process_diagram.jpghttp://www.avweb.com/newspics/battery_process_diagram.jpghttp://www.avweb.com/newspics/battery_process_diagram.jpghttp://www.avweb.com/newspics/battery_process_diagram.jpg -
7/29/2019 Basics of Battery
3/6
In automotive use, "maintenance free" generally means a
wet-cell battery with simply more electrolyte and different
chemical additive. An exception to this is the true
maintenance-free battery from Optima, and a few other
specialty companies, which will run from $150-200 -- and
worth it.
Aviation-type maintenance-free batteries for certified
airplanes -- available fromGillandConcorde-- are also
called sealed, valve-regulated batteries and are truly
maintenance free from adding liquids. Terminals still require
periodic cleaning. They can be damaged by overcharging
with excess voltage (over 14.7 volts for a long time).
They may be installed at any angle, have superior resistance
to self-discharge, and can discharge more completely under
high loads, such as with an alternator failure.
Capacities
Being chemical beasts, the ability to supply current is not really linear. The greater the draw in amps, the less
efficient the typical wet cell battery. For example, a battery that has a 25 amp-hour rating of 60 minutes will not
sustain a 50 amp load for 30 minutes, but substantially less time due to the inefficiencies of the plate design.
A battery has an internal resistance of fractions of an ohm, but it's meaningful. The greater the amp drain per unit of
time, the more energy is lost as internal battery heat due to that small resistance. As batteries age and/or sulfate,
this resistance goes up.
Bottom line: The discharge curve is very sharply sloped. Every battery has an optimal maximum rate for current
draw. This concept is important to understand because in an "alternator-out" situation, you will get much more than
double the battery time if you cut the current drain on the battery in half when running electronics.
Capacity Testing
A classic, wet-cell, 12-volt, Gill battery is still the
most popular format. Newer AGM sealed batteries
are internally maintenance free but need a properly
adjusted charging system to last long. They provide
good cold-weather starting and resist self-discharge.
This is what the plate material looks like, pasted to the
grid. Because the chemical reaction must propagate
http://www.gillbatteries.com/http://www.gillbatteries.com/http://www.gillbatteries.com/http://www.concordebattery.com/http://www.concordebattery.com/http://www.concordebattery.com/http://www.concordebattery.com/http://www.gillbatteries.com/ -
7/29/2019 Basics of Battery
4/6
Capacity Testing is a greatly ignored annual requirement,
because most shops don't have the proper equipment. You
must have an annual capacity test performed on your
starting battery. It's part of the Instructions for Continued
Airworthiness that should come with any new battery.
Our conversations with the FAA indicated they interpret the rules that the capacity test is mandatory for Part 23certified airplanes and recommended for CAR 3 certified airplanes. That's a rule interpretation we don't understand
and don't agree with. Do yourself a favor and do the test -- it's your neck.
The test must show that your battery is capable of sustaining 80 or 85 percent of the amp-hour rating of your
battery (depending on the type of test protocol performed). See each maker's ICA for the specifics -- they do differ.
If you want the best chance of passing this test (and longest-lived battery), then the best bet is to use a multi-stage
battery charger that reaches up to 14.5 volts, but not more, at the conclusion of the charging cycle.
We recommend the BatteryMINDer fromVDC Electronics. They have a line of 12 and 24-volt chargers optimized
for aircraft batteries. With battery costs rising, it makes economic sense to use a proper charger -- it will make the
battery last significantly longer.
Recharging
Once you've discharged a storage battery, promptly recharge it. In the simplest terms, if you apply a voltage to the
battery that's higher than the voltage the battery produces, you reverse the chemical action that produced the
electricity. This converts the lead sulfate and water back to lead, lead dioxide and sulfuric acid.
If the process were 100-percent efficient -- which it isn't -- you'd have to put back exactly as much electricity as you
drew from the battery when you were discharging it to return it to a fully charged condition. In real life, lead sulfate,
lead dioxide and lead are materials with different densities and different rates of expansion and contraction with
changing temperature. When one replaces another, small pieces can flake off the plates and fall to the bottom of the
cell. Then too, the chemical reactions that occur during charging take time to occur.
If you try to charge a battery faster than it can accept a charge, some of the electrical energy goes to producing
additional lead dioxide that will flake off and fall uselessly to the bottom of the cell. Some will also break down the
water portion of the sulfuric acid electrolyte into gaseous hydrogen and oxygen and vent out the caps. An
undesirable but unavoidable consequence of recharging causes some of the electrical energy you supply to a battery
to be lost as heat.
As a rule of thumb, you have to supply about 120 percent of the electrical energy you've taken from the battery to
recharge it. It should be supplied in a controlled manner specific to the battery if you are to achieve maximum
battery life.
Ideal Charging
One of the best ways to charge a battery is at a constant current rate in amps equal to 20 to 40 percent of thebattery's capacity in amp-hours (Ah) until the battery reaches an optimal voltage for its type. The level of charge at
this point is equivalent to about 75 percent of the battery's capacity. This first phase of charging is called bulk
charging.
Once the bulk charging is complete, the charging device should maintain the charging voltage at a constant value
and allow the charging rate in amps to drop steadily. When the battery accepts current at only about 1 percent of its
capacity (e.g., a 25-Ah aircraft battery accepting 0.25 amps), it can be considered fully charged. A current of 5
percent of capacity represents about an 85 percent charge. This phase, which can take several hours, is called the
acceptance phase of the charge cycle.
through the plate, high-current demands are limited to
relatively short current bursts primarily dictated by
surface area. The higher the current demand, the less
effective the battery is at supplying it.
http://www.vdcelectronics.com/http://www.vdcelectronics.com/http://www.vdcelectronics.com/http://www.vdcelectronics.com/ -
7/29/2019 Basics of Battery
5/6
Once a battery is charged, a float cycle helps to hold it in that condition. A float cycle is simply a voltage maintained
slightly above the battery's rested, open-circuit voltage (13.1 volts nominal for a 12-volt battery). This float voltage
doesn't really charge the battery but helps maintain a charge to compensate for internal losses in the battery. If your
charger is trickle charging at more than 13.2 volts, it is slowly cooking the battery -- don't leave it on all the time.
This phase charging is what a "smart" charger does. A typical alternator/regulator charging system is not as
sophisticated in recharging a battery, nor is the typical automotive battery charger.
A quick start takes very little charge out of a battery, so sophisticated charging is not critical. Batteries that sit idle,
though, need more capable battery chargers for long life to reverse any sulfation that always take place when a
battery sits.
Never recharge a battery that's low on electrolyte (plates showing to air). Add distilled water before starting the
charging cycle, not after. Only fill it to the bottom (or a bit below) of the split rings, not to the top, or else the
electrolyte will overflow during the charging cycle. Tap water is not good since it contains minerals that tend to
accelerate adverse reactions. Adding more battery acid is even worse, and usually kills the battery in a matter of
days.
Checking the Charge
A hydrometer is a simple instrument used to measure the
state of charge of a battery. It does this by measuring the
specific gravity -- the weight as compared with water -- of
the electrolyte. You can get a suitable hydrometer at a
local auto parts store, but be sure that it only requires an
ounce or two of electrolyte for a reading. That's about all
you'll get from an aircraft battery. Also, get a numerically
calibrated one; do not buy a pith-ball type.
Checking the charge is simply a matter of taking a sample
of electrolyte from a cell and reading the value where the
fluid line meets the hydrometer's floating scale. Write the
number down. A specific gravity reading between 1.300and 1.275 indicates a high state of charge; between 1.275
and 1.240 is a medium state of charge, and between 1.240
and 1.200 is a low state of charge.
A battery in a low state of charge has 50 percent or less of
its capacity left and needs recharging. Check all the cells;
then compare the readings. Usually, if there's a spread of
more than 0.050 between the worst cell and the average
of the others, the battery is on its way out.
When testing a battery with a hydrometer, observe temperature factors; the temperature of the battery can make a
big difference in the readings. When its temperature is between 70 and 90 degrees F, the readings can be used as-is.
Outside of this range, a correction chart is needed. Some hydrometers have this chart printed on floating scales;others have the chart on or in their packaging. Also, hydrometer readings are taken after the battery has been off
the charger for a couple of hours minimum; overnight is even better for accuracy.
How Batteries Fail
Batteries, as we've noted, are relatively simple beasts, although some complex things happen to them during their
operation. Battery failures, for the most part, are also relatively simple to understand.
Overfilling beyond the bottom of the split ring
promotes the gassing out of the excess electrolyte,
which forms conductive bridges on the surface of the
battery. Also, when initially placing in service, fill to a
little below the split rings or you can boil away
electrolyte, making a mess. Fill to split ring after the
initial charge is complete.
-
7/29/2019 Basics of Battery
6/6
The flaking and separation of lead sulfate -- and, to a lesser extent, lead dioxide -- from the plates that occurs even
in normal operation can eventually cause a sludge in the bottom of the battery that actually short-circuits the plates.
That can't be helped, but this process is exacerbated by overcharging and by excessively high charging rates. Even
worse is when a battery is left badly discharged (more about this in a minute).
Gassing and subsequent loss of electrolyte is another possible failure mode. It's caused largely by overcharging the
battery and by neglecting to check electrolyte levels from time to time. Check the vent caps once a month forelectrolyte level. If you find that every couple of months you need to add water to the battery, have the airplane's
charging system checked. It shouldn't be gassing that much, and the charging voltage may be set too high -- i.e.,
over 14.3 or 28.6 volts -- or the regulator is malfunctioning. If the charge voltage is too low, it can sulfate the
battery.
The most common cause of battery failure is sulfation. It occurs when a battery is left in a discharged state, in which
much of the plate area is covered with a fine deposit of lead sulfate. This deposit grows into larger, harder crystals of
lead sulfate that clog the spongy surface of the lead plates, acting as insulators. The crystals don't readily break up,
and so the battery loses effective plate area, and therefore capacity.
It's a very poor idea, in terms of longevity, to discharge a battery down to less than 50 percent of its capacity. In
fact, a battery that remains in a low or discharged condition for a long period of time will be permanently damaged.
Simply put, it's not so much what needs to be done as what should not be done, to get the most out of your
investment.
A special note: If you buy an aviation battery in a dry-charged state via mail order and plan to buy automotive acid
locally, don't. Auto acid is not the same specific gravity and will reduce battery life.