understanding motorcycle batteries
Posted: Fri Jan 09, 2009 5:22 pm
I thought the group might like to read this.
Tom
SW Pennsylvania
http://www.yuasabatteries.com/motor_battery.asp
UNDERSTANDING MOTORCYCLE BATTERIES
By Stu Oltman - Technical Editor, Wing World Magazine
Reprinted with permission
How Batteries Are Made
A 12-volt motorcycle battery is made up of a plastic case containing
six cells. Each cell is made up of a set of positive and negative
plates immersed in a dilute sulfuric acid solution known as
electrolyte, and each cell has a voltage of around 2.1 volts when
fully charged. The six cells are connected together to produce a fully
charged battery of about 12.6 volts.
That's great, but how does sticking lead plates into sulfuric acid
produce electricity? A battery uses an electrochemical reaction to
convert chemical energy into electrical energy. Let's have a look.
Each cell contains plates resembling tiny square tennis racquets made
either of lead antimony or lead calcium. A paste of what's referred to
as "active material" is then bonded to the plates; sponge lead for the
negative plates, and lead dioxide for the positive. This active
material is where the chemical reaction with the sulfuric acid takes
place when an electrical load is placed across the battery terminals.
How It Works
Let me give you the big picture first for those who aren't very detail
oriented. Basically, when a battery is being discharged, the sulfuric
acid in the electrolyte is being depleted so that the electrolyte more
closely resembles water. At the same time, sulfate from the acid is
coating the plates and reducing the surface area over which the
chemical reaction can take place. Charging reverses the process,
driving the sulfate back into the acid. That's it in a nutshell, but
read on for a better understanding. If you've already run from the
room screaming and pulling your hair, don't worry. Maybe next month's
topic will be more to your liking.
The electrolyte (sulfuric acid and water) contains charged ions of
sulfate and hydrogen. The sulfate ions are negatively charged, and the
hydrogen ions have a positive charge. Here's what happens when you
turn on a load (headlight, starter, etc). The sulfate ions move to the
negative plates and give up their negative charge. The remaining
sulfate combines with the active material on the plates to form lead
sulfate. This reduces the strength of the electrolyte, and the sulfate
on the plates acts as an electrical insulator. The excess electrons
flow out the negative side of the battery, through the electrical
device, and back to the positive side of the battery. At the positive
battery terminal, the electrons rush back in and are accepted by the
positive plates. The oxygen in the active material (lead dioxide)
reacts with the hydrogen ions to form water, and the lead reacts with
the sulfuric acid to form lead sulfate.
The ions moving around in the electrolyte are what create the current
flow, but as the cell becomes discharged, the number of ions in the
electrolyte decreases and the area of active material available to
accept them also decreases because it's becoming coated with sulfate.
Remember, the chemical reaction takes place in the pores on the active
material that's bonded to the plates.
Many of you may have noticed that a battery used to crank a bike that
just won't start will quickly reach the point that it won't even turn
the engine over. However, if that battery is left to rest for a while,
it seems to come back to life. On the other hand, if you leave the
switch in the "park" position overnight (only a couple of small lamps
are lit), the battery will be totally useless in the morning, and no
amount of rest will cause it to recover. Why is this? Since the
current is produced by the chemical reaction at the surface of the
plates, a heavy current flow will quickly reduce the electrolyte on
the surface of the plates to water. The voltage and current will be
reduced to a level insufficient to operate the starter. It takes time
for more acid to diffuse through the electrolyte and get to the
plates' surface. A short rest period accomplishes this. The acid isn't
depleted as quickly when the current flow is small (like to power a
tail light bulb), and the diffusion rate is sufficient to maintain the
voltage and current. That's good, but when the voltage does eventually
drop off, there's no more acid hiding in the outer reaches of the cell
to migrate over to the plates. The electrolyte is mostly water, and
the plates are covered with an insulating layer of lead sulfate.
Charging is now required.
Self Discharge
One not-so-nice feature of lead acid batteries is that they discharge
all by themselves even if not used. A general rule of thumb is a one
percent per day rate of self-discharge. This rate increases at high
temperatures and decreases at cold temperatures. Don't forget that
your Gold Wing, with a clock, stereo, and CB radio, is never
completely turned off. Each of those devices has a "keep alive memory"
to preserve your radio pre-sets and time, and those memories draw
about 20 milliamps, or .020 amps. This will suck about one half amp
hour from your battery daily at 80 degrees Fahrenheit. This draw,
combined with the self-discharge rate, will have your battery 50
percent discharged in two weeks if the bike is left unattended and
unridden.
When A Battery Is Being Charged
Charging is a process that reverses the electrochemical reaction. It
converts the electrical energy of the charger into chemical energy.
Remember, a battery does not store electricity; it stores the chemical
energy necessary to produce electricity.
A battery charger reverses the current flow, providing that the
charger has a greater voltage than the battery. The charger creates an
excess of electrons at the negative plates, and the positive hydrogen
ions are attracted to them. The hydrogen reacts with the lead sulfate
to form sulfuric acid and lead, and when most of the sulfate is gone,
hydrogen rises from the negative plates. The oxygen in the water
reacts with the lead sulfate on the positive plates to turn them once
again into lead dioxide, and oxygen bubbles rise from the positive
plates when the reaction is almost complete.
Many people think that a battery's internal resistance is high when
the battery is fully charged, and this is not the case. If you think
about it, you'll remember that the lead sulfate acts as an insulator.
The more sulfate on the plates, the higher the battery's internal
resistance. The higher resistance of a discharged battery allows it to
accept a higher rate of charge without gassing or overheating than
when the battery is near full charge. Near full charge, there isn't
much sulfate left to sustain the reverse chemical reaction. The level
of charge current that can be applied without overheating the battery
or breaking down the electrolyte into hydrogen and oxygen is known as
the battery's "natural absorption rate." When charge current is in
excess of this natural absorption rate, overcharging occurs. The
battery may overheat, and the electrolyte will bubble. Actually, some
of the charging current is wasted as heat even at correct charging
levels, and this inefficiency creates the need to put more amp hours
back into a battery than were taken out. More on that later.
How Long Will My Battery Last?
There are many things that can cause a battery to fail or drastically
shorten its life. One of those things is allowing a battery to remain
in a partially discharged state. We talked about sulfate forming on
the surface of the battery's plates during discharge, and the sulfate
also forms as a result of self-discharge. Sulfate also forms quickly
if the electrolyte level is allowed to drop to the point that the
plates are exposed. If this sulfate is allowed to remain on the
plates, the crystals will grow larger and harden till they become
impossible to remove through charging. Therefore, the amount of
available surface area for the chemical reaction will be permanently
reduced. This condition is known as "sulfation," and it permanently
reduces the battery's capacity. A 20 amp hour battery may start
performing like a 16 amp hour (or smaller) battery, losing voltage
rapidly under load and failing to maintain sufficient voltage during
cranking to operate the bike's ignition system. This last condition is
evident when the engine refuses to fire until you remove your finger
from the start button. When you release the starter, the battery
voltage instantly jumps back up to a sufficient level. Since the
engine is still turning briefly, the now energized ignition will fire
the spark plugs. In the next installment, we'll see exactly why
increased internal resistance due to sulfation causes less power to be
delivered to the starter.
Deep discharging is another battery killer. Each time the battery is
deeply discharged, some of the active material drops off of the plates
and falls to the bottom of the battery case. Naturally, this leaves
less of the stuff to conduct the chemical reaction. If enough of this
material accumulates in the bottom of the case, it'll short the plates
together and kill the battery.
Overcharging is an insidious killer; its effects often aren't apparent
to the innocent purchaser of the ten-dollar trickle charger who leaves
it hooked to the battery for extended periods. A trickle charger
charges at a constant rate regardless of the battery state of charge.
If that rate is more than the battery's natural absorption rate at
full charge, the electrolyte will begin to break down and boil away.
Many a rider has stored a bike all winter on a trickle charger only to
find the battery virtually empty in the spring. Also, since charging
tends to oxidize the positive plates, continued overcharging can
corrode the plates or connectors till they weaken and break.
Undercharging is a condition that exists on many Gold Wings. Your
voltage regulator is set to maintain your system voltage at around 14
to 14.4 volts. If you're one of those folks who rides the interstate
highways with your voltmeter showing only 13.5 volts because you're
burning more lights than Macy's Christmas display, you should be aware
that that voltage is sufficient to maintain a charged battery but
insufficient to fully recharge a depleted one. Remember, we said that
gassing occurs when all or most of the lead sulfate has been converted
back to lead and lead dioxide. The voltage at which this normally
occurs, known as the gassing voltage, is normally just above 14 volts.
If your system voltage never gets that high, and if you don't ever
compensate by hooking up to a charger at home, the sulfate will begin
to accumulate and harden just as plaque does in your mouth. Consider a
thorough occasional charging to be like a good job of flossing and
brushing your teeth. If you practice poor dental hygiene, you can go
to the dentist, and have him blast and scrape at the yucky stuff. When
your battery reaches that stage, it's curtains!
What Type Of Charger, And Why
Your alternator and a standard automotive taper charger have a lot in
common; they seek to maintain a constant voltage. Here's the problem
with trying to quickly charge a deeply discharged battery with either
one. Remember, we discussed how a heavy current draw would make a
battery appear dead. Then, as the acid diffused through the cells, the
concentration at the plates' surface would increase and cause the
battery to spring back to life.
In similar fashion, the voltage of a battery during charge increases
due to the acid concentration that occurs at the plates' surface. If
the charge rate is significant, the voltage will rise rapidly. The
taper charger or vehicle voltage regulator will taper the charge rate
drastically as the voltage rises above 13.5, but is the battery state
of charge commensurate with the voltage? No! Once again, it takes time
for the acid to diffuse throughout the cells. Although the voltage may
be high, the electrolyte in the outer reaches of the cells is still
weak, and the battery may be at a much lower state of charge than the
voltage would indicate. Only after charging for an extended period at
the reduced current will the full capacity be reached. This is the
reason you must not judge a battery's state of charge by measuring
voltage while charging. Test it only after allowing the battery to sit
for at least an hour. The voltage will reduce and stabilize as the
acid diffuses throughout the cells.
Within the past several years, several companies have developed
chargers that can charge a depleted battery quickly, and then hold the
battery at a voltage that will neither cause it to gas nor allow it to
self-discharge. These are sometimes referred to as "smart chargers" or
multi-stage chargers. Here's how they work.
We said that a battery could accept a much higher rate of charge when
it's partially depleted than when it's near full charge. These
multi-stage chargers take advantage of that fact by beginning the
charge in a constant current, or "bulk charge" mode. Typically, they
provide a charge rate of between 650 milliamps and 1.5 amps, depending
on make and model. This bulk charge is held constant (or should be)
till the battery voltage reaches 13.5 volts, thus allowing the battery
to absorb a larger amount of charge in a short time and without
damage. The charger then switches to a constant voltage or
"absorption" charge. The idea here is to allow the battery to absorb
the final 15 percent of its charge at its natural absorption rate to
prevent undue gassing or heating. Finally, these chargers switch to a
"float" mode in which the battery voltage is held at a level
sufficient to keep it from discharging but insufficient to cause
overcharging. The various companies disagree generally on what this
float voltage should be, but it's usually between 13.2 and 13.4 volts.
Actually, the float voltage should be temperature compensated between
13.1 volts at 90 degrees Fahrenheit to 13.9 volts at 50 degrees. Most
of the very expensive high power multi-stage chargers for use on
larger RV batteries are temperature compensated, but none of the
motorcycle units are to my knowledge; they use a compromise float setting.
So, I can just set it and forget it, right? Well, not exactly. For one
thing, you need to monitor the battery occasionally for correct fluid
level (unless you own a sealed battery). Another problem is that of
exercising the battery. Even if held at 13 volts, the unwavering
voltage will allow the battery to eventually begin to sulfate. With
most of these units, I recommend that you unplug the charger at least
once every 60 days during seasonal storage. Allow the battery to rest
for a couple of days, and then plug the charger in again. One charger
that I'm aware of, the 1.5 amp Yuasa unit, has a feature found mainly
on the aforementioned high priced RV chargers. It drops off the float
charge and sends the battery through a complete new charge cycle every
28 days, thus eliminating the need to do that manually. There may be
other motorcycle units that do that, but I'm not aware of any.
Still Here?
If you're still reading this, you're a real trooper. I realize that
the subject can be confusing or even boring, but take heart; I went
easy on you. There's far more left untold than what appears here. This
was "Battery's Greatest Hits." I hope that it was enough to get you
interested without sending you into information overload, and, maybe,
now that you know how many ways there are to shorten a battery's life,
you know why no one can predict how long a battery will last. A lot of
riders who believe they take excellent care of their batteries are
actually killing them with kindness.
Dooden
A15 Green Ape
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