|
FAST AND SLOW,
CHARGE AND DISCHARGE
The storage capacity of a lead-acid (automotive type) battery is not
fixed, but varies according to how slowly the battery is discharged.
Similarly, the charge capacity of a battery is not fixed, but varies on
how slowly a battery is charged. With batteries, the slower the
charge the better!
When a battery is charged or discharged, this initially affects only the
chemicals closest to the gap between the battery plates and the
electrolyte. With time, the charge stored in the chemicals spreads by
diffusion* of these chemicals throughout the battery.
* Diffusion describes the spread of
particles through random motion from regions of higher concentration to
regions of lower concentration.
Fast Charging
(Not the best for a battery)
If a battery has been completely discharged (e.g. the car lights were
left on overnight) and next is given a FAST charge for only a few
minutes, then during the short charging time there will only be a charge
nearest the battery plates. Charging a battery too fast can also
cause a dangerous condition called Thermal Runaway** If using a high-amperage battery charger (or
even charger with monitoring electronics set to a high amperage
setting), the charger's meter will falsely show that the battery is
fully charged. Why? Because after a few hours this charge
will spread to the volume of the plates and electrolyte, resulting in a
charge that may be so low as to be incapable of starting a vehicle.
**
Thermal Runaway is a very dangerous condition that can occur if
batteries are charged too fast. One of the byproducts of Gassing are
Oxygen and Hydrogen. As the battery heats up, the gassing rate increases
as well and it becomes increasingly likely that the Hydrogen around it
will explode.
Slow Charging
(Best for the battery)
On the other hand, if the battery is given a SLOW charge using a low-amperage float charger, the charge will certainly take longer, but the
battery will become more fully charged. During a slow charge, the charge
current has time to distribute to the entire volume of the plates and
electrolyte, while continuously being replenished by the battery float
charger. The battery voltage will remain below the charger's voltage
throughout the charging process allowing charge current to flow into the
battery until the battery is truly fully charged, and the float
charger stops charging the battery. Article continued below... |
|
|
|
...continued from
above.
Fast Discharge
A similar occurrence happens when a battery is quickly
discharged. We have all experienced the situation of continuously
cranking the engine, trying to start our car. Eventually,
the battery dies - so it appears. But when we come back an hour
later, the battery has sufficient charge to crank the engine a few more
times.
Why does this happen? If a battery is subject to a fast discharge (such
as starting a car with a current draw of 100 amps or more) for a few
minutes, the battery will appear to go dead. However, it may have only
lost its charge nearest the plates. If the discharge is stopped for
awhile, the battery may resume normal operation at the appropriate
voltage and power.
Slow Discharge
On the other hand, if a battery is subject to a slow, deep discharge
(such as leaving the car lights on, a current draw of 7 amps or less)
for hours, then any observed reduction in battery performance will
likely be permanent. |
|
|
|
HOW DOES A FLOAT CHARGER WORK?
A float charger
(also called a storage charger, maintenance charger, or smart charger) will charge a battery at a
similar rate as a battery self-discharges, thus maintaining a full capacity
battery. However, the main difference between a trickle charger and a
float charger is that a float charger has circuitry to prevent battery
overcharging.
Below are two examples of the internal printed circuit board
of a battery float charger.
A. These
wires carry the 6 or 12 volt DC output current that goes to the battery.
B. This is
the printed circuit board that contains the "brains" of the float charger.
This circuit board will do many things. One, it will reduce the input
voltage of 110 volts AC (alternating current) to 6 or 12 volts DC (direct
current)*. Two, it will continuously monitor the battery's charge
status; fully-charged or undercharged. Third, if a battery becomes
undercharged, it will automatically turn the charging cycle on.
* The output current to charge a 12 volt
battery is actually around 13.8 volts DC. But for simplicity, we'll state 12 volts.
C. These wires carry
the 110 volt AC (house current) into the printed circuit board. The
voltage will later be reduced by the circuitry.
D. These wires go to
the selector switch to allow the user to switch between 6 or 12 volt battery
charge.
E. These LED
indicators let the user know the battery's charge or charging status.
Some float chargers use 1 indicator to show that there is power going into
the PC board. Other chargers us multiple or multi-color indicators to
indicate power to the PC board as well as the on / off state of charger
going to the battery.
Here is another example of the internal printed circuit board
of a battery float charger.
A float charger senses when a battery voltage is at the
appropriate float level and temporarily ceases charging; it maintains the
charge current at zero or a very minimal level until it senses that the
battery output voltage has fallen, and then resumes charging. It is
important to note that the appropriate float voltage varies significantly
with the construction of the battery and the ambient temperature. With the
appropriate voltage for the battery type and with proper temperature
compensation, a float charger may be kept connected indefinitely without
damaging the battery.
NOTICE! As with
the use of any type of battery charger, it is best to periodically check the
electrolyte (fluid level) of your battery. On most batteries, you can
remove the vent caps to add water. However, on some maintenance-free batteries,
you can't. If the water level
should drop below the top of the cells, add sufficient distilled water to
cover the cells. Do not use tap water. Some maintenance-free
batteries are sealed and water cannot be added to them.
|
|
HOW CAN BATTERY LIFE BE INCREASED?
Battery manufacturers define the end-of-life of a battery when it
can no longer hold a proper charge (for example, a cell has shorted) or when
the available battery capacity is 80% or less than what the battery was
rated for. The life of lead acid (automotive-type) batteries is usually
limited by several factors. Some you can't control. Some you can.
■ Cycle Life is a measure of how many
charge and discharge cycles a battery can take before its lead-plate
grids/plates are expected to collapse and short out. There's not much
you can do about the life cycle of a battery. When you need to use
your battery, you should use it. That's why you bought it.
■ Age also affects batteries as the
chemistry inside them attacks the lead plates. As with battery life cycle,
there's nothing you can do about the age of a battery - it is what it is.
But lead-acid batteries like to
be kept fully charged, all the time. If you don't plan to use your
battery for weeks, months or years, try to keep it a cool/cold place. Since
lead-acid batteries will not
freeze if kept fully charged, store your battery on a float charger, in
a cool/cold place to maximize its life.
■ Construction. The thicker the
lead plates, the more abuse, charge and discharge cycles they can take. The
heavier the battery for a given group size, the thicker the plates, the
longer it will last. So
you can use weight as one guide to buying lead-acid batteries. The heavier,
the better.
■ Sulphation is a constant threat to
batteries that are not fully re-charged. A layer of lead sulphate can form
in these cells and inhibit the electro-chemical reaction that allows you to
charge/discharge batteries. Many batteries can be saved from the
recycling heap if they are kept fully charged. A battery float charger
will keep your battery fully charged, without over charging. Also,
make sure you periodically check the fluid level of your battery (if
possible). The fluid level should always cover the internal plates.
|
|
WHAT IS BATTERY
SELF-DISCHARGE?
Battery self-discharge is when a battery's charge slowly
decreases over time, with no load on the battery. "No load" means that
there is no power being consumed from the battery as would a vehicle's
clock, radio display, computer, newer alarm systems, and the like.
The battery self-discharge rate is a measure of how much batteries
discharge on their own. The self-discharge rate is governed by the
construction of the battery and the metallurgy of the lead used inside.
For instance, flooded cell batteries as used for automotive applications, typically use lead alloyed with Antimony* to
increase their mechanical strength. However, the Antimony also increases the
self-discharge rate to 8-40% per month. This is why flooded lead-acid
batteries should be used often, or if this is not possible, be left on a
float-charger.
* Antimony is a metallic element having four allotropic forms, the most
common of which is a hard, extremely brittle, lustrous, silver-white,
crystalline material. It is used in a wide variety of alloys, especially
with lead in battery plates.
|
|
|
Did you
know...
Charge current from a battery charger, needs to match the
ability of the battery to absorb the energy. Using too large of
a charge current on a battery can lead to boiling and venting of
the electrolyte*.
In the above graphic, the battery case has bulged due to the
high gas pressure developed during overcharge.
Use of a low amperage float charger
would have avoided this problem.
* Electrolyte is the water
and sulfuric acid mixture in the battery. |
CLICK
PHOTO TO
ENLARGE
Did you know....
There is a safe way to connect a float charger (or any battery
charger) to a vehicle's battery.
First of all, what
do we mean by "safe way"? What we want is to avoid a spark
near the battery which could ignite hydrogen gas emitted by the
battery, and possibly cause the battery to explode. A spark could
be generated when the charger's clamps are connected to the
battery.
Typically, float
chargers of less than 1 amp would not cause a spark, but
chargers of 2 or more amps could - and it pays to be safe.
First, connect the
RED clamp from the float
charger to the positive
terminal of your battery. Second, connect the BLACK
clamp from your float charger to any negative grounded location in your
engine compartment (except the negative terminal of your battery
cable)*. In the above photo, the negative clamp from the
float charger is connected to a nearby nut & bolt (click to
enlarge).
Lastly, plug the
float charger into the wall outlet.
In this case, if
there were a spark from the float charger, there is a good
chance that the spark would not be at the battery's terminal.
* These steps would
be for negative grounded vehicles as were most vehicle built
after the 1950s. |
|
Do you know....
Do you know the voltage of your battery?
How can you determine if your battery is 6 volt, 8 volt or 12 volt?
Simply count the number of cells. You can do this by counting
the number of round individual cell caps, or removing the
'strip' type cell vent covers and counting the access holes for
each cell. Each cell accounts for 2 volts (actually 2.2 volts).
So...
3 cells = 6 volt battery
4 cells = 8 volt battery
6 cells = 12 volt battery
and so on.
Do NOT use an 8 volt charger on a 6 volt battery. Bad things
will happen. Do not use a 6 volt charger on an 8 volt battery,
it will actually discharge it.
Click below for hard-to-find
8 volt float charger. |
|
