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There
are two basically different types of batteries in common use:
Those based on lead electrodes in sulphuric acid,
and those that use a
nickel electrode in potassium hydroxide, with the
other electrode being
cadmium or a special metal alloy that carries
hydrogen gas as the
electrode. The two batteries have different
behaviour, so I will treat
them separately.
Before we start, I would like to explain to
nonelectric people a few
important concepts:
Electric TENSION, measured in VOLT (abbreviated V),
is the "pressure"
with which the electrons try to get from one wire
into the other through
the device being powered. Tension is often called
"voltage" in the US.
Electric CURRENT, measured in AMPERE (abbreviated A,
not Amp, a common
mistake), is the amount of electrons that pass
through a given place in
a wire during a given time. A current of 1A means
that roughly
6,240,000,000,000,000,000 electrons cross a given
spot during each
second!
Electric RESISTANCE, measured in Ohm (abbreviated
normally with the
uppercase Omega sign, but can be abbreviated R when
the Omega sign isn't
available), is how much a given object opposes the
flow of current.
Since more tension will make a higher current flow
in a given
resistance, the three things are related. Ohm's law
states that the
tension divided by the resistance defines the
current.
POWER, measured in WATT (abbreviated W), can be
applied to electricity,
mechanics, heat, etc. It is the work per unit of
time. Tension
multiplied by current gives power.
Batteries are rated mainly by the average tension at
which they work,
and the amount of current they can deliver during a
certain time (given
in Amperehours, abbreviated Ah). Other ratings are
the highest current
they can deliver without excessive drop in the
tension, which is a
little subjective ("cranking amperes"), how much
charge they loose when
stored, and so on.
1) Lead-acid batteries.
Each cell has a nominal tension of 2V. The most
common incarnation of
these batteries is as a rectangular package of 6
cells, giving a 12V
battery. Capacities range typically from 2Ah to
200Ah, but smaller and
larger ones are available. The cells can have a vent
hole and screw cap
each, so that one can look inside and eventually add
water, change
electrolyte, or get a finger burned. Most older car
batteries are of
this kind. They can also have no fill holes, but
still have a small
vent. These batteries have a catalyst capsule in
each cell that reduces
water loss, so they don't need refilling during
their lifetime. Many
modern car batteries are of this type, called
"maintenance free". They
will spill some acid when turned over, which should
not be done. Others
have each cell completely sealed, so that they can
be used in any
position. Many paramotor batteries are of this kind.
They must never be
overcharged, since the pressure formed inside can
make them explode. The
last type of lead acid battery has the electrolyte
gelled into a thick
goo, and is otherwise similar to the third type.
These deliver less peak
current, so they are not good as starter batteries,
but have longer
lifetime. They are often used in alarm systems and
for backup power.
A fully charged 12V lead acid battery that has been
resting for at least
some hours tends to have a tension of around 12.8V.
The exact value
changes with temperature and acid concentration, but
not very much. When
current is drawn from the battery, there is an
immediate reduction from
these 12.8V, given by the internal resistance of the
battery's plates,
joints and the electrolyte. This resistance can be
as low as 0.005 Ohm
for a good new car battery, and would be around 0.1
Ohm for a typical
paramotor battery. This means that a SOLO starter
using 30A would make
the battery voltage drop immediately to about 9 or
10V.
When current is drained for a little longer, the
tension drops further
because of the accumulation of chemical products at
the plates. Stirring
or shaking the battery can reduce this voltage drop!
After stopping
current consumption, the resistance-induced drop
recovers instantly,
while the chemically caused drop recovers slowly, as
the chemicals mix
evenly again. As the battery is depleted, the
average chemical
constitution of the battery changes so that the
tension becomes lower
and lower. At the same time, the electrolyte becomes
depleted of acid,
increasing the internal resistance. If the tension
reaches a stable
state of around 11.8V, the battery is considered to
be discharged.
Drawing more current from it will make it go quickly
down to zero, and
the internal resistance will become so high that it
is hard to make the
battery accept charge afterwards.
A lead acid battery should not be discharged more
than 1/3 of its total
capacity, for best life, and should be recharged
promptly and stored in
fully charged condition. It should NEVER be fully
discharged.
To charge a battery, one merely applies a power
supply of higher tension
than the battery has, using the same polarity
(positive to positive).
That will make a current flow in reverse through the
battery, reversing
the chemical processes and thus recharging it. The
current will be
limited by the tension difference divided by the
internal resistance of
the battery. But since this is so low, there could
be a very high and
dangerous charging current, so it is necessary that
the charger changes
its tension according to the current. Good chargers
for lead acid
batteries produce a regulated output tension, up to
a certain maximum
current, and if the battery resistance would allow a
higher current than
this limit, they drop the tension a little so as to
hold the current at
the maximum permitted value.
The best limiting charging tension for a 12V battery
is 13.8V at room
temperature, varying by 1V for every 30 degrees
Celsius; lower
temperature requires higher charging tension. In
practice, it is not
necessary to care much for this temperature
compensation, unless the
temperature gets very low.
The limiting current should be between 1/10th and
1/5th of the battery's
Ah rating. 1/10th is a very safe value, while 1/5th
is tolerated well by
most batteries and allows faster charging. Open cell
batteries can even
be charged a lot faster at higher ratings.
A typical charger of 13.8V and 1/10th the Ah rating,
when connected to a
battery, will charge it to better than 90% in about
15 hours. The charge
state of the battery at the start of the process
matters less than what
you would expect, because the charging is fast at
first and then becomes
ever slower, so that most time is used for the last
bit of the charge.
Achieving 100% charge at 13.8V takes at least 100
hours, but is usually
not needed. If 100% charge is desired quickly, the
charge tension
should be increased to 14.2 or 14.5V, but then must
be dropped to 13.8V
as soon as the battery starts gassing. This is a bit
hard to do and not
recommended with sealed batteries.
Most car type chargers do not have any voltage
regulation at all. They
are suitable ONLY for large open cell batteries, and
require someone
watching them and unplugging them when the battery
starts gassing. Such
a charger can kill a paramotor battery quickly and
effectively.
It is fine to leave a 13.8V charger permanently
connected to a battery.
It will keep the battery in top charge and let it
achieve a long
lifetime. Anything above 13.8V should not be left
connected for much
time, and much less than 13.8V will not keep it
charged. Storing without
a charger connected is OK, but once a month a few
hours of charge should
be given, at least.
A battery that is left discharged for a long time
develops a layer of
dense lead sulphate on the plates, which is an
insulator. Such a battery
develops a very high internal resistance, and
becomes useless. Sometimes
they can be recovered in part by leaving a charger
connected for several
weeks, but they will never recover fully enough to
serve as starter
batteries. If you have ever heard the term "sulphated
battery", this is
what it means.
A lead acid battery, properly treated, can live for
3 to 4 years.
Mistreated, it can die in a few months or even
faster.
2) Nickel-Cadmium batteries:
The nominal cell tension is 1.2V. They are commonly
available as single
cells, almost always in round shape. Often they are
sold as packs of
several individual cells. Capacity varies typically
from about 0.1Ah to
7Ah, but larger and smaller ones exist. They are
always fully sealed,
with the electrolyte soaked into a fibrous
substance. An emergency valve
avoids explosion if overcharged. They are used in
all kinds of gadgets:
Radios, notebook computers, cellphones, cordless
shavers and
toothbrushes, and certainly some paramotors.
A fully charged cell has about 1.35V. It is
considered discharged at
about 1.0V, but usually recovers to almost 1.2V
after a while. The
tension falloff at the end of discharge is very
sharp, much more so than
for the lead acid battery. This implies that the
tension is quite
constant during most of the discharge cycle, staying
around 1.25V.
Individual nickel cadmium cells can safely be
discharged to zero, and
actually benefit if this is ocassionally done.
Battery packs should not
be discharged further than 1.2V below the nominal
pack tension, because
this can revert polarity on the weakest cell,
weakening it further. A
reasonably new battery can be stored in discharged
state without ill
effect, but older batteries tend to grow dendrites,
which are little
needle-like structures that perforate the fibrous
plate separator and
short the plates. The effect of this is individual
cells having zero
Volt and not accepting charge.
The internal resistance of nickel cadmium batteries
can be extremely low
and doesn't change very much with charge condition.
For this reason
usually a lower Ah rating is sufficient for
paramotor starting
applications, compared to lead acid batteries.
Nickel cadmium batteries discharge themselves faster
than lead acid
ones. After one month in storage, a good part of the
charge is lost. So
they need to be kept in permanent charge, or charged
shortly before use.
Charging this battery requires a different (and
simpler) approach than
the lead acid one: The charger merely has to put a
controlled current
through the battery, equivalent to 1/10th the Ah
rating. The tension can
vary from zero to about 1.45V per cell, and the
charger should be able
to hold the current reasonably constant over the
entire range. This will
fully charge the battery in 14 hours. If the charger
is left connected,
the current will not cause significant damage. But
the best way to keep
the battery fully charged in storage is to have two
current levels,
charging at 1/10th the Ah rating and then switching
down to about 1/50th
or 1/100th. That's enough to keep full charge and
poses absolutely no
risk of damage.
Fast charging is possible with many nickel cadmium
batteries, but
shortens battery life and requires a more complex
charger. It should be
avoided when possible.
Nickel cadmium batteries can easily live for 6 to 10
years, and are
harder to kill than lead acid ones. They are a
little lighter for a
given rating. But they are more expensive.
3) Nickel hydride batteries:
These are almost like nickel cadmium ones. The main
differences are:
Nickel hydride ones offer more capacity per size,
but have higher
internal resistance and thus offer less maximum
current (not good for
starter applications); they are reputed to have
slightly shorter
lifetime, but even my oldest ones (8 years) are
still going strong. They
don't tend towards dendrite formation. They are less
suited to fast
charging, but it can be done.
4) Lithium ion batteries:
These are the new kids on the block, being around
only for a few years.
For the moment they are reserved mostly to
cellphones and notebook
computers. They have 3.6V per cell, pretty high
capacity per size, but
are expensive. Not suitable as starting batteries.
Charging them is
pretty much like charging lead acid batteries, but
with a catch: Each
cell needs to have separate voltage regulation! They
are very easy to
kill by wrong charging.
Now, a few practical hints:
- Charging lead acid paramotor batteries from a car:
It can be done very
well by using a special charger, which takes the 12
to 14V from the car,
and converts it to a regulated 13.8V at limited
current. It is not hard
to build. I made one for about three dollars in
parts.
If you are in an emergency, you CAN charge you
battery directly from the
car, but it is not a good thing to do. If you have
to, do this: Connect
the battery to the car battery with a LONG,
relatively THIN wire.
Positive to positive, of course. 1mm diameter and
10m length may be
suitable. This wire will add enough resistance to
avoid major danger.
Don't place the wire near anything that can catch
fire, just in case.
Connect it with the engine stopped. After a while,
and making sure the
wire is not getting warm, start the engine. Check on
the car's voltmeter
to make sure you have nominal tension - many cars
don't give normal
voltage at idle speed! Leave connected for several
hours. That can of
course be done while driving. Once the battery is
reasonably charged,
the danger is over, since it will have a voltage
equal to that of the
car system and take very little current.
Don't use the cigarette lighter jack. It usually has
a 10A fuse, and
that can easily blow if the battery is discharged
enough. If you have to
use it, use an even longer wire!
Using a car headlamp between the car electrical
system and the paramotor
battery provides a reasonable safe way to charge,
but it makes the
process a bit slower. When driving on a long trip
away from other
charging possibilities, it's an option.
- Charging a nickel cadmium pack from the car: This
is easy and safe if
you have access to the midpoint of the cells! If you
split the 12V
starter pack in half and connect each 6V pack half
to the car through
properly sized resistors, all will be fine. For a
typical 12V 2.5Ah
pack, the charging voltage for each half would be
around 7V. Since the
car gives 14V while running, you have another 7V to
burn up. For 0.25A
charging current that gives 28 Ohm. 27 Ohm is the
nearest standard
value, so use that. The resistor will produce almost
2 Watt of heat, so
use at least a 3 Watt resistor to be safe. Remember
you need two, one
for each half of the pack.
To charge the paramotor battery from the parked car,
consider just 12V,
so you have 5V difference and the resistor needs to
be only 20 Ohm. A 6
Volt, 1.5 Watt flashlight bulb would be fine too.
Some nickel cadmium paramotor batteries deliver more
than 12V. If
necessary, they could be split up into three
sections for parallel
charging.
A 12V nickel cadmium pack cannot be charged by
direct connection to the
car, because the charging voltage required is higher
than the car system
voltage. But a special charger like that described
above for the lead
acid battery can be used, of course, and it doesn't
even need voltage
regulation, only current limiting!
by Manfred Mornhinweg- Chile
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