How are they made and how do they
work ?
The lead acid battery is made up of a series of identical cells
each containing sets of positive and negative plates.
In semi traction cells flat plate construction is used. Each
positive plate is a cast metallic frame which contains the lead dioxide active
material. The negative plates contain spongy lead active material. also on a
similar frame. Both plates usually have the same surface areas.
In practice a typical cell is constructed with many more plates
than just two in order to get the required current output. All positive plates
are connected together as are all the negatives. Because each positive plate is
always positioned between two negative plates, there is always one or more
negative plate than positives.
The resultant voltage of lead acid cell is normally 2 volts In
order to achieve the voltage required for the application each cell is then
connected in series by substantial metal straps to form a battery. In a typical
monoblock battery, such as that used in a car for starting, the voltage
required is 12 volts, achieved by connecting six cells together in series and
enclosing them in one plastic box. Leisure batteries where a sustained current
requirement is needed and a deep cycle, the ability to be discharged to 90%,
have a different make-up to that of a traction battery that is used in a car.
The cell containing the plates is filled with an electrolyte
made up of sulphuric acid and distilled water with a specific gravity of 1.270
at 60deg F (15.6deg C). Sulphuric acid is a very active compound of hydrogen
and sulphur and oxygen atoms. Sulphuric acid is a very reactive substance and
because of its instability it is able to distribute itself very evenly
throughout the electrolyte in the battery. Over time, this action ensures that
an even reaction can occur between all the plates. producing voltage and
current. The chemical reaction between constituent parts of the electrolyte and
the 1. spongy lead of the negative plates and 2. the lead dioxide at the positive
plates turns the surface of both plates into lead sulphate. As this process
occurs the hydrogen within the acid reacts with the oxygen within the lead
dioxide to form water. The net result of all this reaction is that the positive
plate gives up electrons and the negative plate gains them in equal numbers,
thereby creating a potential difference between the two plates. The duration of
the reactions producing the cell voltage is limited if there is no connection
between the two plates and the voltage will remain constant.
If a connection (a load) is placed between the positive and
negative plates the chemical reaction is able to continue with electrons
flowing through the circuit from the negative plate to the positive. The flow
of electrons is in fact the current produced by the cell. Only when the supply
of electrons becomes depleted i.e. when the active material on the negative
plate has been used up, and the within the electrolyte has mostly been turned
into water will the battery fail to produce any current. During the chemical
process different levels of heating can occur and the faster a battery is
exhausted the greater will be the heating and thus the efficiency of the system
will be reduced.
Care and Maintenance of Flooded Lead
Acid Batteries
The most important aspects of care
for these and all other types of batteries concern both charge and discharge as
well as the mechanical treatment of the batteries i.e. keeping them topped up
with water etc.
It is important to consider that
lead acid (pb) batteries are, as we have seen above, quite delicate chemical
factories.
Discharge
Lead acid batteries should never be
run flat. The maximum recommended discharge is 75% of the total. This means
that the battery should have a minimum of 25% of charge remaining when it is
put on charge.
Lead acid batteries once filled with
electrolyte, should always be regularly charged even if they are not in use.
When left idle a filled battery will self discharge because of its own internal
resistance. left long enough a battery can go completely flat without ever
having been put into service. Storage also affects the rate of discharge. A
battery should never be stored directly on the ground and especially not on
concrete. The best storage method is wooden pallets which do not conduct or
allow damp paths and do allow good air circulation. During storage, most
manufacturers recommend a freshening charge once every two months or so.
How to find the state of charge
It is extremely difficult to
accurately measure the state of charge of a lead acid battery and to predict
the remaining capacity.
To determine the SOC (State Of
Charge) of a lead acid battery, the classic voltmeter approach does not work
well. The terminal voltage will vary widely between batteries as a function of
things like ambient temperature and the
relative age of the battery. A full set of temperature profile tables would show big differences in the open circuit terminal voltage over a wide temperature range. This is why a good charger must incorporate a temperature
compensation network to avoid ‘over’ or ‘under’ charging the battery at different ambient temperatures. To test a lead acid battery’s SOC, the best indicator is a hydrometer. When you test a battery’s SOC with a hydrometer, you are actually measuring the amount of sulfuric acid left in the electrolyte solution. As more energy is drained from the battery, the ratio of sulfuric acid to water decreases and the created lead sulfate byproduct begins forming on the
electrode plates. A low hydrometer reading means the chemical makeup that generates the free electrons is diminished so not as much energy is stored for use.
The term ‘specific gravity’ is often used to benchmark a lead acid battery’s SOC. The specific gravity of a substance is a comparison of its density to that of water (1.000). Imagine a one gallon bottle filled with water and a second
filled with feathers. There are equal volumes of material present in both but the bottle with the feathers will weigh less than that containing the water. The resultant specific gravity value of the bottle of feathers would be less than that of the bottle of water. With lead acid batteries, the sulfur atoms break down and leach out of the electrolyte solution as it discharges. The breakdown of the electrolyte reduces its overall ‘weight’ as the sulfur is removed from the solution thus reducing the specific gravity measurement. Take a look at the table below:
relative age of the battery. A full set of temperature profile tables would show big differences in the open circuit terminal voltage over a wide temperature range. This is why a good charger must incorporate a temperature
compensation network to avoid ‘over’ or ‘under’ charging the battery at different ambient temperatures. To test a lead acid battery’s SOC, the best indicator is a hydrometer. When you test a battery’s SOC with a hydrometer, you are actually measuring the amount of sulfuric acid left in the electrolyte solution. As more energy is drained from the battery, the ratio of sulfuric acid to water decreases and the created lead sulfate byproduct begins forming on the
electrode plates. A low hydrometer reading means the chemical makeup that generates the free electrons is diminished so not as much energy is stored for use.
The term ‘specific gravity’ is often used to benchmark a lead acid battery’s SOC. The specific gravity of a substance is a comparison of its density to that of water (1.000). Imagine a one gallon bottle filled with water and a second
filled with feathers. There are equal volumes of material present in both but the bottle with the feathers will weigh less than that containing the water. The resultant specific gravity value of the bottle of feathers would be less than that of the bottle of water. With lead acid batteries, the sulfur atoms break down and leach out of the electrolyte solution as it discharges. The breakdown of the electrolyte reduces its overall ‘weight’ as the sulfur is removed from the solution thus reducing the specific gravity measurement. Take a look at the table below:
State
of Charge as related to Specific
Gravity and Open-Circuit Voltage |
||
State
Of Charge
|
Specific
Gravity (sg)
|
Open-Circuit
Voltage (approximate) |
100%
|
1.265
|
12.63
|
75%
|
1.210
|
12.30
|
50%
|
1.160
|
12.00
|
25%
|
1.120
|
11.76
|
0%
|
1.100
|
11.64
|
Great care should be taken to avoid
discharging a battery beyond the 75% SOC point. Once the specific gravity drops
below the 1.210 level, excessive sulfate deposits form on the electrode plates.
This process is called ‘sulfation’ and leads to the hardening of the electrode
plates. If the battery is kept in a low charge state for long a period of time,
the sulfation process will eventually reduce the ability of the battery to
generate ion charge carries to the point that it no longer provides the needed
power. This point is otherwise known as a DEAD BATTERY!
When you recharge the battery, the process is reversed and the sulfur returns to the electrolyte solution. Proper cycling of the battery will ensure a long and functional life. If the battery is abused by allowing sulfation of the electrode plates on a regular basis or over an extended period of time, the charging process will not be able to restore the battery to its former full potential. Time to make a costly battery replacement!
When you recharge the battery, the process is reversed and the sulfur returns to the electrolyte solution. Proper cycling of the battery will ensure a long and functional life. If the battery is abused by allowing sulfation of the electrode plates on a regular basis or over an extended period of time, the charging process will not be able to restore the battery to its former full potential. Time to make a costly battery replacement!
Sulfation is mainly due to
undercharging. Here the sulfate ions have entered into deep bonds with the lead
on the cell's plates. The sulfate ions can bond with the lead at three
successively deeper energy levels. Level One is the bond we use when we
normally charge and discharge the cell. After a month or so at Level One, some
of the bonds form Level Two bonds which require more electric power to break.
After several months of being at Level Two bond, the sulfate ions really cozy
up to the lead and form Level Three bonds. Level Three bonds are not accessible
electrically. No amount of recharging will break Level Three bonds. The longer
the lead sulfate bond stays at a level the more likely it is to form a closer
acquaintance and enter the next deeper level. This is why it is so important to
fully, regularly, and completely, recharge lead-acid cells.
Equalization Charges
If the loss in capacity is due to Level Two bonding, then a repeated series of equalizing charges will break the Level Two bonds. Under equalization the Level Two bonds will first be transformed into Level One bonds, and then the sulfate ion can be kicked loose of the lead entirely and reenter the electrolyte solution. If your lead-acid cells have lost capacity, then a regime of equalizing charges is the first procedure to try. An equalization charge is a controlled overcharge of an already fully recharged cell. First recharge the cell and then continue to charge the cell at a C/20 rate for five to seven hours. During equalization charges, the cell voltage will become very high, about 2.7 VDC per cell. This overcharge contains the necessary power to break the Level Two bonds and force them to Level One. Once they reach Level One, the bond is easily broken and the sulfate ions reenter into solution in the electrolyte.
If the loss in capacity is due to Level Two bonding, then a repeated series of equalizing charges will break the Level Two bonds. Under equalization the Level Two bonds will first be transformed into Level One bonds, and then the sulfate ion can be kicked loose of the lead entirely and reenter the electrolyte solution. If your lead-acid cells have lost capacity, then a regime of equalizing charges is the first procedure to try. An equalization charge is a controlled overcharge of an already fully recharged cell. First recharge the cell and then continue to charge the cell at a C/20 rate for five to seven hours. During equalization charges, the cell voltage will become very high, about 2.7 VDC per cell. This overcharge contains the necessary power to break the Level Two bonds and force them to Level One. Once they reach Level One, the bond is easily broken and the sulfate ions reenter into solution in the electrolyte.
Battery capacity is not comparable
to a tank full of petrol. A filled petrol tank contains a finite amount of
energy which can be used either slowly or fast according to the energy
required. Battery capacity is not so simple.
In a battery, the rate of which
energy is drawn affects the overall amount of energy available from the
battery. For example , a 100 Ah battery rated at 20 hour rate means that over
20 hours there are 100 Ah available, or to put it another way you can expect to
draw up to 5 Amps per hour for up to 20 hours, 20x5=100Ah.
If you try to draw the battery down
more quickly you cannot expect to get the same amount of Ah from it. For
example if you draw it down over just one hour the approximate capacity will
become 100 x 0.59 which =59Ah. Putting this another way means that you can
connect a load on the battery which will draw 59 Amps for just one hour. If you
discharge over just half an hour you can only expect to get around half (47%)
the capacity from your batteries.
With most uses the rate at which a
battery is discharged varies enormously , you can see that any battery
condition indicator has to be quite a clever piece of equipment if it is ever
going to get close to giving an accurate reading.
Charging Lead Acid Batteries
Battery vent caps should always be
kept in place and tight during both charging and discharging.
For best battery life, i.e. greatest
number of charge/discharge cycles and years service most battery manufacturers
recommend that you should aim to recharge the batteries when they have reached
around 50% discharge. This level of discharge, of course, must be measured
according to the rate at which the battery is discharged, which as we have
already seen, varies the available total capacity of the battery.
In addition, some battery
manufacturers specify that best life will be achieved only if the batteries are
discharged sufficiently for a 4 hour bulk charge to take place before the
batteries are fully recharged.
A reasonable rule of thumb is that
you should aim to charge the batteries only when they are between 70% and 40%
discharged. If you charge them then they are only lightly discharged i.e. less
than 40% you will end up boiling them unnecessarily which wastes energy in the
form of heat and gassed off hydrogen and in turn shortens the life of the batteries.
In effect the batteries are being overcharged which can cause degradation and
buckling of the plates. In the process some active material is forced off the
plates and drops down to the bottom of the battery. If this occurs frequently
the eventual result is a build up of a bridge between the plates which in turn
can cause a possible short across the plates. This situation leads to the
destruction of a cell which then reduces the capacity of the battery.
To confuse matters further, a
battery will operate at its most efficient the deeper it is discharged, up to
around 75%. The bulk phase of the charge cycle is the most efficient and is
proportionately longer the deeper the discharge.
There are many battery chargers
available. Today, manufacturers supply automatic chargers which are supposed to
ensure optimum battery life. there are numerous charge profiles available and
now, with the advent of the electronic switch mode chargers, it is possible to
have a fully programmable charger on board your car capable of charging almost
any type of battery.
Lead acid batteries must be charged
carefully. If the charge is too violent and uncontrolled the batteries can
overheat and cause thermal run-away which can result in a possible explosion.
Too gently charging will take too
long to get the batteries fully charged with the result that the batteries will
end up being used in an under charged state eventually leading to premature
failure due to sulphation.
The latest electronic chargers
mostly make us the IUI charge profile for standard flooded lead acid batteries.
This means that the current drawn by the batteries is allowed to flow at a
constant (I) rate while the voltage is allowed to rise of its own accord, which
it will do as the battery starts to be charged up. This first part of the
charging cycle is known as the bulk charge phase. When a preset voltage has
been reached, normally the voltage at which the batteries just start to gas,
the charger will switch into the constant voltage (U) phase and the current
drawn by the battery will gradually drop until it hits another preset level.
This second part of the cycle is really the finish charge where the battery
just topped up to the brim very carefully at a much gently diminishing rate.
Finally the charger will switch again into the constant current mode (I) and
the voltage is allowed to rise again, up to a new higher preset limit, in order
to achieve a successful equalization charge.
The Bulk Charge
In this first part of the charge the
battery is allowed to have a large draw on the available current. Usually the
limit to this current level is determined by the availability of a suitably
sized mains outlet, especially on large batteries. It is however, worth noting
that that the life of a battery will be greatest if even this first bulk phase
of charging is started off gently and the maximum current is limited. If the
current is too high the result will be excess heating within the battery which
is wasteful and could lead to buckling of the plates and destruction of the
battery. Sizing of the charger to suit the batteries is important.
Finish charge
Once the bulk phase has been
completed, the finish phase commences and the battery charge is topped off.
This phase is very important. If the battery is not topped up gently it will
overflow in the form of waste heat and violent gassing of the plates which
again can lead to the plates buckling and the battery being destroyed. If the
battery is not topped up fully, it will become sulphated after only a few charges
and the result will be premature failure.
Equalization
In any cyclic application, a series
of batteries will always need to be equalized from time to time in order to
ensure that the battery cells remain at the same voltage throughout the pack.
No two battery cells or batteries
are created equal. During both charge and discharge each and every cell/battery
will react in a minutely different way to its neighbour. This could mean that
each battery may be holding a different quantity of charge. In order to get the
most out of the total battery pack it is necessary to make sure, as far as
possible, that each and every battery is holding a similar amount of charge.
During the charge cycle the voltages
of the different batteries will very. In order to bring them all to the same
level it is necessary to give some a slight overcharge in order to bring the
other up to full charge.
Equalization is done by allowing the
voltage to rise while allowing a small constant current to the batteries. The
voltage is allowed to rise above the normal finish voltage in order to allow
the weaker batteries/cells to draw more current. The stronger batteries will
not be adversely affected providing the current is gently and the period and
frequency of overcharging are not too high and great respectively. The stronger
batteries will absorb the overcharge by giving off heat by gently boiling and
gassing more heavily. Once the weaker batteries have absorbed the required
current, the equalization charge can be halted. The equalization time should be
long enough to bring all the batteries up to a full state of charge. As the
time factor will very the most reliable way to check the charge states is by a
voltmeter on each cell or individual battery.
Really sophisticated battery
charging and monitoring systems do not require the use of an equalization
charge and are able to charge all the batteries fully including the weaker ones
without overcharging the strong ones.
In these systems, each battery is
fitted with an electronic clamp, which gradually reduces the amount of charge
going into the fully charged batteries as the finish charge progresses. This
means that the weaker batteries receive more current to bring them up to a full
state of charge and the strong batteries are prevented from being overcharged
unnecessarily. The drawback with these sophisticated systems is their cost. The
price of each battery clamp can be in the order of 1/5 the cost of each
battery.
Watering
Traction and semi traction batteries are generally supplied
with removable vent caps so that they can be kept topped up with water. The
action of charging the batteries causes gassing when a certain voltage is
reached, usually somewhere around 2.35 to 2.4 volts per cell. The result is
that water is depleted of its constituent parts by liberation of the Hydrogen
gas plates. Hydrogen is of course much less dense that air and the electrolyte
and consequently floats out of the batteries at the earliest opportunity. This
water must therefore be replenished from time to time. If you do not gas a
battery the chances are that it will become sulphated due to fact that not all
the sulphate will be fully removed during the non gassing phase of charging.
It is very important to note that only very pure water is
suitable for topping up. It must contain no mineral traces and especially no
metallic solids, especially iron. The most suitable form of water is distilled
water or water that has been chemically treated and demineralised. Only these
types of water should ever be used to top up the batteries. Tap water will
quickly corrode the battery and should never be used. The frequency of topping
up required for the batteries depends on how they are used. If the are
frequently and heavily discharged they will need to be topped up regularly.
Perhaps every two weeks to a month.
If the batteries are not charged and discharged frequently then
it is likely that they will not require topping up for longer periods of time,
say once every two or three months or so.
If the batteries are regularly charged after only short
discharges they will use much more water than normally. This type of treatment
of the batteries should be avoided. It is most important to make sure that the
tops of the plates in a battery never become exposed. They should always be
covered by electrolyte or they will quickly sulphate and the battery will fail.
If watering is too much of a chore, It is possible to fit either automatic
watering systems or catalytic caps. With the former, the battery vent caps are
replaced with special caps, which incorporate float valves.
A series of hoses connect the float caps to a single filling
point. The filling point can be arranged so that distilled water is
poured into it from time to time or a reservoir can be fitted to constantly top
up the batteries. The catalyst caps prevent the battery charging gases from
venting to atmosphere and by chemical action combine the gases back into water.
Thus there is little or no water loss. Both the above systems work but there
are drawbacks. Lots of hoses interconnecting battery cells and batteries can
pose a serious safety threat if one of the cells goes into thermal run-away and
the vent caps block and the tubes have no water in them. The result is that
ignition from one cell will almost instantaneously result in ignition in all
the rest of the cells as the tubes will fill very quickly with hydrogen gas
which is extremely combustible and explosive, i.e. like the H bomb! A study by
Lucas back in the 1970s showed what can happen. In a demonstration, two
batteries were connected via their vent float caps by a 3-mm bore tube of a
length of over 60-ft. Combustion was artificially initiated in one battery and
within an almost immeasurably short time combustion occurred in the second
battery. The moral of this tale is that only vent float caps in first class
condition should be used and at around £5-00 to £8-00 each they are not cheap!
In addition, should the tubes ever become blocked or the floats fail to operate
correctly, it is highly likely that the batteries will not receive the correct
supply of water and the result will be failure of at least one cell.
The catalyst caps are possibly a better bet. They are however,
even more expensive and do not last for ever. Most sealed batteries make use of
a built in catalyst system.
The cheapest method of watering is by hand. It is also the most
reliable provided it is done regularly.
Temperature
Temperature affects charging as well as discharge. As a rough
rule of thumb, the cooler the temperature the more charge a battery will absorb
and the warmer the temperature the more it will discharge. There are of course
limits. However it is senseless to charge a battery that has just been
discharged fast and has been left out in the sun. In these circumstances it
must be allowed to cool before charging is initiated. The ideal charging
temperature varies but a reasonable guide is between 15 and 25 degrees
Centigrade. Ideal discharge temperatures are between 20 and 30 degrees
centigrade. Unfortunately discharging a battery at a higher temperature, while
desirably affecting its performance, will adversely affect its life. In fact,
so far as I can determine almost every battery manufacturer can work out the
number of Ah which can be drawn from any battery over its life under given
constant discharge conditions. There does appear to be a finite number of Ah
that can be drawn from a battery for a given discharge profile. Predicting this
discharge profile is the difficult bit!
No comments :
Post a Comment