Good Engineering Practice/Batteries

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• LEAD-ACID BATTERIES
• INTRODUCTION
• BASIC INFORMATION
• CHEMICAL PRINCIPLE
• FACTORS AFFECTING LEAD-ACID BATTERY LIFE
• LEAD-ACID BATTERY RATINGS
• CHARGE STATE
• PRECAUTIONS
• BATTERY CHARGING
• BATTERY DIAGNOSTICS
• SUMMARY

LEAD-ACID BATTERIES

INTRODUCTION

BASIC INFORMATION

1. A 12 volt lead-acid battery is made-up of six cells. Each cell contains a number of plates made from two different types of material. The plates are very close together, but prevented from contact with one another by sheets of separator material.
2. One type of plate is termed negative (the source of electron flow), and the other type positive (the collector of electron flow). The plates are assembled in such a way that an electrolyte can flow freely around them. (An electrolyte is a liquid that readily conducts electricity).
3. Chemical energy is converted into electrical energy or electrical energy into chemical energy by a transfer of electrons through the electrolyte from one plate type to another. During the transfer of electrons the chemical composition of the two types of plate changes.
4. The six individual cells are located within an outer container made of a shock-resistant and acid-resistant material, and the cells are electrically linked inside the case by terminal connecting strips. Vents are fitted to each cell to allow gas to escape without leakage of electrolyte. Plugs are also fitted to each cell of some batteries to allow replenishment of distilled water and for checking the Specific Gravity (SG) of the electrolyte. Specific Gravity is a measure of the density of a liquid. So a liquid with an SG of 1.26 (a typical figure for a fully charged car battery) means that a particular volume of the liquid will weigh 1.26 times heavier than an equal volume of pure water.

CHEMICAL PRINCIPLE

1. In a fully charged battery the active material of the positive plates is lead peroxide (PbO2?, also known as lead dioxide) and the material of the negative plates is spongy lead (Pb - metallic lead made into a sponge-like structure to increase its surface area). The electrolyte is sulphuric acid, heavily diluted with distilled water or de-ionised water so that the solution starts out at 25% acid to 75% water by volume.
2. With the battery connected to an external circuit, electrons flow from the negative terminal to the positive terminal through the car's circuit. At the same time ions (Oxygen and Sulphate molecules) transfer inside the cell from the positive plates through the electrolyte to the negative plates. This process reduces the chemical energy stored within the battery. To forestall any controversy over the last statement - although most modern vehicles have a "negative earth" system and the system is also called "earth return", electron flow is from the battery's negative terminal, through the car's circuit and back to the positive terminal of the battery.
3. During discharge, lead sulphate is formed on both the positive and negative plates as sulphate ions from the sulphuric acid migrate to the two plates. The acid content of the electrolyte is decreased as the sulphate ions leave the solution and its water content is increased because oxygen ions from the lead peroxide combine with the hydrogen ions in the sulphuric acid to form H2O - water. As discharge continues, the amount of lead sulphate on the plates increases until the sulphate coatings become so thick that the now weakened electrolyte cannot effectively reach the active material (lead and lead peroxide) and the two plates become too similar (both lead sulphate) for an electrical potential to exist between them. When this happens, chemical reaction is retarded and the output of the cell is reduced. In practice, the battery should not be permitted to discharge to this extent because thick coatings of lead sulphate are difficult to remove in charging, a cell approaching a state of total discharge is of little use because the high internal resistance caused by the sulphate coatings on its plates reduces the current to a value too low for practical use.
4. The chemical energy in the cell can be restored by passing a "charging current" through the cell in the reverse direction to that of the discharge current. In this way the cell can be discharged and recharged many times.
5. During recharging, electrical energy is converted into chemical energy which is retained until the cell is discharged again.
6. When a cell is being charged, sulphate ions are removed from both the positive and negative plates and the water is broken into hydrogen and oxygen ions. The sulphate ions recombine with the hydrogen ions to form sulphuric acid again and the oxygen combines with lead to form lead dioxide. This process is not perfect and some hydrogen is freely liberated - the bubbling of a charging battery is hydrogen - don't smoke near it - it WILL explode. Also some lead sulphate will precipitate out and drop to the bottom of the battery where over a long period it can accumulate sufficiently to short out the cells and thus kill the battery.
7. immediately after charging, the 'off-load' voltage of a lead-acid cell, that is, its voltage when no electrical devices are connected, just a voltmeter, will be approximately 2.2 volts. After a 'rest' period of about 12 hours the 'off-load' voltage of a fully charged cell will have dropped to approximately 2.1 volts (ie: 12.6v total for the 6 cells). The 'off-load' fully-charged voltage is the same for every lead-acid cell regardless of its plate size and falls in a linear fashion directly proportional to the charge remaining in the battery. This fall in off-load voltage is not true for all types of batteries however. The tiny silver-oxide or mercury cells used in cameras etc hold their open circuit voltage till near full discharge and then drop dramatically.
8. The 'on-load' voltage of a cell, that is, its voltage when electrical devices are being supplied by the battery, decreases gradually as the cell is discharged. This gradual decrease in terminal voltage is due to a gradual increase in the internal resistance of the cell caused by sulphation of the plates.
9. To give a high discharge current and a high terminal voltage under load, a battery must have low internal resistance. The battery can be modeled as a perfect voltage source with a small resistance in series with that source. We have access to the terminals after the resistance - we don't have access to the perfect voltage source inside. The lower the internal resistance, the higher the current that can be supplied. A low internal resistance characteristic can be achieved through extensive plate area. Therefore, each cell contains several sets of plates. All the positive plates of a cell are connected by one connecting bar, and all the negative plates by another. Thus the plates are connected in parallel, further decreasing the internal resistance of the cell. The 'off-load' cell voltage is not affected; it remains the same as that of a single pair of plates.

FACTORS AFFECTING LEAD-ACID BATTERY LIFE

1. Over discharging, which causes excess sulphation. (If your car fails to start within a reasonable length of time/number of start attempts, continuing to engage the starter until the battery will not even operate the solenoid, can permanently damage your battery). When supplying a heavy starter current and the engine doesn't fire, lots of water is produced around the plates due to sulphuric acid breakdown which prevents proper battery action. Leaving the battery for a minute or two before trying again, allows time for the water to mix with the acid again and restore proper battery action. That's why an apparently deadish battery seems to work again if you leave it a while.
2. Too rapid charging and discharging, resulting in overheating and buckling of the plates and shedding of active material. The accumulation of shed material, in turn, can cause shorting of the plates and results in internal discharge.
3. A battery that remains in a low or discharged condition for a long period of time (one month) may be permanently damaged. In addition to causing deterioration of the battery, these factors also decrease battery capacity.

LEAD-ACID BATTERY RATINGS

1. The voltage of a battery is determined by the number of cells connected in series to form the battery. A battery rated at 12 volts consists of 6 cells connected in series.
2. The capacity of a battery is rated in ampere-hours (amperes furnished by the battery times the amount of time current can be drawn). THEORETICALLY, this rating indicates how long the battery may be used at a given rate before it becomes completely discharged.
3. THEORETICALLY, a 30 ampere-hour battery will furnish 30 amperes for 1 hour, 15 amperes for 2 hours or 6 amperes for 5 hours. ACTUALLY, the ampere-hour output of a particular battery depends on the rate at which it is discharged. Heavy discharge current heats the battery and decreases its efficiency and total ampere-hour output. so the ampere hourage rating is based on an unrealistic 1 amp continual discharge. In use, the battery can be completely discharged within a few minutes, or it may never be discharged if the alternator provides sufficient charge.
4. The ampere-hour capacity of a battery depends upon its total effective plate area and, from a practical point of view, can only be used for comparative purposes when shopping for a new battery.
5. Other ratings on a battery are:

its 20 hour rate: the number of amps that can be withdrawn at a constant rate for 20 hours before voltage drops to 10.5 volts. (@ 27 deg C)
(ii) Reserve capacity: the number of minutes for which 25 amps can be withdrawn before voltage drops to 10.5 volts. (@27 degC)
(iii) Cold Cranking Capacity: The discharge in amps that can be maintained for 20 seconds at zero deg F (-17.8 deg C) before voltage drops below 7.2 volts. Useful if you use your 7 in mid-winter.

CHARGE STATE

1. A battery loses water by evaporation and by 'gassing' at the plates when fully charged.
2. The level of the electrolyte should be maintained at all times approximately 1/4 inch above the top of the plates by topping up with distilled water or de-ionised water. NEVER TAP WATER.
3. The state of charge of a lead-acid cell can be most reliably determined by measuring the strength of the electrolyte solution. This is done with a HYDROMETER which measures the specific gravity(SG). A fully charged cell will have an SG of about 1.27 and a discharged cell will have an SG of about 1.12. Check the hydrometer because SG readings are temperature dependent and a correction factor (see hydrometer instructions) has to be applied. The hydrometer is calibrated at a certain temperature and for example a reading of 1.23 on a hot day will read about 1.25 on a cold day.
4. When the battery is connected to an external circuit and current is flowing, lead sulphate is formed at both plates and the specific gravity will fall as the acid becomes weaker. When the SG has fallen to 1.17 the battery is only about 1/4 charged and should be recharged. (The actual SG is dependent upon the temperature of the electrolyte). A hydrometer is not easy to read accurately and usually has coloured bands to make it easier to determine the state of charge).
5. To charge a battery it is connected to a battery charger (an external charger or the car's alternator). which applies a slightly higher voltage (about 14 volts) and causes current to flow in the 'reverse' direction. This current is limited by (deliberate) resistances in the charger's circuit and the value of the battery's voltage. The battery's voltage rises as it charges and so the current is reduced as it depends on the difference between the charger voltage and the battery voltage. That's why your charger shows a higher current when you start charging overnight and a much lower value next morning. While this is happening the lead sulphate which had been deposited on the plates is removed, the water is turned back into acid and the SG of the electrolyte rises to 1.27.
6. In addition to SG, the other reliable and 'practical' indication of a battery's ability to do the job of starting your car is its 'on-load' voltage ie: when cranking the engine. The on-load voltage tells you nothing whatsoever though about the state of the battery's charge only about the state of its internal resistance. The 'on-load' voltage of a good lead-acid battery should not drop below 9 Volts. Also be aware that each cell must have the required SG. One cell showing a bad SG is an indication of a 'dead' battery.

PRECAUTIONS

1. The electrolyte (sulphuric acid) is highly corrosive and if spilled can cause extensive damage to your Se7en, your clothes, your skin and in particular, your eyes. The neutralising agent to be used is a sodium bicarbonate solution (Baking Soda and water).
2. The performance of a battery is affected by temperature. In low temperatures the rate of discharge is decreased because of higher internal resistance but the available cranking amps are therefore reduced for the same reason. In warm temperatures the battery rate of discharge will increase. In general the battery performs better in warm temperatures.
3. As a lead acid battery discharges the SG of the electrolyte reduces. In freezing temperatures, a discharged battery MAY freeze. It is therefore very important to maintain the battery in a fully charged state during winter operations. (It would have to be REALLY cold for this to occur, but it's best not to leave a discharged battery exposed to low winter temperatures for a long time and still expect it to work as before). As an example a fully discharged battery (SG=1.12)will freeze at around -10 degC while a fully charged battery (SG=1.26) won't freeze till the temperature drops to -58 degC. Some difference - so keep it charged in winter - don't leave it.
4. When the chemical energy of a battery is being reduced by connection to an external circuit, both plates become coated with lead sulphate. This is removed during recharge and recombines with hydrogen ions to form sulphuric acid. However, some falls to the bottom of each cell, where it then remains. During the life of the battery the lead sulphate sediment slowly increases. It can be seen from the second illustration that this is allowed for by the Supporting Ribs at the base of each cell. The supporting ribs maintain the plates clear of the base of the cell to provide space into which the lead sulphate can fall. Once the lead sulphate touches the bottom of the plates, that cell will no longer work - the plates are short circuited and the battery will need to be replaced.
5. If a battery is left in a discharged state for too long, the lead sulphate deposit hardens and can no longer be completely removed by the charging process. This process, if repeated too often, will needlessly shorten the useful life of the battery. A battery left discharged for a month will probably never recover.
6. When a battery is being charged, it generates a certain amount of hydrogen and oxygen. Since this is an explosive mixture, it is important that steps be taken to prevent ignition of the gas mixture. The vent caps, if fitted, should be loosened and left in place. No open flames, sparks, or other source of ignition should be permitted in the vicinity. Before disconnecting or connecting a battery to the charger, always turn off the power to the charger by means of a remote switch to avoid sparks at the crocodile clips which can cause a big bang as the hydrogen ignites.
7. Before servicing or handling a lead-acid battery, remove all metal articles from clothing as well as personal jewellery, also keep metal tools away from the terminals. If the battery terminals are bridged by a heavy metal object it could 'weld' in place and cause severe burns or the battery case to split.
8. A lead-acid car battery is quite heavy for its size and should be handled carefully and lifted only when a good firm grip is possible. Use any carrying handle if supplied. If the battery is dropped, the outer case could easily split.

BATTERY CHARGING

1. Both vehicle and battery manufacturers recommend that a battery be electrically disconnected from the car's electrical system before connecting to an external charging device. (There has been a great deal of debate on this subject. To date, the technical reason(s) for this almost universal recommendation have not been discovered.) Afterall, your alternator charges the battery at an exceptionally high rate (30A) and the vent caps are not undone then, nor is it disconnected from external equipment.
2. Any battery charger will supply 14 volts, which "shouldn't" give trouble to the car's electrical system. However:
1. there are some delicate microprocessors in the ECU, that MIGHT not appreciate reverse current flow. (Although one might expect the ECU manufacturers to have thought of this and provided protection devices within).
2. there are diodes, designed as electronic one way valves, which convert the ac (sinusoidal) output of the alternator to a dc (one way) current; plus various transistors. (There have been several instances of alternators fitted to Se7en's failing after a comparatively short period, "speculatively", this COULD be an accumulative effect of charging a battery without first disconnecting it electrically.
3. It may merely be advice to "prevent" a driver from forgetting the battery was placed on charge the night before, starting the engine and driving away with the battery charger still connected. "MURPHY'S LAW" states: if it's at all possible, some "Plonker" will do it.
3. There are some "THREE STAGE BATTERY CONDITIONERS" on the market whose technical literature states that they can be safely left connected to a battery indefinitely, without electrically disconnecting the battery from the car's electrical system. The better examples of this device contain algorithms which automatically monitor the charge state of the battery (by monitoring its open-circuit voltage)and are capable of applying one of three charging levels to the battery, as appropriate. One popular model is the "CSI Airflow Battery Conditioner". The instruction leaflet that comes with the "Airflow" states that the battery cell caps should be unscrewed and left loose in the holes whilst the battery is being charged.
4. Charging is only about 70% efficient so if you roughly know the state of charge you can calculate how many hours are needed to charge it. eg: a 30 amp-hour Banner battery at 50% charge needs 15 Ampere-hours input to charge it. With a 2 amp charger this should in theory take 7.5 hours but due to the 70% efficiency factor it will actually take 11 hours. Don't overcharge a battery - it will overheat, can boil and be ruined. Even a fully discharged battery shouldn't take more than about 22 hours to recharge on a 2 amp charger.

BATTERY DIAGNOSTICS

1. Buy a hydrometer, they only cost 3 quid.
1. Remove the cell caps and check the SG of each cell.
1. If one or more of the cells is low - charge the battery.
2. Have someone check the voltage on the battery while you are trying to crank the engine. If it falls below 9v, the battery is gone.

SUMMARY

1. If you totally ignore your battery, you shouldn't expect "Brum-Brum" every time you go for a start.
2. If your battery has removable caps, keep a regular eye (weekly) on the electrolyte level and keep it up to the recommended max. level (approx 1/4 inch above the top of the plates). The level is very difficult to see by peering into the cell cap from above. If the battery has a translucent case the level might be possible to see against a max. mark on the outside. NEVER DIP ANYTHING INTO THE CELL TO DETERMINE THE LEVEL.
3. Allowing the electrolyte level to drop and partially uncover the plates, reduces the plate area that is 'working'. Your battery may not now be able to crank the engine fast enough for a start.
1. The uncovered plate area may dry-out and that portion of plate area may become unrecoverable - result "Ere mate, how much is a new battery then?"
4. It is not a good idea to stand with your face directly above, and too close to an open cell. Battery acid in the eye WILL ruin your day.
5. If you often go for short Blats, the alternator will not have enough time to return the battery to full charge.
6. Keep an eye on the alternator drive belt tension - keep it at the car manufacturers recommended value. The alternator has a 'fair' resistance to being turned when it is supplying lots of electrical services, particularly at night. A loose belt will tend to slip, which polishes the inside of the belt, so even after you subsequently tighten it up, it may still slip slightly at night. The battery will then be partially feeding the system and may not have enough charge in the morning to start the car - sound familiar?
7. Don't keep "grinding away" at the starter if the engine fails to start as usual - you are "knackering" the battery needlessly. I know it's annoying when you want to go, but batteries are not cheap and there is probably something wrong with the engine which needs to be fixed before a start is possible.