BATTERY TECHNOLOGY

I still get some interest in this subject, what with all the digital cameras and remote controls, clocks and even battery driven tools. Battery use is endless and can be expensive too. It pays to know the best way to cope. There has been tremendous progress in both primary & secondary battery technology over the last few years. I once wrote a little treatise for my sons which I will reproduce here. Actually, it came about because of all their battery driven toys. Since then we have seen alkaline, Mercuric-Oxide, Zinc-Air, Silver Oxide, Nickel MetalHydride, Lithium, Li-Ion and today I spotted a Nickel-Zinc type for digital cameras in Boots. Anyway here's how it was a few years ago with one or two updates: -

RE-CHARGABLE BATTERIES
(A collection of information)
Resume
First of all a recap.
The "battery" is made up of a group of "cells".
There are two classes of such cells: -

PRIMARY, those which cannot be re-charged because the chemical action is not reversible.

SECONDARY, those that can be re-charged because it is.
Efficiency is related to the energy that it takes to produce a charged battery, and what energy can then be recovered. It is measured in Amp hours or mAh

ELECTRO-MOTIVE FORCE (EMF). This refers to the UN-LOADED terminal voltage, or the actual potential (Voltage) that the cell delivers before there is any load whatsoever. In practice a very light load as would be drawn by a high resistance volt-meter reads it OK. EMF falls as the current drawn is increased and is in proportion to the internal resistance of the cell. This can change over time or with the state of charge, hence the drop off of voltage as cells "flatten." The internal resistance can, in some chemistries, be used as an indication of the state of charge.

POTENTIAL DIFFERENCE (PD) Refers to the terminal output voltage under load - when a fairly substantial current is being drawn, say at a 1 Hr rate. This means, for example that for a 50Ah battery the load would be 50 Amps; and for a 1600mAH battery, 1600mA.

SURFACE CHARGE There is an apparent increase in EMF when a cell is freshly and fully recharged. It reduces quickly after standing or at first discharge. PD (Potential Difference) is the EMF or Voltage that is present when the cell is being made to work (discharge). This is the parameter that counts in practical use. Fortunately, most cells have a fairly constant and predictable terminal voltage which is maintained over their discharge cycle.

CAPACITY refers to the total energy that a cell contains in Ah (Ampere - hours). This can be too big for some little cells and so we then use mAh (for milli or one thousandth part). i.e. 1000mA = 1A.
A specific discharge rate is implied usually over 10 hours for large capacity cells, and over 1 hour for small cells. This is sometimes referred to as the "C" or "C1" / "1C" rate. For example, a 1.2 Ah cell will supply 1.2A for 1 hour.
The CAPACITY of a cell will vary with the discharge rate, reducing as the process is speeded up. The discharge time at the HOUR RATE, meaning the current that can be drawn over that period that would just render the cell to be FLAT - or discharged. This is sometimes quoted as a PD that is about 15% under the nominal voltage. This gives us 1.05V for Ni-Cad & 1.75V for Lead-Acid.

WATTS PER HOUR
If you prefer to think in the more familiar Watts measurement for power, multiply the terminal voltage by the current in amps. A 12V car battery of say 50 Ah capacity, can deliver 12 x 50 = 600Watt-hours. It's not much really. In practice, at that rate of discharge, it might well be rather less.

RECHARGING
Optimum re - charging is quoted in relation to the ten hour rate at, but with an added EFFICIENCY factor in per cent (%). If the EFFICIENCY is quoted at 40% over the TEN HOUR RATE, then we would proceed with the charging for that extra period of time: 10 hrs + 40% = 14 hrs.
Now if the CAPACITY is say 1Ah at the 10 hour rate (that's 100mA over 10hrs), we charge at that rate over 14 hrs (assume 40% extra for in-efficiency), to achieve full charge. This is the most usual example.
Fast charge / discharge results in reduced efficiency because the chemical action cannot keep up with the demand and therefore energy is wasted, usually as heat or gas. Carried to extremes this will cause damage. This is similar to what happens when you go on charging for too long and is one reason why it is better to start the re - charge from when the cell is flat. However, there are dangers to having some cell types in a flat condition for long. Most cell types must never be reverse charged. This can happen when cells are connected in a serial stack (or BATTERY), as one or more cells become fully discharged before some of the others.

IT IS VERY IMPORTANT TO UNDERSTAND THIS LAST POINT!

EXAMPLE
A circuit of cells connected in series (and +ve to -ve to increase the voltage) is to supply a bulb (we call this the LOAD). At first all the cells are in a more or less fully charged condition. As time goes by one, (or more), cells is the first to be discharged to zero Volts at its output terminals. At this point it begins to absorb energy in reverse polarity from the other cells in the circuit that are not yet flat and receives the damaging reverse charge. It is difficult or impossible to rescue cells that are damaged in this way.

The 2Volts / cell LEAD - ACID (as used in car) batteries SULPHATE when areas of their lead plates are no longer in a chemically charged state. {Re-charge every 3 months or keep trickle-charged}.

LEAD-ACID CELLS SHOULD THEREFORE BE STORED IN A CHARGED CONDITION. There are two types of lead involved and it is the negative plate that suffers if they are not fully charged. The white lead-sulphate covers the surface and prevents it being in contact with the electrolyte which is dilute - SULPHURIC ACID - H2SO4. Sulphate is very hard to remove once it has formed although CALCIUM in the acid can help. For this reason it is far better to make sure that LEAD - ACID batteries are always kept in a charged condition or are re - charged at very frequent intervals. Sulphation leads to a loss of CAPACITY, (Ah), as not so much area of material is available for conversion - to absorb the charge. This can lead to inadvertent overcharge-ing which although they are tolerant of a small "trickle" over charge, tends to force material from the plates where it falls to the bottom, is lost for chemical conversion and can cause shorts. Perhaps more serious is that when a fully charged lead - acid cell is still receiving charge current, it produces highly explosive HYDROGEN gas !!
Some heavy duty, slow discharge units produced say, for telephone exchange back - up, can last for 30 years !

Nickel-Cadmium (Ni - Cad) cells have some different problems. In many ways they are superior to other SECONDARY re - charge-able cells, and they are certainly less hazardous if only because they contain no acids. They are however, POISONOUS to all life! Their PD is only 1.2V per cell and for a given Ah capacity will need to be larger in size and heavier than other types.

Some reports say that they are tolerant of persistent overcharging, others say they are not! It may depend upon the construction and re-sealable venting. In actual practice, these batteries do NOT thrive on being continually charged - or over-charged at high current rates. However, they seem happy to be trickle charged. Capacity is adversely affected by high temperatures during charging.

Ni-Cad cells tend to keep their output PD right up to the end of their full discharge, rolling off very suddenly as they go entirely flat. Little warning is given by the terminal voltage and the only real means of knowing the state of charge is to monitor the discharge rate and period. Because these cells should be stored in a discharged state, they are easier to maintain and keep. Their chief draw-back is that they grow DENDRITE hairs of conductive metal when left in a charged state which shorts out the material within and prevents re - charging. One possible solution is to "blow" the hairs away (melt them) with a very high instantaneous current pulse of correct polarity, limited duration and magnitude. For cells up to say, 4Ah use a charged up electrolytic capacitor (say to 12V from a car battery via a series resistor of about 1 to 10 ohms, (to resrict the surge current).
When it is charged, "splash" the capacitor across the cell terminals briefly, then try a few seconds of high charge within normal limits for the cell. As a guide use say 50% of capacity. Once the cell can support its normal 1.2 Volts PD for a light current draw, charge normally, but at 14 hour rate. (One tenth of total capacity for 14 Hrs). Remember that this treatment formula applies to individual cells and NOT battery packs. If individual cells cannot be isolated a higher voltage will be required, say double (24 for V for a 12 Volt battery, with the risk of damage to perfectly good cells in the pack.
NI-CAD cells also suffer from a phenomenon called 'THE MEMORY EFFECT' If a cell is boost charged before it has become exhausted, it behaves as though it has a reduced capacity, subsequently discharging to the level it was at when the boost started and then behaving 'FLAT'. This may be related to the dendrite growth that was mentioned earlier. However, official procedure is to make sure the cell is fully discharged before the re - charge begins. This is reasonably easy with a single CELL as a modern 'intelligent' charger can effect a discharge until the cell voltage first reduces and thereby commence the full charge cycle. Because of the reverse charge dangers that can occur in a BATTERY of several cells, this procedure is not without some problems when say, camcorder batteries are to be re - charged. In any case these 'intelligent' chargers have to be told what the full voltage should be, and the Ah capacity. They are completely duped by a battery with a faulty cell in its stack !
STORE Ni-Cad BATTERIES/CELLS IN FULLY DISCHRGRED STATE!

Nickel-Metal-Hydride
The Ni-M-H type of cell has become popular & much cheaper over revcent years. These are very similar to the older Ni-Cad having 1.2V cells, but without the terrors of the memory effect, or indeed long term inactivity. Shelf life charge is improved and occasional top up boosting does not cause a problem. Nor does a small continuous trickle charge. It would appear that these types don't grow dendrite hairs. CAN BE STORED IN A CHARGED STATE WITH A MODERATE SHELF LIFE. For long term store discharged.
B.J.Greene. Apr99

LI-ION TYPE CELLS ! {26May2005}
These Lithium-ion batteries are best STORED IN A FULLY CHARGED CONDITION. They are also fussy about the temperature, not liking to be charged in any extremes below freezing or above 40°C. Such operation or even storage will reduce capacity and life.
There is no memory effect and the power to weight ratio is very favourable.

Lithium-ion batteries (sometimes abbreviated Li-ion batteries) are a type of rechargeable battery in which a lithium ion moves between the anode and cathode. The lithium ion moves from the anode to the cathode during discharge and in reverse, from the cathode to the anode, when charging.
Lithium ion batteries are common in consumer electronics. They are one of the most popular types of battery for portable electronics, with one of the best energy-to-weight ratios, no memory effect, and a slow loss of charge when not in use. In addition to uses for consumer electronics, lithium-ion batteries are growing in popularity for defense, automotive, and aerospace applications due to their high energy density. However certain kinds of mistreatment may cause Li-ion batteries to explode.
The three primary functional components of a lithium ion battery are the anode, cathode, and electrolyte, for which a variety of materials may be used. Commercially, the most popular material for the anode is graphite. The cathode is generally one of three materials: a layered oxide, such as lithium cobalt oxide, one based on a polyanion, such as lithium iron phosphate, or a spinel, such as lithium manganese oxide, although materials such as TiS2 (titanium disulfide) were originally used.[3] Depending on the choice of material for the anode, cathode, and electrolyte the voltage, capacity, life, and safety of a lithium ion battery can change dramatically. Lithium ion batteries are not to be confused with lithium batteries, the key difference being that lithium batteries are primary batteries containing metallic lithium while lithium-ion batteries are secondary batteries containing an intercalation anode material.