David Gibson

Looking through some caving journals from 20 years ago I was amused to read of a design for an 'emergency caving lamp' which discussed at great length how to make a waterproof torch in true 'Blue Peter' style. In those days there were no Tekna, Petzl or Maglite torches. Neither were there the same quality of rechargeable cells, and there was a profusion of articles on cell fettling. Today there are high quality cells, and much of the need for fettling has disappeared. However, many of the charging myths have remained.

If you are using a rare vintage of "wet" cell from a junk shop, or a low-cost "high street" cell then you may well need to observe special rules. But cost and performance are such that unless you are in the position of having to equip an entire caving club, you should really be using the latest type of sintered-plate, sealed NiCd such as those manufactured by Ever Ready and distributed by RS Components and Farnell Electronics then the following simple rules apply.

• The cells can be overcharged at full current without damage.
• You do not have to charge cells with a so-called constant current source, but merely a controlled-current source.
• The cells can be discharged at high current without significant loss of capacity (unlike a lead-acid battery).
• It does help if you discharge the cell before charging it, and the cells can be left in a discharged state.
• Batteries should not be discharged until completely flat.

EVER READY "AN" RANGE

Ever Ready manufacture a range of cells, and are probably typical of most manufacturers of high-quality cells. The AN range are designed for general and cyclic use (rather than stand-by or high temperature use). A recent data-sheet I obtained is subtly different to one I had in 1985, and thus shows the continual improvements that are being made to cell design. The figures listed below are based on the Ever-Ready datasheet, but should apply to other high quality cells. WARNING: many of the Nickel-Cadmium cells you can buy in the High Street are not high-quality, but low-cost imports. Often, for instance, a low capacity 'C' cell is packaged in a 'D' size pack.

'AN' stands for "advanced negative" and refers to the special design of the negative electrode. When an overcharged cell gases at the positive electrode, the oxygen is re-absorbed at the negative electrode at the same rate. The cell will not 'vent' under normal conditions and is capable of continuous over-charging.

DEFINITION OF CAPACITY - THE 'C' TERM

The capacity of a cell is measured in Amp-hours, that is the amount of current it will deliver for one hour. It is convenient to call this the C rate and to describe the current in terms of this rate. For example, a current of 5C would discharge the cell in 12 minutes; a current of C/5 in 5 hours. Current [Amps] x time [hours] = C.

The capacity also depends to some extent on the rate at which the cell is discharged (though this is nowhere near as marked as that for a lead-acid cell). The capacity of the cell when it takes five hours to discharge is known as C5.

The capacity has been measured, over the years, in slightly different ways. The method presently used by Ever-Ready is to charge at a current of C/10 for 16 hours, leave the cell to stand for an hour and then to discharge at C5/5 to an end-point of 1.0V. This takes a nominal five hours, and effectively exhausts the cell. The test is also performed at a discharge current of 5C to an end-point of 0.8V.

The nominal capacity of a cell is quoted such that the average capacity of a batch is higher than the nominal (e.g. a 7Ah cell is likely to deliver 7.35Ah). The capacity is affected by charging rate and standing time, so it is important always to use the same technique. Ever-Ready used to specify a standing time of 16 - 24 hours; and prior to that they specified a charge rate of C/8 for 12 hours, so there is certainly no unique way of specifying capacity.

DISCHARGING THE CELL

Discharge current

The nominal capacity of the cell is achieved when it is discharged at the C/5 rate to an end-point of 1.0V. At a discharge rate of 5C to 0.8V the cell will deliver about 80% capacity. The maximum continuous discharge current is around 10C or 35A whichever is lower. Much higher discharge currents can be given for short periods (3 - 4s) with adequate cooling.

Discharge Voltage

The discharge voltage varies slightly with load and temperature but remains substantially constant at 1.2V until discharge is almost complete, when it drops rapidly to an end-point of 1.0V, or 0.8V at high discharge currents. Discharging below the end-point will not give any extra capacity and may cause cells in a battery to become reverse-charged, which will damage them.

Depth of discharge - cycle history - memory effect

A cell which is repeatedly shallow-discharged to a high end-point voltage may exhibit a temporary loss of capacity, whereby the cell becomes conditioned into giving a shallow discharge. This is known as the memory effect. The capacity can be restored by fully-recharging and deep-discharging, or by fully discharging the cell before charging. The memory effect can also occur after a history of cycling at low current or charging at high temperature or with low overcharge factor. Some capacity is also lost over the first 10-12 cycles of the cell. If a cell is not giving its required performance it may be due to the cycle history, and can usually be corrected by a few full charge/deep discharge cycles at the recommended rate.

Self-discharge

A fully charged cell left to stand will gradually discharge. The loss of charge is strongly dependent on temperature and is about 0.5%/day at 20°C, 2%/day at 30°C, and 3%/day at 40°C.

CHARGING THE CELL

Charging can be done with very simple circuitry. The term constant-current is misleading; it is only necessary to use a controlled-current within fairly wide limits. Controlling the current accurately does mean, though, that you know when charging is complete; and is essential at the higher charging currents where overcharge is not allowed.

Normal Charging

The cells should be charged at between C/20 and C/10 to 160% of capacity, at temperatures from 5° to 45°C. Continuous overcharging is allowed, though in order to achieve good life expectancy this should be at the lowest rate that is appropriate. During overcharge the temperature should be between 10° and 30°C.

At low temperatures, down to -30°C the supply voltage should be limited to 1.6V, and the current to C/20 with no overcharge. At high ambient temperatures ventilation may be needed to keep the cell temperature within limit.

Fast Charging

The AN range can be fast-charged provide there is no overcharge. This requires that the cell is fully discharged first and that the current is controlled to around 5%. Charging is more efficient at high current. The rates are:

At high charge rates, limiting the charge to less than a full charge avoids a large increase in cell temperature at the end of a charge. The temperature limit is 10° to 40°C for fast-charging.

It is possible to monitor rate of rise of cell voltage or temperature to indicate when charging is complete, but this is not necessary if the cell is deep-discharged first and the charge timed.

Low Current charging

This is not as efficient as high-current charging and can cause a reduction in capacity - see "cycle history". Cells may be trickle charged at rates from C/50 to C/20. They should not be trickle-charged outside this range.

Charge Voltage

At room temperature during normal charge conditions the cell voltage increases from an initial 1.2V to an end-point of about 1.45V. The rate of rise increases markedly as the cell approaches full charge. The end-point voltage decreases slightly with increasing temperature.

Rejuvenation of a damaged cell

If a cell becomes reverse-charged, or is left flat for a long time, small metal whiskers can grow across the plates. The symptom of this is that the cell will not accept a charge, its voltage remains below 1.2V even when on charge. A common solution is to pulse the cell with a very high current. The current should be limited to, say, 1C and the cell voltage monitored until the fusing of the whisker is detected (by the cell voltage jumping to >1.2V). This method has worked for me in practice but I do not know if this is recommended by the manufacturers.

SELECTED SPECIFICATIONS

DESIGNING A CHARGER

Because the cells will cope with an overcharge you do not need to control the current very accurately. For size F cells you should charge at 700mA for 16 hours, or say 640mA +/- 10% for 19 hours.

The best facility you can give to a more-than-basic charger is something to discharge the cell at around C/5 (or 1.4A for an F cell) to an end-point of 1.0V per cell before charging. A timer could then also allow you to do a timed charge, though this is of slightly less benefit than a controlled discharge.

You could add a pulsing feature to eliminate whiskers whereby the cell voltage, if monitored low, caused a high current to flow until the cell recovered. If your charger is not capable of supplying a high enough current then you could use a capacitor discharge circuit to supply a brief high-current pulse.

There is a school of thought which says that cells should be charged from an a.c. waveform with a very slight negative bias so that they are charged and then slightly discharged on each mains cycle. This is supposed to stop whiskers forming but is less appropriate for high current charging than for trickle charging.

My home-made charger detects open-circuit, short-circuit and reverse connection which is useful when dealing with a mass of crocodile clips. It also prevents the battery from leaking back into the charger during a power failure.

Solar-powered chargers are now possible, with or without fancy voltage converters. The important point is to make sure that the current does not dip below the lower limit for trickle-charging, of C/50 (or 140mA for an F cell) since this could introduce a memory effect. You only need a reverse-blocking diode and no current-regulating components. Solar chargers which claim to take 14 days to charge a size D cell are really a waste of money. (14 days implies around 20mA or C/200 for a D cell).

A switched mode converter would allow cells to be charged more efficiently from a car battery, so that two F cells draw about 3Ah instead of about 12Ah as they would with a straightforward series regulator.

I would not like to tarnish my reputation by actually producing a practical charger design with the above features, but you never know.