All types of nickel-cadmium (Ni-Cad) batteries and cells can be charged and discharged in any position and can be satisfactorily used in comparatively hostile environments. Unlike lead acid batteries, Ni-Cads can be stored for periods in excess of 5 years without the need of periodic recharging to ensure good life. Cell performance is influenced by variations In the following:
Temperature on charge/discharge
Charge time
Charge rate
Cycling regime
As a general guide, the optimum overall performance for cells charged and discharged at the same temperature is given at 15 c to 25 c and there will be some decrease in capacity and/or reduction in service life as the operating temperature range is extended.
The charge time should be set to give the degree of overcharge required. A shortening of this time may lead to a reduction in capacity and a increase may lead to overheating under some conditions. Increasing the charge rate generally produces an increase in capacity and more stable performance, but precautions must be taken to protect the cells under fast charge conditions.
Normal Charging
The one hour discharge current, or 'C' rate, in amps of the cells or battery is defined to be equal numerically to the capacity of the cell in ampere hours.
Charging rates are usually expressed in multiples or sub-multiples of the one hour or 'C' rate, ie. 2C, C/10, C/20.
For the majority of cyclic applications charge rates between C/10 and C/20 may be used if the cell temperatures are in the range +10c to 45c. Under these conditions the cells should be charged to 150% of their nominal capacity, eg. the recommended charge rate is C/10 for 15 hours, or the C/20 rate for 30 hours.
Charging at C/10, commonly referred to as 'overnight charging', has become the accepted norm and is convenient for most applications. Optimum capacity is obtained if the cell temperature is maintained between +l5c and +25 c. Efficiency is adversely affected by increasing temperature and a lower returned capacity results. Repeated high temperature charging, with cells above +45c will also reduce the useful life; +45c should therefore be regarded as the safe upper limit during charge.
Fast Charging
For applications where normal charge times are inconveniently long, fast charging may be employed. An advantage of the fast charge mode is that it avoids the gradual reduction in returned capacity normally encountered with prolonged repetitive cycling at low charge rates.
There is one important proviso for fast charge, the need to avoid overcharge conditions. As the cell approaches the fully charged state, oxygen is produced at the positive electrode, the rate of production varying directly with the rate of charge. When the cell is driven into overcharge oxygen is produced faster than it can combine at the negative electrode; the gas pressure rises to a level sufficient to operate the resealable safety vent; the cell overheats due to the energy released by the recombination reaction and the negative electrode polarises still further to a value sufficient for hydrogen to be generated. At lower temperatures, high charge rates accentuate the problems associated with gas evolution at the electrodes and at higher ambient temperatures the heat generated in the cell may take the cell temperature above the safe maximum.
So for fast charging keep the cell temperature between +10c and +45c, bearing in mind the thermal insulation provided by' the battery casing. (FX2 owners should note gas venting problems also).
The following provides a guide to the charge rates and times which should be employed on FULLY DISCHARGED CELLS.
C/10 charge: Normal charge 16 hours; Maximum charge indefinite.
C/4 charge: Normal charge 5 hours; Maximum charge 6 hours.
C/2 charge: Normal charge 2 1/4 hours; Maximum charge 21/2 hours.
C charge: Normal charge 1 hour; Maximum charge 1 1/4 hours.
2C charge: Normal charge 27 minutes; Maximum charge 30 minutes.
4C charge: Normal charge 12 minutes; Maximum charge 12 minutes.
8C charge: Normal charge 5 minutes; Maximum charge 5 minuets.
It will be noticed that the given charge times for rates above C are adjusted so that the ampere hour input is reduced by 10% for each doubling of the charge rate. This is to minimise the increase in cell temperatures which would be expected near the completion of charge.
High Temperature Charging
The cells may be charged continuously at currents up to the C/10 rate at cell case temperatures not exceeding +45c. It should be noted that this will be a few degrees above ambient temperature depending on the charge rate and ventilation provided.
Low Temperature Charging
As the cell temperature decreases, charge efficiency and on-charge voltage increase. On prolonged overcharge at the C/10 rate there is a risk of hydrogen evolution at temperatures of +5 c and below, with consequential venting and deterioration. For a FULLY DISCHARGED CELL in the range 0 c to +10c charge rates up to C/10 should be used and the capacity input restricted to 100% of normal. In the temperature range -10c to 0c the capacity should be restricted to 100% using charge rates up to C/20. In the temperature range -30c to -10 c, charge rates as low as C/50 should be used and the capacity restricted to 100% of normal.
Due to the good charge retention characteristics of the nickel-cadmium system AT LOW TEMPERATURES, continuous overcharge is not usually required.
Note that good storage characteristics does not mean that the cell remains fully charged for five years. Cells usually drop to 60%. of their capacity within 120 days at 0c, 40 days at +20c, and 11 days at +40c. So in hot weather remember .to boost your battery before using it.
A Brief Note On Chargers
Charging may be accomplished with very simple circuitry. It is recommended that the current be maintained approximately constant or limited to a known value. The direct use of constant potential charging systems is not advised due to the possibility of drawing very large currents, with the consequent internal overheating and possible thermal runaway of the cells.
Nickel-cadmium cells should be charged in series. Parallel charging can cause unequal sharing of the charging current.
At normal charge rates, of C/10 and normal temperatures +20c to +25c, the cell has an on-charge voltage of 1.45 Volts, -4mV/c change in temperature.
Charging From A Constant Voltage Source
This can either be a constant voltage charger or, say, a car battery. With this type of charger, the voltage source is connected to the cell via a dropping resistance R. The resistance should drop about half the supply voltage to guarantee adequate current control.
Example :- FX2 from a 12 Volt car system.
The FX2 is a 2.4 Volt (ie 2 cells in series) 7Ah battery.
The supply voltage will be about 14 Volts.
The recommended C/10 charge rate will be 7 / 10 Amps = 0.7 Amps.
The on-charge voltage will be 1.45 x 2 = 2.9 Volts.
The voltage across R needed will therefore be 14 - 2.9 = 11.1 Volts.
The value of R needed will be 11.1 ÷ 0.7 = 15.8 ohms (nearest value is 15 ohms).
The power to dissipate will be 0.7 x 0.7 x 15 7.35 watts.
Thus a 15 ohm, 11 watt resistor should be used and, for best heat dissipation. should be sited in an unrestricted area.
Charging From An AC Source
I will not go into details here on the various circuit designs, but describe the types of charger design you can have. If you want to build your own, then approach me in the pub sometime if you don't know how.
1) Simple charger: So called because you must be simple to use it! Consists of a rectifier, non-electrolytic capacitor and resistor. It works, but there is NO ISOLATION from the mains.
2) Simple transformer charger: FX2 chargers are of this type. Charger output is pulsed current due to rectification of the transformer output. A series resistance, P. is selected to give a correct average current.
3) Constant current charger: A circuit which gives a true constant current output. It does not take much effort to convert a simple transformer charger ((2) above) into this type.
Conclusion
If you have got this far, then you will have probably realised that getting the best from your Ni-Cads needs a little thought. However, they are quite robust, and if you charge them at the C/10 rate you can't go wrong (unless it's cold when you must not overcharge them for TOO long). And for those who fancy charging their FX2 's in 5 minutes by pumping 60 Amps into them - save it for bonfire night as I can't tell what the effect of the encapsulation will be.