NiCads should, ideally, be charged with a constant current.
A commonly suggested value is at the “ten hour rate”. That is, at a
tenth of the current quoted in its capacity. For example, a battery whose
capacity is 4Ah (one which can deliver 4 amperes for one hour, or 2 amperes
for 2 hours etc), should be charged at one tenth of 4 amperes, i.e. 400mA, the
current which it can deliver for 10 hours. However, due to inefficiencies
- principally energy lost as heat - during charging and discharging, the
current has to be applied for 1.4 times the theoretical duration, i.e. for 14
hours, not 10.
This assumes that the battery is charged from a fully discharged state.
There are other, more complicated, systems in use. One method measures the battery’s temperature and terminates the charge when it rises to a pre-determined figure. Another monitors the battery voltage. Curiously, this diminishes slightly as full charge is reached and this reduction is detected and used to terminate the charge.
If an older battery is frequently only partially discharged it loses capacity. This is often called the “memory”effect. (Modern cells/batteries do not, apparently, suffer from this shortcoming.) The cure is to discharge the battery, to 1 volt per cell, and re-charge it. It may be necessary to do this several times.
If a battery has been completely discharged, to much less than 1 volt per
cell, one or more cells may have reached zero volts whilst still having current
passed through them. This will result in them developing a reverse voltage
or “reverse charge” state.
The affected cell/s should have a fairly heavy current, say 5 amps, passed through
them in the normal charge direction for 20 seconds or so. Leave them for
a couple of minutes and then check the polarity of each affected cell. If
it is correct it may be charged again. If not you can try doing this a
few times before discarding.
Occasionally one encounters a cell which has developed an internal short. This is evident from it always reading zero volts. Such cells may sometimes be recovered by using a similar technique to the previous one. In this case the current generally needs to be higher in an attempt to “blow” or rupture the short. One way is to use a fully charged battery, or a high current d.c. power supply, applying it to the affected cell with normal charging polarity for a couple of seconds. Again it may be necessary to do this a few times.
If you are trying to make up a battery from used cells or to re-condition
a neglected battery a good procedure is to first attend to each cell individually.
Disconnect them all. Charge each one. Then fully discharge it –
connect a resistor across it until it reads zero volts. Do this several
times for each cell.
This should reveal any which have permanently reduced capacity and ensure that
the ones you end up with are reasonably well matched.
N.B. Charging NiCads can be harzardous. They can explode, even when subjected to the rather benign ten hour rate charging regime. Some manufacturers and suppliers have recommended that they have a heavy cover placed over them.
A stored NiCad will self-discharge. The higher the temperature the
quicker the discharge.
A fully charged cell will, over 30 days, lose approximately 10 – 12% of its
capacity at 0 degrees C, 30 – 33% at 20 degrees, and 40% at 30 degrees.