NiCad:
Charging
NiCd batteries can charge at several different rates, depending on how the cell was manufactured. The
charge rate is measured based on the percentage of the
amp-hour capacity the battery is fed as a
steady current over the duration of the charge. Regardless of the charge speed, more energy must be supplied to the battery than its actual capacity, to account for energy loss during charging, with faster charges being more efficient. For example, the typical "overnight" charge, called a C/10 charge, is accomplished by applying 10% of the batteries total capacity for a period of 14 hours; that is, a 100Ah battery takes 140Ah of energy to charge at this rate. At the "fast charge" rate, done at 100% of the rated capacity, the battery holds roughly 80% of the charge, so a 100Ah battery takes 120Ah of energy to charge (that is, approximately 1 hour and fifteen minutes) The downside to faster charging is the higher risk of
overcharging, which can damage the battery.
[3]
The safe temperature range for a NiCd battery in use is between −20°C and 45°C. During charging, the battery temperature typically stays low, around 0°C (the charging reaction absorbs heat), but as the battery nears full charge the temperature will rise to 45–50°C. Some battery chargers detect this temperature increase to cut off charging and prevent over-charging.
When not under load or charge, a NiCd battery will self-discharge approximately 10% per month at 20°C, ranging up to 20% per month at higher temperatures. It is possible to perform a "trickle charge" at current levels just high enough to offset this discharge rate; to keep a battery fully charged. However, if the battery is going to be stored unused for a long period of time, it should be discharged down to at most 40% of capacity (some manufacturers recommend fully discharging, or even short-circuiting), and stored in a cool, dry environment.
[edit] Inspecting
The battery should have no external damage and depending on the number of cells it should have 1.2V per cell when fully charged and about 0.8–1V when discharged .
[edit] Charge condition
High quality NiCd’s have a thermal cut-off so if the battery gets too hot the charger stops. If a NiCd is still warm from discharging and been put on charge, it will not get the full charge possible. In that case, let the battery cool to room temperature then charge. Watch for the correct polarity. Leave charger in a cool place or room temperature when charging to get best results.
[edit] Charging method
A NiCd battery requires a charger with a slightly different
voltage charge level than a lead-acid battery, especially if the NiCd has 11 or 12 cells. In addition, the charger requires a more intelligent charge termination method if a fast charger is used. Often NiCd
battery packs have a
thermal cut-off inside that feeds back to the charger telling it to stop the charging once the battery has heated up and/or a voltage peaking sensing circuit. At room temperature during normal charge conditions the cell voltage increases from an initial 1.2 V to an end-point of about 1.45 V. The rate of rise increases markedly as the cell approaches full charge. The end-point voltage decreases slightly with increasing temperature.
NiMh
Charging
NiMH Charge curve
The
charging voltage is in the range of 1.4-1.6 V/cell. A fully charged cell measures 1.35-1.4 V (unloaded), and supplies a nominal average 1.2 V/cell during discharge, down to about 1.0-1.1 V/cell (further discharge may cause permanent damage). In general, a constant-voltage charging method cannot be used for automatic charging. When
fast-charging, it is advisable to charge the NiMH batteries with a smart
battery charger to avoid
overcharging, which can damage batteries and cause dangerous conditions. A Ni-Cad charger should not be used as an automatic substitute for a NiMH charger.
[edit] The delta-V charging method
One of the preferred charging methods is the "delta-V" method, according to
Panasonic and other battery manufacturers. This is illustrated in the "NiMH Charge curve" figure. The battery is rapidly charged at
constant-current, at a high rate of "1C". After the battery is fully charged, and as it begins to overcharge, the voltage polarity of the electrodes inside the battery will begin to reverse, and this will cause the battery voltage to decrease slightly. A "delta-V" type battery charger will sense this drop in voltage, and when a set threshold is exceeded, the charge cycle must end, and the charge current must be stopped. In some cases, a very small "trickle charge" may remain. The "charge curve" graph also shows that the charge voltage will change depending on the charge current. (Incidentally, it also changes with temperature and battery age.) This generally means that
a constant voltage charging method cannot be used automatically, because it will either be unsafe, or it will not charge batteries reliably and consistently. This is unlike a
lead-acid battery for example, which can in theory be more easily charged at a suitably chosen constant-voltage.
[edit] The delta-temperature charging method
The delta-temperature method is similar in principle to the delta-V method. Since the charging voltage is nearly constant, if constant current charging is used, then a near constant power is entering the battery. When the battery is charging, most of this power will be converted to chemical energy. However, when the battery is fully charged, most of the charging power will then be converted to heat. This results in an increase in the rate of change of temperature, which can be detected by a sensor measuring the battery temperature. This signal is monitored by the battery charger, which then stops the charging current.
[edit] Manual charging
If a suitable battery charger is not available, constant-voltage or constant-current charging can be done
manually, at a moderately high charging rate, if careful attention is given. For proper charging, the voltage and/or current must be set to a suitable charging rate for the particular battery, and a timer should be set. Periodic monitoring is strongly recommended to avoid overcharging (resulting in a voltage drop), or overheating (resulting in an excessive temperature rise and possibly an overpressure condition).
[edit] Trickle charging
Some equipment manufacturers consider that NiMH can be safely charged in simple fixed low-current chargers with or without timers, and that permanent overcharging is permissible with currents up to 'C'/10 h. This may be what happens in some types of cheap
cordless phone base stations and the cheapest
battery chargers.[
citation needed] Although this may be safe when the current is low enough, it decreases the battery capacity and longevity. According to the
Panasonic NiMH charging manual (link below), permanent
trickle charging (small current overcharging) can cause battery deterioration, and it is the least preferred charging method concerning battery performance. If it is used, the trickle charge rate should be limited to between 0.033×
C per hour and 0.05×
C per hour for a maximum of 20 hours to avoid damaging the batteries.
For a slow charge, or "trickle charge" process,
Duracell recommends "a maintenance charge of indefinite duration at C/300 rate".
[3] Some chargers do this after the charge cycle, to offset the natural self-discharge rate of the battery. To maximize battery life, the preferred charge method of NiMH batteries (and most types of batteries), uses low
duty cycle pulses of high current rather than continuous low current.
This may help and it may not. In my experiences in the past I used a Cad charger on HiMh's. NiMh retain memory and need to be treated with more care than NiCads. With the wrong charger they can get messed up. I only know what I have done, but I am sure others can say with more efficiency.
