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Chargers for NiMH batteries are very similar
to those of the NiCd system but the electronics is generally
more complex. To begin with, the NiMH produces a very small
voltage drop at full charge. This NDV is almost non-existent
at charge rates below 0.5C and elevated temperatures. Aging
and cell mismatch works further against the already minute
voltage delta. The cell mismatch gets worse with age and increased
cycle count, which makes the use of the NDV increasingly more
difficult.
The NDV of a NiMH charger must respond to a voltage
drop of 16mV or less. Increasing the sensitivity of the charger
to respond to the small voltage drop often terminates the
fast charge by error halfway through the charge cycle. Voltage
fluctuations and noise induced by the battery and charger
can fool the NDV detection circuit if set too precisely.
The popularity of the NiMH battery has introduced
many innovative charging techniques. Most of today’s
NiMH fast chargers use a combination of NDV, voltage plateau,
rate-of-temperature-increase (dT/dt), temperature threshold
and timeout timers. The charger utilizes whatever comes first
to terminate the fast-charge.
NiMH batteries which use the NDV method or the
thermal cut-off control tend to deliver higher capacities
than those charged by less aggressive methods. The gain is
approximately 6 percent on a good battery. This capacity
increase is due to the brief overcharge to which the battery
is exposed. The negative aspect is a shorter cycle life. Rather
than expecting 350 to 400 service cycles, this pack may
be exhausted with 300 cycles.
Similar to NiCd charge methods, most NiMH fast-chargers
work on the rate-of-temperature-increase (dT/dt). A temperature
raise of 1°C (1.8°F) per minute is commonly used to terminate
the charge. The absolute temperature cut-off is 60°C (140°F).
A topping charge of 0.1C is added for about 30 minutes
to maximize the charge. The continuous trickle charge that
follows keeps the battery in full charge state.
Applying an initial fast charge of 1C works well.
Cooling periods of a few minutes are added when certain voltage
peaks are reached. The charge then continues at a lower current.
When reaching the next charge threshold, the current steps
down further. This process is repeated until the battery is
fully charged.
Known as ‘step-differential charge’,
this charge method works well with NiMH and NiCd batteries.
The charge current adjusts to the SoC, allowing high current
at the beginning and more moderate current towards the end
of charge. This avoids excessive temperature build-up towards
the end of the charge cycle when the battery is less capable
of accepting charge.
NiMH batteries should be rapid charged rather
than slow charged. The amount of trickle charge applied to
maintain full charge is especially critical. Because NiMH
does not absorb overcharge well, the trickle charge must be
set lower than that of the NiCd. The recommended trickle charge
for the NiMH battery is a low 0.05C. This is why the original
NiCd charger cannot be used to charge NiMH batteries. The
lower trickle charge rate is acceptable for the NiCd.
It is difficult, if not impossible, to slow-charge
a NiMH battery. At a C-rate of 0.1C and 0.3C, the voltage
and temperature profiles fail to exhibit defined characteristics
to measure the full charge state accurately and the charger
must depend on a timer. Harmful overcharge can occur if a
partially or fully charged battery is charged on a charger
with a fixed timer. The same occurs if the battery has lost
charge acceptance due to age and can only hold 50 percent
of charge. A fixed timer that delivers a 100 percent
charge each time without regard to the battery condition would
ultimately apply too much charge. Overcharge could occur even
though the NiMH battery feels cool to the touch.
Some lower-priced chargers may not apply a fully
saturated charge. On these economy chargers, the full-charge
detection may occur immediately after a given voltage peak
is reached or a temperature threshold is detected. These chargers
are commonly promoted on the merit of short charge time and
moderate price.
Figure 4-2 summarizes the characteristics
of the slow charger, quick charger and fast charger. A higher
charge current allows better full-charge detection.
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Charge
C-rate |
Typical
charge time |
Maximum
permissible charge temperatures |
Charge
termination method |
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| Slow
Charger |
0.1C |
14h |
0°C to 45°C
(32°F to 113°F) |
Fixed timer. Subject to overcharge.
Remove battery when charged. |
| Quick
Charger |
0.3-0.5C |
4h |
10°C to
45°C
(50°F to 113°F) |
NDV set
to 10mV/cell, uses voltage plateau, absolute temperature
and time-out-timer. (At 0.3C, dT/dt fails to raise the
temperature sufficiently to terminate the charge.) |
| Fast
Charger |
1C |
1h+ |
10°C to 45°C
(50°F to 113°F) |
NDV responds to higher settings;
uses dT/dt, voltage plateau absolute temperature and time-out-timer |
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Figure 4-2:
Characteristics of various charger types.
These values also apply to NiMH
and NiCd cells.
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