The critical specification for a battery is its
Amp-hour (A-hr)
rating,
which is numerically equal to the maximum amount
of current the battery can supply to a load for one hour
before the cell reaches its
end-of-life voltage
(usually tak-
en as 1.0V/cell for Ni-Cd and Ni-MH batteries.
When a battery is charged or discharged at a current that is
equal to its A-hr rating,
this is known as the ‘‘c’’ rate.
Most Ni-Cd and Ni-MH batteries can be safely charged at a
1c rate, as long as they are not overcharged. However, the
battery temperature must be within a range of about 15§Cto
45§C (the reasons are detailed later in this paper).
OVERCHARGING: THE SILENT KILLER
The nemesis of all rechargeable batteries is overcharge . . .
although some battery types tolerate it better than others,
the results of overcharge range from minor damage to cata-
strophic failure.
In the case of Ni-Cd, which is the most popular rechargea-
ble battery type presently in use, sustained overcharge
causes increasing pressure within the battery that eventual-
ly causes the cell’s vent to open and release oxygen. This
has a detrimental effect on the battery, although it may still
retain some useful capacity.
If Ni-MH batteries are overcharged, they will also build up
pressure and release gas: however, the gas released will be
hydrogen, which is extremely explosive near spark or flame.
One battery manufacturer created an interesting euphe-
mism for some of the unfortunate accidents in cases where
Ni-MH batteries were overcharged:
Rapid Spontaneous
Disassembly.
DETECTING END-OF-CHARGE
There are several ways to detect end-of-charge for Ni-Cd or
Ni-MH batteries, but one way that is both simple and reliable
is called a DT detector. It measures both the ambient tem-
perature and the battery temperature and cuts off the high
current charger when the battery rises a pre-set amount
above ambient. This design uses a 10§C rise as the cutoff
point (which is recommended by most battery makers), but
can be easily adjusted by changing resistor values.
Ni-Cd cells are perfectly suited for DT cutoff techniques,
because their charge process is
endothermic
(they get
slightly
cooler
when a discharged battery is being re-
charged). Even at fast charge rates, the battery will not be-
gin to heat until it is nearly fully recharged. At that point, the
battery is no longer converting the electrical current into a
chemical reaction, so it must be dissipated as heat. The
resulting increase in temperature provides a very accurate
indicator that it is time to stop charging.
The Ni-MH battery is not quite as accommodating: the re-
charge cycle is
exothermic
(the battery gets slightly
warm-
er
during recharge) but still shows a fairly well defined in-
crease in temperature when the battery is fully charged. Us-
inga10
§
CDT detection point will give good results in most
cases, and is recommended by the battery makers.
NOTE: WARNINGS ABOUT FAST CHARGING NI-MH
AND NI-CD BATTERIES
Since the Ni-MH battery normally gives off heat during re-
charge, the 10§C ‘‘window’’ may have to be adjusted to suit
the characteristics of the specific cell: The window must be
wide enough to prevent premature cutoff from ‘‘normal’’
heating, but narrow enough to detect the temperature rise
which occurs at full charge (and execute appropriate charge
termination).
Any new design that uses Ni-MH batteries should be care-
fully evaluated to verify accurate end-of-charge termination
because of the potential for battery explosion if hydrogen is
released.
IMPORTANT:
With Ni-Cd or Ni-MH cells, the 1c (fast)
charge rate can only be safely used if the battery tem-
perature is in the range of about 15§Cto45
§
C.
At
low temperatures,
gas recombination within NiCd and
NiMH batteries does not occur as easily, which limits the
amount of charging current that can be safely used before
venting will occur. If low-temperature (k15§C) recharging is
required, consult the battery maker for safe charging current
levels.
A battery that is recharged at
elevated temperature
will
retain substantially less energy than a battery recharged at
25§C. At high temperatures (l35§C) gas generation within
the cell occurs at a much lower state of charge, meaning
that the cell will not accept as much charge (compared to
25§C) for a given amount of cell temperature rise.
The poor charging efficiency seen at high battery tempera-
tures means that extremely long recharge times (at low
charging currents) are required to deliver full (25§C) capacity
of charge to a ‘‘hot’’ battery.
TRICKLE CHARGE CURRENT
All batteries lose charge internally due to self-discharge,
usually occurring due to leakage paths through the battery
separators (insulators). The amount of leakage is depen-
dent primarily on battery age and usage, with leakage in-
creasing dramatically in batteries that are old or have com-
pleted many cycles of charge and discharge.
Trickle charging is a continuous low-level charging current
that tops off the total charge in the battery, and prevents
any energy loss that would occur due to leakage.
The
maximum safe trickle charging current for a typical
Ni-Cd cell is about 0.1c,
this being the maximum charge
rate at which all of the gas developed internally is able to
recombine (so there is no internal pressure buildup that
would cause venting).
For Ni-MH batteries, the maximum (safe) trickle charge
rate is lower
(one manufacturer specifies c/40). This is an
important difference between Ni-Cd and Ni-MH batteries,
and must not be exceeded for continuous charging.
In this design, the trickle charge current is provided by the
resistor labeled RTR (see
Figure 1
). This current flows any
time VIN is present, regardless of operation of the high-cur-
rent charger. When the high-current charger is operating,
the total charging current is the
sum
of the trickle current
and the current provided by U1.
Once the input voltage VIN and the desired trickle charge
current ITR are known, the value for RTR is found using
Ohm’s Law:
RTR e(VIN b7b0.7)/ITR
The maximum power dissipation in RTR must also be calcu-
lated (when selecting a resistor, make sure the power rating
is greater than the value calculated below):
PMAX (RTR)e(VIN b4b0.7)2/RTR
Note that the power dissipation in the resistor is dependent
on the battery voltage. As the battery voltage increases, the
voltage drop across RTR decreases (causing the power dis-
sipation to decrease).
In the above equation, a battery voltage of 4V is assumed
as a worst-case minimum value for battery operating volt-
age for a five-cell battery pack (which would provide the
maximum power dissipation for RTR).
2