Smart Charger Module for NiMH Battery Packs
Model No.: NHC-01
Smart Charger Module for NiMH Battery Packs
(Rev. 2.0)
The NHC-01 is a professional, processor-controlled charger
module for NiMH battery pack for AA and AAA cells
(1000…2800 mAh) intended to build into mobile devices,
mobile educational robots and other apparatus. It has the
possibility to set different charge currents and number of
cells. This charger module can charge 2..8 pcs series NiMH
Because of small dimensions of the NHC-01 (60 mm x 30 mm x 17 mm) it is ideally suited to build
into many devices (for OEM and DIY) which normally require inconvenient and frequent removing
of battery cells from battery holder and moving them to an external charger. The possibility of
building the NHC-01 charger module into different devices is very unique, practical and economical
solution for many users and designers in the field of electronics and robotics.
The NHC-01 charger module requires only external power supply (AC/DC adapter or other power
supply source; (230VAC/9..18VDC, 1.5A). The charger module employs several algorithms to detect
the end of the charge. They ensure high reliability and make the battery to be fully charged without
the risk of overheating and overcharging. Configuring the charger is very simple and boils down to
proper setting of jumpers build into the charger.
Indication of current work mode is signaled by a LED diode. It is also possible to connect an optional
NTC thermistor to measure temperature of the battery cells. The thermistor ensures the most
accurate detection of the end of charge at higher charge currents.
FEATURES: Smart DC Charger Module for OEM and DIY
Basic NHC-01 parameters:
Charging current: 125 mA, 250 mA, 500 mA, 1000 mA
Cell capacity: 1000..2800 mAh (cells are connected in series)
Number of cells: 2, 3, 4, 5, 6, 7, 8 (2.4V − 9.6V)
Power supply voltage: 9..18 VDC (from external power supply : 230V AC/9..18 VDC, 1.5A)
Small dimension (LxWxH): 60 mm x 30 mm x 17 mm
• Ambient operation temperature: 0°C to +40°C
• Ambient operation relative humidity: 20% to 85%
• Applications: excellent for building in battery charging function inside your device or build
your own battery pack charger
Examples of use:
mobile robots
measurement instruments
mobile devices
AMEX Research Corporation Technologies
Address: PL 15-692 Bialystok,
Elektronowa Str. 6, POLAND
Tel.: +48 602723295, Fax: +48 856530703
[email protected]
NiMH battery charging methods
Basically, charging of NiMH cells is performed by injecting constant current for a given period of time.
The higher the current the shorter the charging time. Additionally, the charger monitors voltage
across the cells and as an option their temperature. Depending on the charging current value the
cells voltage may change differently. Proper detection of NiMh cells voltage is very important at
higher charging currents (0.5..1C; C – cell capacity). The NHC-01 uses different end-of-charge
detection algorithms depending on charging current.
To assure reliable charging, NiMH chargers must include electronic filtering to compensate for noise
and voltage fluctuations induced by the battery and the charger. NHC-01 smart charger module
achieves this by combining NDV (Negative Delta V), voltage plateau, delta temperature (dT/dt),
temperature threshold and time-out timers into the full-charge detection algorithm. These “orgates” utilize whatever comes first depending on battery condition.
Standard charging with current 0,1C (also called ”overnight charging”)
This charging method is the simplest and basically does not require any precise measurement
circuits. Charging is performed by forcing constant current with a value of 0.1C for 14..16 hours. This
method was very common in NiCd battery chargers (it is also used to charge NiMH batteries).
Because of low charge current there is almost no risk of overheating and overcharging even though
the battery will not be disconnected from the charger for much longer time. However, the big
disadvantage of the method is quite long charging time which is unacceptable in many cases.
When low charging currents are selected (125 mA and 250 mA) the –dV detection is extremely
difficult or not possible at all, because the voltage drop value is incredibly low. The only way to
detect the end of charging is to measure voltage across the cells pack (the cells are connected in
series). Additionally, charging time is counted by a timer. When time expires the charging process is
stopped even though the required voltage across the cells pack is not yet reached (too low voltage
may indicate battery worn-out or damage)
Fast charging
In order to fast charge a NiMH cell high current with a value of 0,5..1C is used. NHC-01 charger
module uses current of 1A to charge AA cells and 0.5A for AAA ones. High charging current shortens
the entire process to 1..3 hours. However, this approach requires precise detection of cells voltage
changes and optionally cells temperature because high charging current can easily cause
overcharging and overheating that can destroy the cells and even lead to explosion as a
When fast charge is used, the cell voltage initially rises and when the cell is almost fully charged the
voltage value is stabilized for a short time (see Fig. 1). Then the voltage starts to drop slightly (“-deltaV” algorithm) . At the same time the temperature of the cell starts to rise rapidly (typically 0.8..1.5°C
per minute – it is used in “delta-T” or “dT” algorithm). The voltage drop is very low and it is typically
in the range of 5 to 10 mV per cell (depending on the charging current).
The above phenomenon is a sign that the cell is fully charged. However, to detect such low voltage
drop is very difficult task because of external noise, power supply voltage fluctuations and errors
introduced by measurement circuits. The voltage drop is very low relative to high cell voltage which
makes its detection even more difficult. Because of that, it is recommended to use a temperature
sensor attached to the cell. NHC-01 has an additional connector to install NTC 10kΩ thermistor
(B25/85=3691 K). Thermistor installation is described further.
Cell capacity (percents)
Fig. 1. An example of cell voltage waveform
at different charging currents
The –delta-V (or –dV for short) is precisely and reliably detected by the NHC-01 thanks to the use of
proper analog and digital filtering and precise measurements of the cells voltage. Digital filtering
allows to reject false –dV which can occur due to sudden changes in the power supply voltage value
and noise that can be present in the battery wires.
Overcharge protection
Under normal conditions (fast charging) charging process is stopped when –dV reaches 8 mV per cell
or dT reaches about 0.8...1.5 C/minute. However it may happen that –dV or dT are not detected.
In such case there must be some other ways to detect the end of the charging. The charger
constantly measures the voltage across cells packet. If the voltage reaches about 1.7V per cell the
charging is stopped. When the thermistor is used and appropriate dT value is not detected the
charging is also stopped when the temperature reaches 55°C. Additionally, a timer is employed to
count charging time. The timer has the highest priority in deciding of stopping the charging. When
the time expires the charger turns off the current regardless of –dV, dT and overall voltage across
cells pack.
Short circuit and overload protection
NHC-01 has built-in output overload and short circuit protection. The protection is activated when
charging current exceeds 1.5 A. Then the charging is turned off for 2 seconds. After this time the
charging is turned on again (it is a “hiccup” mode protection). When the overload or short circuit is
detected again the cycle repeats. As long as the failure condition is detected the timer is frozen.
It should be noted that the protection is usable only for the charger output itself. It does not protect
the charged battery against short circuit which can cause very large currents to flow through the
Work mode indicator
NHC-01 can work in several modes which can be signaled by LED diode indicator. The LED does not
require current limiting resistor as it is built into the charger PCB and it has value of 330 Ω. The LED
supply voltage is 3.3V (typ.).
Four work modes are defined:
no charging (LED is off)
charging (LED lights continuously)
short circuit or overcharge (LED blinks fast with a frequency of about 3 Hz)
end of charging (LED blinks slowly with a frequency of about 0.5 Hz)
The LED diode signal can also be used to connect to external circuits for example microcontrollers,
processors etc.
Table 1. NHC-01 charging methods
125 mA
250 mA
500 mA
1000 mA
Charging mode
Full charge detection methods used
(for AAA cells)
(for AA cells)
(for AAA cells)
(for AA cells)
timer, max. voltage across cells packet,
max. cell temperature (note 1)
timer, max. voltage across cells packet,
max. cell temperature (note 1)
timer, max. voltage across cells packet,
max. cell temperature, dT, -dV (note 1)
timer, max. voltage across cells packet,
max. cell temperature, dT, -dV (note 1)
Max. cell
voltage (note 2)
16 h
1,55 V
16 h
1,55 V
1,7 V
1,7 V
1) Maximum cell temperature and dT method are used only when thermistor is used. The
thermistor is detected automatically when the charging process starts. The thermistor is
not required but is highly recommended when high charging currents are used (500 mA
and 1000 mA)
2) Maximum voltage across cells pack is equal to number of cells multiplied by max. cell
It is recommended to use a proprietary battery packs made of selected NiMH cells.
The selection among others is based on a choice of NiMH cells with similar internal resistance. It
guarantees similar voltage values across the cells during charging. Internal cells resistances have
different values among different manufacturers. It makes different voltages to appear across cells
pack in the final charging phase. The voltage across cells pack also depends on charging current.
Powering the NHC-01
It is best to use stabilized external power supply source with output current higher than 1A. The use
of non-stabilized power supply is not recommended as its output voltage depends on load current
and may exceed the allowable power supply voltage range accepted by the charger. Non-stabilized
power supply output voltage contains ripple that may substantially disturb charging process. The
minimum value of power supply voltage required depends on the number of cells selected (see Table
Table 2. NHC-01 parameters
Power supply voltage (note 5)
quiescent current (note 1)
charging current (note 4)
charging current deviation
output voltage for LED diode
(note 2)
overload/short circuit
protection threshold
number of cells
required thermistor
(note 3)
Power supply current drawn by NHC-01 without cells pack
LED output is equipped with internal built-in current limiting resistor (330 Ω)
Thermistor is optional but it is recommended when 500 mA or 1000 mA charging currents are used.
Charging current is set by jumpers and can have the following values: 125 mA, 250 mA, 500 mA and
1000 mA (see charger installation description)
5) The actual power supply voltage value depends on number of cells chosen (see charger installation
Typical installation of the NHC-01
Figure 2 shows typical installation of the NHC-01 charger module (see notes below the picture). If
the charger module is to be built into mobile robot for example the use may want to mount a twoposition switch (ON/OFF-Charge) and a socket for power supply (AC/DC adapter) – see Fig. 3.
Fig. 2. Typical installation of the NHC-01. Thermistor should be placed inside the cells packet (between two
cells and must strictly adhere to one of the cells). LED diode should be visible to the user (for example on the
panel of the device
• Jumpers should be set before powering up the charger. When charging takes place,
changing jumpers setting will have no effect.
• Ambient temperature should not be higher than +40° C when using the charger module.
• Use short wires with proper cross-section to connect battery to the charger. Too long or
too thin wires will add unwanted voltage drop which can cause charging process stop
prematurely. Total resistance of battery wires should not be higher than 0.05 Ω.
• Remove the AC/DC power before connecting or disconnecting the battery pack.
• Some components in the charger module may become hot when charging current is set to
1000 mA. It is absolutely normal and should not arouse a concern.
Charging current jumpers setting:
(NHC-01 top view as shown below)
125 mA
250 mA
500 mA
1000 mA
Charging current
Number of cells jumpers setting:
(NHC-01 top view as shown below)
2 cells
3 cells
4 cells
5 cells
6 cells
7 cells
Number of
cells selection
8 cells
Table 3. Minimum power supply voltage vs. number of cells
Number of
Minimum power
supply voltage of the
NHC-01 charger
10 V
11 V
12 V (see note below)
15 V
17 V
Note: some NiMH cells may require higher voltage (min. 13 V)
Final remarks concerning NHC-01 charger module usage
Before charging the battery the user should set appropriate number of cells and charging
current. Incorrect selection of the number of cells can cause overcharging or undercharging.
Thermistor is an optional component and when it is used it must strictly adhere to one of the
cells (it should adhere to the cell which is in the middle of the pack at best)
Stabilized power supply
(230VAC/9..18VDC, 1.5A)
OFF - Charge
NHC-01 charger
− +
LED indicator
NiMH cells pack
Fig. 3. Schematic diagram of the NHC-01 equipped with external power supply, power supply
voltage connector, NiMH battery pack, thermistor, LED diode and a switch
If the two-position switch is in ON position (Fig. 3) then the battery voltage is present across
BAT+ and BAT- terminals. When the switch is in OFF position (charge) then charging will start
when power supply is connected to the power supply connector. When charging is finished
(it is indicated by slowly blinking LED) it is necessary to disconnect the power supply from
the charger. The power can be drawn from the battery when the switch is ON position.
Example of using NHC-01 charger for charging NiMH battery pack
(6 x AA) in a mobile robot
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