AVR463: Charging Nickel-Metal Hydride

Batteries with ATAVRBC100

8-bit
Features Microcontrollers
• Fully Functional Design for Charging Nickel-Metal Hydride Batteries
• High Accuracy Measurement with 10-bit A/D Converter
• Modular “C” Source Code
• Easily Adjustable Battery and Charge Parameters Application Note
• Serial Interface for Communication with External Master
• One-wire Interface for Communication with Battery EEPROM
• Analogue Inputs for Reading Battery ID and Temperature
• Internal Temperature Sensor for Enhanced Thermal Management
• On-chip EEPROM for Storage of Battery and Run-Time Parameters

1 Introduction
This application note is based on the ATAVRBC100 Battery Charger reference
design (BC100) and focuses on how to use the reference design to charge Nickel-
Metal Hydride (NiMH) batteries. The firmware is written entirely in C language
(using IAR Systems Embedded Workbench) and is easy to port to other AVR®
microcontrollers.
This application is based on the ATtiny861 microcontroller but it is possible to
migrate the design to other AVR microcontrollers, such as pin-compatible devices
ATtiny261 and ATtiny461. Low pin count devices such as ATtiny25/45/85 can also
be used, but with reduced functionality.

Rev. 8098A-AVR-09/07
2 Theory of Operation
Battery charging is made possible by a reversible chemical reaction that restores
energy in a chemical system. Depending on the chemicals used, the battery will have
certain characteristics. A detailed knowledge of these characteristics is required in
order to avoid inflicting damage to the battery.

2.1 NiMH Battery Technology

Nickel-Metal Hydride (NiMH) battery technology is the successor of Nickel-Cadmium
(NiCd), and is considered a stepping-stone to lithium-based batteries. Its
characteristics are quite similar to NiCd, but offers approximately 50% higher energy
density at the cost of reduced cycle life [1] (see References page 25). NiMH does not
suffer so much from the memory effect as NiCd, and therefore needs fewer exercise
cycles during storage. In addition, NiMH batteries are more environmentally friendly
as they contain no cadmium, and therefore pose less of a problem regarding
disposal. Currently, it is being used in everything from cameras, PDAs and power
tools to hybrid cars.
Although Li-Ion batteries offer almost twice the energy density, higher cell voltage and
shorter charge times, they are not as stable as nickel-based batteries. Also, since
nickel-based batteries have a voltage close to the regular 1.5V Zinc Carbon and
Alkaline batteries, they are more suitable for use in existing equipment powered by
such batteries.
Compared to NiCd, the drawbacks of NiMH batteries include a slightly higher self-
discharge rate (approximately 20% per month at room temperature), lower
overcharge tolerance, shorter cycle life (300-500 charge cycles) and longer charge
time as it generates more heat.

2.1.1 Safety
Nickel-based batteries are quite stable, but might vent gas or explode if severely
mishandled (short circuited, charged with the wrong charger, etc.) due to internal
pressure and heat build-up. Most batteries come with a safety vent, thermal
fuse/switch or pressure switch to help prevent this.

2.2 Charging NiMH Batteries

The most common method for recharging nickel-based batteries is with a constant
current. In general, the charge current is recommended in the range 0.5 – 1.0 C 1 (fast
charge) since slow charging causes crystalline formation, also known as memory
effect, and lesser charge efficiency [7]. Greater currents than this, on the other hand,
may cause damage. It is also more difficult to detect a full charge of NiMH batteries at
low charge currents.
NiMH batteries are between 70% and 90% charge efficient, meaning you must
charge for longer than one would calculate from a given charge current and battery
capacity.

1
C represents the battery’s capacity per hour, i.e. mA.
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2.2.1 Safety
Towards the end of a charge cycle, the positive electrode of the battery will reach full
charge first, causing it to produce oxygen. To avoid pressure build-up in the cell, the
oxygen is recombined at the negative electrode to produce water. This process does
however generate heat, and it relies on the charge current to be limited so that the
oxygen generation rate is not greater than the recombination rate.
Should a pressure build-up occur, most batteries have a safety vent or even a
pressure switch that will cut off the charge current once the pressure gets too high.
The venting of gas deteriorates the battery as electrolyte is lost. More advanced
batteries have safety electronics that activate once pressure or temperature exceeds
a threshold.

2.2.2 Battery Temperature

As mentioned, NiMH batteries generate heat during charging and it is therefore
important to monitor the battery temperature to prevent damage. Suggested
temperature ranges for fast charging are in the area 5 - 45°C, but temperatures
between 45°C and 60 °C are acceptable for a short period at the end of a charge. [5]
[6].
Naturally, great currents cause a faster increase in temperature. High temperatures
decrease cell voltage and charge efficiency, and cause a higher rate of self-
discharge. Cooling the batteries with a fan is sometimes done to maximize efficiency,
though lower temperatures slow the oxygen recombination, possibly causing a faster
pressure build-up.

2.2.3 Charge Time

Determining for how long to charge is not so easy since a battery’s state of charge
cannot be reliably determined from its voltage. In addition, it depends on the battery’s
capacity and the charge current.
Due to charge inefficiency, the ideal charge input lies in the area 120 – 150%,
depending on if maximum cycle life (120%) or capacity (150%) is wanted. Assuming a
1.0 C charge current, a full charge of an empty battery should therefore take between
1.2 hours and 1.5 hours. Since NiMH batteries are not as tolerant of overcharge as
NiCd ones, measures must be taken to detect a full charge.

2.2.4 Full Charge Detection

At about 60% charge input the heat generation in NiMH cells starts to increase, and
may be used to detect a full charge. It is recommended to end the charge once the
rate of temperature increase reaches 1 °C per minute [5] [6].
Another sign of a full charge is a drop in voltage due to the oxygen recombination,
though this is not as pronounced at high temperatures or low charge currents.
Suggested limits are in the range 6 - 20 mV per cell. A low limit will reduce the
amount of overcharging, but also heightens the risk of prematurely ending the charge
due to electrical noise or other normal drops in voltage during charging.

2.2.5 Typical Fast Charge Characteristics

Battery specifications should always be verified from manufacturer’s data sheets.
Below is a summary of typical nickel-metal hydride battery fast charge characteristics.

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 Table 2-1. Typical Charge Characteristics for Fast Charge
Parameter Typical Value
Charge time 1.5 - 3 hours
Charge current 0.5 – 1 C
Charge efficiency 70 – 90 %
Charge voltage 1.4 – 1.6 V
Temperature range 5 … 60 °C (max)

2.2.6 Typical Battery Characteristics

The table below summarises general data for NiMH batteries [5] [6].
Table 2-2. Typical NiMH Battery Characteristics
Parameter Typical Value
Nominal voltage 1.2 V
Open circuit voltage 1.25 – 1.35 V
Typical end voltage 1.0 V

2.2.7 Three Step Fast Charge

The chosen charge procedure for this application is a fast charge followed by a
topping charge, as recommended in literature and by battery & charger
manufacturers. This does however require that the batteries’ specifications are
defined at compile time since we otherwise don’t know them. The procedure is as
follows:
• Prequalification – Charge at low current (0.1 C) for at most 2 (TBD) minutes,
until battery voltage reaches 1.0 V. Timeout means battery is quite likely
damaged.
• Fast charge – Charge at high current (C) for at most 1.5 hours, until either the
rate of temperature increase reaches 1 °C per minute or the voltage drops
with 15 mV per cell.
• Low rate charge – Top up the battery with a low current (0.1 C) for 30
minutes, to ensure a full charge. This current is low enough for oxygen
recombination to keep up, in case of an overcharge.
For safety, a temperature limit of 50 °C has been set for the last two stages, and 35
°C for the first.
It is also recommended to leave the batteries on a trickle charge (0.003 C – 0.05 C)
for an indefinite duration, to counteract self-discharge. This has not been
implemented, but may easily be done so by changing the end charge stage in
Charge(). It would be beneficial to switch between the batteries at regular intervals,
which would also allow for detection of uncharged batteries.

2.3 Battery Charger

This application note targets the ATAVRBC100 Battery Charger reference design by
Atmel. The reference design is rather complex and has loads of features but this
application focuses on the core feature of the design, the buck converters, only. For

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more information on the BC100, please refer to application note AVR451 - BC100
Hardware User's Guide [2].

2.3.1 Microcontroller
The BC100 hosts two microcontrollers: a master (ATmega644) and a slave (an
ATtiny85 or ATtiny861, by default). The master microcontroller is outside the scope of
this application but it may be noted that the microcontrollers are capable of
communicating with each other such that the master may request data from the slave
at any time.
The slave microcontroller is fully capable of handling all tasks related to battery
charging and it does not require a master microcontroller to be present. It constantly
scans the connectors for batteries and, if found, charges them when required. The
slave microcontroller also constantly monitors the hardware for any anomalies.

2.3.2 Power supply

This application note does not focus on the power supply. It may, however, be noted
that the firmware constantly monitors the input voltage levels in order to make sure
operation is reliable.

2.3.3 Buck switches

The firmware on the slave microcontroller controls any of the three buck converters
on board the BC100. The default is to use a high-speed PWM output of the
microcontroller to adjust the voltage and current flow to the battery. The voltage (and
current) of the buck converters is directly proportional to the duty cycle of the PWM
signal.

3 Battery Charger Hardware

This application note is based on the ATAVRBC100 Battery Charger reference
design. A detailed hardware description will not be provided in this document. Please
see AVR451 - BC100 Hardware User's Guide for detailed information.

3.1 Configuration
The ATAVRBC100 Battery Charger reference design must be configured as detailed
below.

3.1.1 Microcontroller
The hardware should be populated as follows:
• Make sure socket SC300 is empty
• Populate socket SC301 with an ATtiny861

It is possible to use other AVR microcontrollers but this application has been
optimised for using ATtiny861. Pin compatible replacements such as ATtiny261 and
ATtiny461 [3] may be used if the compiled code size is decreased. This can be done
by increasing the optimisation of the compiler and by removing unwanted features
from the firmware.
Other microcontroller options include ATtiny25, ATtiny45 and ATtiny85 [4]. These (as
well as other 8-pin AVR microcontrollers) use the SC300 socket on BC100. It should

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 be noted that due to reduced pin count the 8-pin microcontrollers provide less
features than the default 20-pin.

3.1.2 Programming Connector

The microcontroller can be programmed via 6-pin connector J301, using either SPI or
debugWIRE.
Please note that in some hardware revisions of BC100 it may be necessary to
remove R303 and tri-state pin 15 of U202 (set /OE low from the mega644). This
procedure frees the /RESET line for use by external programmer or debugger but
removes the possibility for the master microcontroller to reset the slave. Do not alter
the board unless required. Alternatively, the microcontroller can always be
programmed off-board.

3.1.3 Jumpers
The jumpers should be configured as follows:
• J405 & J406: Set jumpers to 1/4 (max measurable voltage 10V)
• For 300 mAh battery: J401, J404 and J408 should be set to enable Buck converter
C (20V / 1A)
• For 1300 mAh battery: J400, J401, J403, J408 and J407 should be set to enable
Buck converter B (30V / 2.5A)

Other configurations are possible, but may require firmware changes. See variable
VBAT_RANGE in file ADC.h.

3.1.4 Battery
For testing of this application, two generic 3-cell NiMH batteries from Minamoto were
used. The only data available were the specific capacity and nominal voltage. Both
batteries were rated at 3.6 V, with the respective capacities 300 mAh and 1300 mAh.
These batteries did not include Resistor ID, EPROM or NTC.

3.1.5 Battery Resistor ID

Some batteries may include a Resistor ID (RID), which is used to identify the battery
type. The charger can then know data such as capacity, maximum charge current
and charge time, instead of keeping these parameters static. If RID is used, the
resistor values and the associated data are available from the battery manufacturer.
This application supports but does not require RID, and if used, the RID lookup-table
should be updated. The resistor should be connected as follows:
Table 3-1. Connecting battery resistor ID to charger
Battery Pin Charger Connector
RID SCL
GND BATTERY-

If a Resistor ID is not connected, or its value causes ADC saturation, default battery
data may be used. See Configuration on page 22.

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3.1.6 NTC
Temperature measurement during fast charge of NiMH batteries is vital, and some
manufacturers therefore often include a thermistor in their batteries. Usually, these
have a negative temperature coefficient (NTC).
For this application, an RH16-3H103FB NTC from Mitsubishi was employed, and the
NTC lookup-table populated accordingly. The NTC should be connected as follows:
Table 3-2. Connecting NTC thermistor to charger
Battery Pin Charger Connector
NTC NTC/RID
GND BATTERY-

In this application, if an NTC is not connected, the ADC will be saturated and
temperature registered as -1 °C. This will halt the charging, unless a sub zero
minimum battery temperature has been set.

3.1.7 Data EPROM

Some batteries are equipped with an embedded EEPROM for storing charge and
manufacturing data. This application supports the reading of EEPROM via a one-wire
interface. The default is a DS2502 EEPROM connected as follows.
Table 3-3. Connecting external EPROM DS2502 to charger
EPROM Pin Charger Connector
DATA 1-WIRE/SDA
GND BATTERY-

If an EPROM is not connected to the battery charger the application will simply
disregard its absence.

3.1.8 Supply Voltage

The higher the supply voltage, the higher the minimum current the buck switches can
provide. For example, if supply voltage is about 9 V and buck charger C is used to
charge a battery at 4.20 V then the minimum attainable current is about 80 mA. At
this point the smallest decrease in PWM duty cycle (i.e. reducing the contents of
OCR1B by 1) will effectively turn off the current to the battery.
It is recommended to use a supply voltage some three volts above battery charge
voltage. In this application the battery voltage will typically max out at 4.5 V during
charging, so the recommended supply voltage is 7.5 V.
Another method to lower the minimum charge current the hardware can provide is to
use a buck switch with a large inductor. In BC100 this means Buck Switch A.

4 Battery Charger Software

The firmware is written in C language using IAR Systems Embedded WorkbenchTM.
Since the firmware has been written entirely in C, it should not be a difficult task to
port it to other AVR C-compilers. Some compiler specific details will probably need to
be rewritten.

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 The table below lists the files that are relevant to the compiler project.
Table 4-1. Project files
File Type Note
ADC.c C source code
Functions related to A/D conversion
ADC.h Header file
battery.c C source code Functions related to battery control &
data acquisition, and definition of default
battery.h Header file battery data
chargefunc.c C source code
Functions related to charging
chargefunc.h Header file
Enumerations for time.c and USI.c,
enums.h Header file which are used in both Slave and Master
(SPI communication)
main.c C source code
Main program / Program entry point
main.h Header file
menu.c C source code
State machine definitions
menu.h Header file
NIMHcharge.c C source code The charge state function for charging of
charge.h Header file NiMH batteries
Definitions of NiMH cell & battery
NIMHspecs.h Header file
specifications
OWI.c C source code
Functions related to one-wire interface
OWI.h Header file
PWM.c C source code Functions related to generating pulse-
PWM.h Header file width modulated output

statefunc.c C source code

The different state functions
statefunc.h Header file
Structs for ADC.c and battery c, which
structs.h Header file are used in both Slave and Master (SPI
communication)
time.c C source code Functions related to timekeeping and
time.h Header file measurement of time

USI.c C source code Functions related to serial interface (SPI

USI.h Header file communcation)

4.1 Overview
The firmware integrates all functions required to charge two NiMH batteries. Batteries
are connected to separate ports such that one may be charged while the other is idle.
The firmware is fully automated and capable of stand-alone battery monitoring and
charging but it may also be used together with a master microcontroller, such as the
one implemented in BC100.
By default, the firmware fits into an ATtiny861 (build option: debug) or an ATtiny461
(build option: release). Memory requirements of the firmware are summarised in the
table below.
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Table 4-2. Memory requirements of firmware
Build option Memory Approximate value
CODE (Flash) 5800 bytes
Debug DATA (SRAM) 270 bytes
XDATA (EEPROM) 130 bytes
CODE (Flash) 3900 bytes
Release DATA (SRAM) 270 bytes
XDATA (EEPROM) 130 bytes

4.2 State Machine

The state machine is rather simple and resides in the main() function. It simply looks
up the address of the next function to execute and then jumps to that function. The
flow chart of the state machine is illustrated in the figure below.

Figure 4-1. Flow chart of main function, including the state machine
main()

Set Current State = INIT

Look up address for Current State

Jump to function of Current State State function

Next State

Look up address for Next State

Set Current State = Next State

Upon return, the state machine expects the function to indicate the next state as a
return argument. The recognised return codes are described in the table below.
Table 4-3. State machine codes (see source code, menu.h)
Label (1) Related Function (2) Description
INIT Initialize() Entry state
BATCON BatteryControl() Check hardware and batteries
PREQUAL Charge() Raise battery voltage, safety check.
SLEEP Sleep() Low power consumption mode
FASTCHARGE Charge() Charge with constant current at 1.0 C.
TRICKLECHARGE Charge() Charge with constant current at 0.1 C.
ENDCHARGE Charge() End of successful charge
DISCHARGE Discharge()
ERROR Error() Resolve error, if possible
Notes: 1. Name of label, excluding leading “ST_”
2. Function name, as declared in source code

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 State functions are described in the following sections.

4.2.1 Initialize()
The initialisation function is the first state function that will be executed after device
reset. The flow chart of the function is shown in the figure below.

Figure 4-2. Flow chart of initialisation function

Initialize()

Set clock prescaler to 1

Initialize One-Wire Interface

Configure I/O pins and disable all batteries

Initialize Serial Peripheral Interface

Initialize Analoguo-to-Digital Converter

Read data from all batteries

Initialize timer functions

Return(BATCON)

The initialisation function always exits with the same return code, pointing to the state
function for battery control.

4.2.2 BatteryControl()
The battery control function verifies that jumpers are set correctly and then checks to
see if there are any enabled batteries present that require charging. The program flow
is illustrated in the figure below.

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Figure 4-3. Flow chart of battery control function
BatteryControl()

JumperCheck()

JumperCheck()
NO
OK?

Return(ERROR)
YES

Any batteries
NO
enabled?

Flag Error: no batteries enabled

YES
Return(ERROR)

Any battery:
Status OK and not NO
charged

Disable (disconnect) all batteries

Refresh data for selected data

Return(SLEEP)

Return(PREQUAL)

4.2.3 Charge()
The charge function contains the charging algorithm divided into stages. For this
application, it has four stages:
• Prequalification – If the battery voltage is between 0.8 V and 1.0 V, charge at
a low rate like 0.1 C for 2 minutes. If the voltage doesn’t rise to 1.0 V within
the time limit, the battery is likely damaged. Limit max. temperature to 35 °C.
• Fast charge – Charge at 1.0 C for 1.5 hours at maximum, and terminate
when voltage drops 10 – 15 mV per cell or the rate of temperature increase
reaches 1 °C per minute. Limit maximum temperature to 50 °C.
• Top-up charge – Top the battery up with a 0.1 C charge current for 30
minutes. Limit maximum temperature to 50 °C.
• End charge – Decide whether to go into the sleep state or to attempt a
charge of the other battery.
ChargeParameters and HaltParameters are central variables in this function. After
each stage, the function returns the next desired state to main(). The program flow of
this state function is illustrated in the figure below.

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Figure 4-4. Flow chart of the charge state function
Charge()

What is the current

ST_PREQUAL ST_ENDCHARGE
state?
ST_LOWRATECHARG
ST_FASTCHARGE
E

Set charge current to 0.1 C. Set charge current to 1.0 C Set charge current to 0.1 C.
Stop PWM output.
(BattData.Capacty / 10) (BattData.Capacity) (BattData.Capacity / 10)

Set ST_FASTCHARGE as the Set ST_LOWRATECHARGE Set ST_ENDCHARGE as the

Flag battery as charged.
next desired state. as the next desired state. next desired state.

Flag that charging should halt Flag that charging should halt
Reset VBATMax.
once voltage reaches limit or on maximum voltage, voltage
time runs out, and that drop, timeout or maximum Is the other battery
NO
timeout means that battery is temperature rise. enabled?
exhausted.
Start charge timer with
30 minute limit.
Set maximum voltage and
YES
Set voltage limit to defined voltage drop limit to the
limit. defined values.
(BAT_VOLTAGE_PREQUAL) (BAT_VOLTAGE_MAX & Callt ConstantVoltage() Set ST_BATCON as Set ST_SLEEP as next
BAT_VOLTAGE_DROP) to continue charging, next state. state.
get next state in return.

Set minimum and maximum

temperature. Set maximum temperature.
(BAT_TEMPERATURE_MIN (BAT_TEMPERATURE_MAX)
& 35)

Start PWM output. Reset VBATMax.

Start charge timer with

Start charge timer with
defined limit.
1.5 hour limit.
(BAT_TIME_PREQUAL)

Call ConstantCurrent() Call ConstantCurrent()

to start charging, get to continue charging,
next state in return. get next state in return.

Return next state to

main().

4.2.4 Sleep()
The application enters sleep mode when all batteries have been fully charged. It
wakes up at regular intervals to check the current status of the batteries. Sleep mode
is terminated as soon as any battery requires charging.

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Sleep mode is illustrated in the flow chart below.

Figure 4-5. Flow chart of sleep function

Sleep()

Sleep for 8 seconds

Set first battery to actual

Enable actual battery

Actual battery
YES
charged?

Return(BATCON)
NO

First battery
actual?

NO
YES

Set second battery to actual

4.2.5 Error()
Program flow is diverted here when an error has occurred. The error handler contains
some simple algorithms that try to resolve the most common problems. Program
execution will exit the error handler when all sources of error have been cleared.
The program flow is illustrated in the figure below.

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 Figure 4-6. Flow chart of error handler
Error()

Stop PWM output

Disable all batteries

Sleep for 8 seconds

Jumper
mismatch YES
error?
Clear bit in error flag

NO Check jumpers

NO
batteries YES
error?

Any batteries
YES
enabled?

NO
Clear bit in error flag
NO

PWM
control YES
error?

Clear bit in error flag

NO

Battery
temperature YES
error?
Clear bit in error flag

Battery
exhausted YES
error?

Clear battery exhausted bit

Change active battery

NO

Clear bit in error flag

Any error
Yes NO
flags set?

Return(INIT)

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4.3 Charging Functions
These functions are called by Charge() after all parameters have been set.

4.3.1 Constant Current/Voltage

These two functions are similar, apart from what ADC measurements they try to keep
within limits. Therefore, only the flow chart for ConstantCurrent() is illustrated in the
figure below. They both make use of the variable ChargeParameters.
If a Master microcontroller is present, it may temporarily stop the charging by flagging
a charge inhibit. This is to prevent battery damage during prolonged serial transfers.

Figure 4-7. Flow chart for ConstantCurrent()

ConstantCurrent()

Wait for
ADC
conversions
to complete.

Were we Remove flag that

Charging of stopped by Master MCU
NO YES
battery inhibited? Master MCU stopped the
earlier? charging.

YES
Start timers
Flag that Master again.
NO NO
MCU stopped the
charging.

Drop PWM output Increment

Current below
to zero. YES PWM duty
hysteresis?
cycle.

Stop timers. NO

Decrement
Current above
YES PWM duty
hysteresis?
cycle.

NO

HaltNow()?

YES

Return next state.

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4.3.2 Charge Halt Determination
Charge halt is determined by HaltNow(). This function is called by ConstantCurrent()
and ConstantVoltage() every time they loop, to decide if a stage of charging is done.
With the variable HaltParameters the user can specify at what terms the charging
should be halted, and if an error should be flagged if e.g. the time limit expires. An
error flag will also result in ST_ERROR being set as the next state, thereby aborting
the charge. If no errors are flagged, the next desired state, set earlier in Charge(), will
apply.
Lastly, the function checks if temperature is within limits, if the battery is OK and if
mains voltage is above minimum. Should any of these tests fail, the next state is set
to an appropriate error handler (ST_ERROR, ST_INIT or ST_SLEEP) and charging is
aborted.

Figure 4-8. Flow chart for HaltNow() part 1.

HaltNow()

Wait for ADC

conversions to
finish.

Output voltage
Halt on voltage Voltage drop above
YES higher than stored NO
drop selected? or equal to limit?
maximum?

YES YES

Store new Set Halt flag.

maximum.
NO

NO

Output voltage
Halt on maximum
YES above or equal to YES Set Halt flag.
voltage selected?
limit?

NO
NO

Output current
Halt on minimum
YES below or equal to YES Set Halt flag.
current selected?
limit?

NO
NO

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Figure 4-9. Flow chart for HaltNow() part 2

Halt on
temperature rise?

YES

Measured NTC
above stored NTC?

NO
YES

Difference between stored

Temperature
NO and measured NTC above or YES YES Store NTC value.
timer expired?
equal to limit?

Store NTC value.

NO
Reset
Set Halt flag. temperature
Start timer.
temperature NO
timer.

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 Figure 4-10. Flow chart for HaltNow() part 3

Halt on timeout?

YES

Charging timer run

out?

YES

Set Halt flag.

Flag battery Stop PWM

YES
exhaustion? output.
NO

Disable battery and

NO flag it as
exhausted.

NO Flag battery
exhaustion error
and set
ST_ERROR as
next state.

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Figure 4-11. Flow chart for HaltNow() part 4
HaltNow()

Wait for ADC

conversions to
finish.

Output voltage
Halt on voltage Voltage drop above
YES higher than stored NO
drop selected? or equal to limit?
maximum?

YES YES

Store new Set Halt flag.

maximum.
NO

NO

Output voltage
Halt on maximum
YES above or equal to YES Set Halt flag.
voltage selected?
limit?

NO
NO

Output current
Halt on minimum
YES below or equal to YES Set Halt flag.
current selected?
limit?

NO
NO

4.4 Other Functions

The program flow is mainly state-based, but some processing takes place in the
background. This includes A/D conversion, time keeping and serial interface handling.
All of these functions are interrupt-driven.

4.4.1 A/D Conversion

The A/D converter uses the multiplexer to read in data from several channels. At the
end of a conversion the ADC Interrupt Service Routine (ISR) is called, as illustrated in
the flow chart below. After the ISR is complete program execution will return to
normal.

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 Figure 4-12. Flow chart of ADC interrupt service routine

ADC_ISR()

Disable ADC

0x01 MUX channel? 0x17

0x02 0x05

0x03

Store NTC reading Store RID reading Format and

Format and Format and
save
save supply save battery
average
voltage voltage
battery
reading reading
Set next MUX = Set next MUX = current
0x02 0x03

// Flag signed Flag unsigned

conversion next conversion next
Supply voltage
low?

Set next MUX = Set next MUX =

0x17 0x01
YES

Flag mains failure.

NO
Flag a compelete
ADC scan cycle

Set next MUX =

0x05

Update MUX

Signed
Set unsigned Set signed
NO conversion YES
conversion conversion
next?

Both halt and

complete scan cycle NO Re-enable ADC
flagged?

YES

Return from
interrupt

4.4.2 Master-Slave Communication

This application is designed to work as stand-alone but it also supports co-operation
with other microcontrollers. The Universal Serial Interface (USI) can be used for
communication between microcontrollers. The basic protocol for this interface has
been developed but some functions need to be finalised.

20 AVR463
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 AVR463
Figure 4-13. Flow chart of USI overflow interrupt service routine.
USI_OVF_ISR()

Update flags

COMMAND Which state? DATA

Save incoming data ADDRESS Read or write?

Save incoming data (not implemented)

Set Read/Write flag

(not implemented)
Set address
Set SRAM/EEPROM flag

Change state to DATA

Change state to ADDRESS Block counter
non-zero?

Decrease counter

NO Counter Zero?

YES

Set COMMAND state

Return from interrupt

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5 Quick Start Guide
This section describes how to configure, create and download the software.

5.1.1 Configuration
The most important compile-time constants are listed in the table below.

Table 5-1. Battery-related compile-time constants (see source files battery.c,

battery.h and NIMHspecs.h)
Label Description
The number of cells in the battery. Each of the defined cell
BAT_CELL_NUMBER voltages gets multiplied with this, to define
BAT_VOLTAGE_MAX, _LOW, _MIN and _PREQUAL.
In case unmatched batteries are to be charged, this constant
CELL_VOLTAGE_SAFETY is subtracted from CELL_VOLTAGE_MAX for every extra cell
in the battery, ie. BAT_CELL_NUMBER – 1.
CELL_VOLTAGE_MAX The maximum voltage to which a cell should be charged.
The lowest voltage at which a cell is considered charged.
CELL_VOLTAGE_LOW
Charging will start when voltage drops below this level.
The lowest voltage at which charging may be initiated.
CELL_VOLTAGE_MIN Should generally be set to the voltage limit under which
further discharge of batteries will cause damage.
The voltage to which a cell should be charged to during
CELL_VOLTAGE_PREQUAL
prequalification.
The highest battery temperature allowed. Charging will stop /
BAT_TEMPERATURE_MAX
not start if above this.
The lowest battery temperature allowed. Charging will stop /
BAT_TEMPERATURE_MIN
not start if below this.
BAT_CURRENT_PREQUAL Charge current during prequalification mode.
Charge current hysteresis. Current will not be adjusted when
BAT_CURRENT_HYST
within plus or minus this value from target.
Charge voltage hysteresis. Current will not be adjusted when
BAT_VOLTAGE_HYST
within plus or minus this value from target.
Target voltage during prequalification stage. If this voltage is
BAT_VOLTAGE_PREQUAL
not achieved the battery will be marked as exhausted.
BAT_TIME_PREQUAL Maximum amount of time to spend in prequalification stage.
DEF_BAT_CAPACITY Default battery capacity.
DEF_BAT_CURRENT_MAX Default maximum charge current.
DEF_BAT_TIME_MAX Default maximum charge time.
DEF_BAT_CURRENT_MIN Default cut-off charge current.
If defined, batteries without RID (or not matching the lookup-
ALLOW_NO_RID table) will cause the charger to use the battery defaults.
Otherwise, charge is halted.
RID[].Low and RID[].High Assume RID resistance match if value within these limits.

22 AVR463
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 AVR463
Label Description
RID[].Capacity Battery capacity for given RID.
RID[].Icharge Charge current for given RID.
RID[].tCutOff Maximum charge time for given RID.
RID[].IcutOff Charge termination current for given RID.
NTC[] Temperature look-up table.

5.1.2 Compilation
Before compiling the code the following configurations should be made.
Table 5-2. Compiler configuration.
Section Tab Field Value
(1)
Processor ATtiny861
Target configuration
Memory model Small
Data stack 0x40
General Options
Return address 24
System stack
Enable bit Selected
definitions …
Require Selected
C/C++ Compiler Language
prototypes
Output Format Other: ubrof8
-y(CODE)
Linker
Extra Options Command Line -Ointel-extended,(DATA)=$EXE_DIR$\$PROJ_FNAME$_data.hex
-Ointel-extended,(XDATA)=$EXE_DIR$\$PROJ_FNAME$_eeprom.hex
Notes: 1. Other options possible. See section 3.1.1 on page 5 for more information.

5.1.3 Programming
The compiled code is conveniently downloaded to the target device using AVR Studio
and a debugger or programming tool of choice, such as the JTAGICE mkII.
Note that the compiled code contains EEPROM data that must be loaded to the target
for the software to work. Answer OK when AVR Studio asks if EEPROM contents
should be loaded. This is illustrated in the figure below.

Figure 5-1. Loading initialised data to EEPROM

23
8098A-AVR-09/07
 The program expects the use of the internal oscillator and that the clock signal is not
prescaled. Some fuse bits must be programmed to ensure proper program execution.
The fuse bit settings that deviate from the default are listed in the table below.
Table 5-3. Non-default fuse bit settings
Fuse Bit Setting Description
CKDIV8 1 (unprogrammed) Do not divide clock by eight
CKSEL3…0 0010 Use internal oscillator

6 Known Limitations
Here are listed known limitations of the design.

6.1 Detecting Battery Presence

Currently, battery presence is detected by measuring the voltage on each battery
connector. In case of fully discharged batteries, the voltage will sink to 0 Volts as
soon as the relay connects it to the sensing circuitry, causing the charger not to
detect them.
One solution is to charge such batteries for a couple of minutes with a power supply
at about 0.1 C, to raise the voltage slightly.
It is also possible to apply a charging voltage, and flag the battery as absent if no
current flows.
Note that fully discharging batteries may damage them, especially if they are multicell
batteries.

6.2 Current Hysteresis and Voltage Drop

When charging just one or two cells, the current hysteresis may cause false full
charge detection due to a voltage drop. Narrowing the hysteresis may help, but keep
in mind that the Buck converter output is not of infinite resolution.

6.3 Battery Current Measurement

Battery current is sensed using a shunt resistor with very low resistance. This means
noise is easily picked up in the measured signal and that even noise with very low
amplitude may disturb the measurements. As a remedy, the battery current measured
is averaged over four samples.
Yet, it is not uncommon to find fluctuations in the order of 1 or 2 LSB. By default (see
section 3.1.3) this means a measurement error of 7 or 14 mA (see function ScaleI() in
file ADC.c). In practice, this may result in premature end of charge cycle.
The suggested solution is to optimise the size of the shunt resistor (R410: the larger,
the better) and the resistor divider (R400…R410, R427, R428, R446 and R447).

6.4 RID Sensing

Battery identification resistor is sensed via pin PA2 (ADC2). The default pull-up
resistor on this line (R305 in ATAVRBC100 Battery Charger reference design) is 4.7
kohm. This limits the size of the sense resistor to about 14.7 kohm.

24 AVR463
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 AVR463
When using Varta PoLiFlex batteries this means the largest battery size that can be
reliably sensed is 1000 mAh. For larger sense resistors / battery sizes the pull-up
resistor on BC100 must be changed. In addition, the software must be updated to
reflect the new pull-up resistor value.

6.5 Buck chargers

The choice of buck charger (and supply voltage) sets a limit on how low the minimum
charge current may be. The higher the supply voltage and the smaller the buck switch
inductor, the higher will the minimum charge current be. This means some
configurations may result in premature end of charge cycle.
The remedy is to use a low supply voltage and a buck switch with a large inductor.

7 References
1. “What’s the best battery?”. Retrieved August 1, 2007, from Battery University:
http://www.batteryuniversity.com/partone-3.htm

2. “AVR451 - BC100 Hardware User's Guide”. Available from Atmel web site:
http://www.atmel.com/products/avr/

3. “ATtiny261/461/861 Data Sheet”. Available from Atmel web site:

http://www.atmel.com/products/avr/

4. “ATtiny25/45/85 Data Sheet”. Available from Atmel web site:

http://www.atmel.com/products/avr/

5. “Duracell Ni-MH Rechargeable Batteries Technical Bulletin”. Retrieved August 1,

2007 from Duracell:
http://duracell.com/oem/Pdf/others/TECHBULL.pdf

6. “Handbook of Batteries, Third Edition”, from McGraw-Hill.

http://www.mhprofessional.com/product.php?isbn=0071359788

7. “Charging nickel-based batteries” Retrieved August 1 2007, from Battery

University:
http://www.batteryuniversity.com/partone-11.htm

25
8098A-AVR-09/07
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8098A-AVR-09/07