order achieve maximum efficiency when charging battery, necessary impl

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Application Note Using DS2770 3-Cell NiMH Charger
order achieve maximum efficiency when charging battery, necessary implement charging techniques geared specifically battery cell chemistry. example, negative dT/dt termination criteria used NiMH cells, while CC/CV pulsed-charged technique appropriate rechargeable lithium cells. DS2770 battery monitor charge controller configured charge either NiMH cells. This application note addresses several things consider when using DS2770 charge NiMH battery pack.
NiMH CHARGING BASICS
NiMH cell nominal voltage 1.2V. Therefore, stacking three NiMH cells together results battery pack with nominal voltage 3.6V maximum voltage 4.5V. Typically, NiMH cell charged with constant current until terminating condition encountered. most common determine when NiMH cell become fully charged either observe drop voltage rise temperature. cell becomes fully charged, voltage drops slightly. same time, temperature rises rapidly less charge source energy goes into actually charging cell more energy turns into heat. DS2770 monitors temperature NiMH battery pack when charging checks change temperature over certain time period (dT/dt) determine when terminate charge.
HARDWARE SETUP
Figure contains recommended schematic charging 3-cell NiMH battery pack. Refer Application Note values components shown schematic. addition following this schematic, there some other things consider when completing application design. charge source application should constant current current limited supply with opencircuit voltage between 12V. current supplied charge source must large enough sufficiently heat battery pack when full charge reached, otherwise, cells heat slowly trip dT/dt primary charge termination limit. Consequently, charge termination always reliant secondary criteria such exceeding maximum temperature maximum charge time. order assure charging will terminate result dT/dt primary termination limit, DS2770 should placed close battery cells with good thermal contact. necessary include thermally conductive putty between cells DS2770 enable efficient heat transfer. Additionally, enclosing cells DS2770 casing allows heat generated cells they charge transferred DS2770 before lost ambient conditions.
091602
AN222
Figure Recommended Schematic 3-Cell NiMH Battery Pack
CHARGE SOURCE PACK+
150W
DS2770
NiMH Cells
100pF 100pF
100pF
PACK100pF 150W
DATA
DEVICE SETUP
Prior starting charge sequence, DS2770 must configured NiMH charge control. chemistry type charged selectable through Charge Type (CTYPE) (bit Status Register. Setting CTYPE value selects NiMH charge control. Status Register also contains Charge Initiation (CINI) (bit which determines charge started. value CINI indicates that charge only started when Start Charge command sent device. value CINI then charge started issuing Start Charge command simply attaching charge source. Also, necessary maximum charge time Charge Time Register (CTR) initialization (memory location 0x34h). When charge started, initialized value that stored memory location 0x34h. initialized value 0x00h, charge begins; however, terminates less than 56s. Each register adds another maximum charge time. example, maximum charge time required, then 0xC0h should written initialize CTR. necessary ensure initialized maximum amount time that charge require. value smaller than that could cause charge terminated prematurely.
STARTING CHARGE
When start charge condition encountered, charge begins unless DS2770 outside valid temperature range, +40°C. Once charge started, DS2770 outside valid temperature range without terminating charge. battery voltage ever below 3.0V charger connected, trickle charge initiated until battery voltage recovered 3.0V. Start Charge command required start trickle charge will begin regardless setting CINI bit. typical charge profile NiMH battery pack using DS2770 charge controller shown Figure When constant current charge begins, voltage rises loading charge source temperature rises heating pack. DS2770 ignores temperature
AN222
rate termination first 5min avoid false dT/dt detection uses most recent temperature measurement select appropriate dT/dt threshold.
Figure Temperature, Voltage, Current During Typical NiMH Charge
Temp (50°C) NiMH Cell Temperature Ambient Temp
Voltage Current
Start Charge
Charge Terminated
Charge Time
ENDING CHARGE
DS2770 provides several ways terminate charge. primary charge termination surpassing dT/dt threshold. constant current continues until rate temperature rise above selected dT/dt threshold 2.8min. effective dT/dt threshold determine full charge 1°C/min regardless starting temperature this occurs approximately same time voltage drop negative secondary charge terminations provided DS2770 shown Figure First maximum temperature threshold, which fixed +50°C. Second maximum charge time, which programmable described DEVICE SETUP section this application note. temperature rises above maximum temperature threshold charge time exceeds maximum limit, charging immediately terminates. These secondary charge termination criteria included prevent permanent damage battery pack over charging. However, important terminate primary termination order prevent shortening life battery pack. When these terminations occur Charge Status (CSTAT0 CSTAT1) bits (bits Status Register will both indicate "Charge Sequence completed" status.
CONCLUSION
DS2770 easily configured provide safe reliable charging 3-cell NiMH battery pack. crucial remember considerations proper implementation this solution. first ensure that there sufficient heat transfer between cells DS2770 accurate temperature reading cells. second ensure charge source providing enough current heat cells rate that will exceed dT/dt termination limit. Once these issues addressed, circuit implemented, DS2770 configured NiMH charging through simple settings onchip registers setting that does expire before charge terminated.

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