advertisement Fast, High Efficiency, Standalone NiMH/NiCd Battery Charging Design Note 380 Fran Hoffart Introduction Although recent popular attention is focused on Lithium Ion batteries, one must not forget that other battery chemistries, such as Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) have advantages in rechargeable power systems. Nickel-based batteries are robust, capable of high discharge rates, have good cycle life, do not require special protection circuitry and are less expensive than Li-Ion. Among the two, NiMH batteries are rapidly replacing NiCd because of their higher capacity (40% to 50% more) and the environmental concerns of the toxic cadmium contained in NiCd batteries. NiCd/NiMH Battery Charging Basics Batteries come in many sizes and capacity ratings. When specifying charge current, it is commonly related to a battery’s capacity, or simply “C”. The letter “C” is a term used to indicate the manufacturers’ stated battery discharge capacity which is measured in milliamp-hours (mAh). This capacity rating becomes important when fast charging because it determines the required charge current for proper charge termination. There are several commonly used methods for charging Nickel batteries. They are all related to the length of the charge cycle which determines the recommended charge current. A slow charge (or low rate charge) consists of a relatively low charge current, typically 0.1C, applied for approximately 14 hours set by a timer. A quick charge applies a constant current of approximately 0.3C to the battery while a fast charge applies a constant current of 1C or higher. Both quick and fast charge cycles require that the charge current terminate when the battery becomes fully charged. The LTC® 4010 and LTC4011 are NiCd/NiMH battery chargers that simplify Nickel-based battery charger design and include power control and charge termination for fast charging up to 16 series-connected cells using a synchronous buck topology. The LTC4011 provides a full feature set in a 20-lead TSSOP while the LTC4010 comes in a 16-lead TSSOP. The LTC4010 removes the PowerPathTM control output, top-off charge indicator, DC power sense input and provides limited thermistor options. , LTC and LT are registered trademarks and PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. IVIN TO SYSTEM LOAD 100k Q3A LEDS V IN 9V to 30V 18V 18 3k 3k 3k 20 3k TGATE VCC INFET BGATE 2 3 13 19 1 10 2Ok 5 CHEM PIN NiMH CHRG NiCd 14 3.3V LTC4011 READY BAT DCIN TIMER VCELL VCDIV CHEM VRT IBATT 10mH Q1 IDISCHARGE 15 Q2 16 1A RSENSE 0.05 (2A) 12 30.1k (4 CELLS) 8 10k 9 10mF + VTEMP 2000mAh NiMH (4 CELLS) 0.1mF 6 9.76k GND INTVDD 17 SENSE 11 TOC 49.9k (1.5 HRS) 4 PGND FAULT Q3B 10mF NTC 10k 28k 7 10k DN380 F01 Q1: FDC658P Q2: Si3434DV Q3: FDR8508P 0.1mF 21 (BACKSIDE) 0.1mF NTC MURATA NCP18XH 0603 SIZE Figure 1. Full Featured Standalone 2A, 4-Cell NiMH Fast Charger with PowerPath Control 01/06/380 Initial battery qualification verifies that sufficient input voltage is present for charging and that the battery voltage and battery temperature are within an acceptable range before charging at full current. For deeply discharged batteries, a low current trickle charge is applied to raise the battery voltage to an appropriate level before applying full charge current. When qualification is complete, the full programmed constant-current begins. Standalone Charge Termination The charge termination methods used by the LTC4010 and LTC4011 utilize battery voltage and battery temperature changes to reliably indicate when full charge is reached as a function of the charge current selected. The charge current must be sufficiently high (between 0.5C and 2C) for the battery to exhibit the voltage and temperature profile required for proper charge termination. Figure 2 shows a typical fast-charge profile displaying charge current, battery temperature and per cell voltage. This profile indicates that the charge cycle terminated due to the rate of temperature rise or ΔT/Δt. During the charge cycle, both the –ΔV and ΔT/Δt termination methods are active. For NiMH batteries, the –ΔV termination requires that the single cell battery voltage drop 10mV from the peak voltage or the rateof-temperature rise (ΔT/Δt) be greater than 1°C/minute. The measurements are taken every 30 seconds and the results must be consistent for four measurements for termination to take place. Typically the ΔT/Δt termination method occurs earlier in the charge cycle. If this occurs, the LTC4010/4011 adds a top-off charge at a reduced charge current for 1/3 of the programmed time. Top-off only occurs when charging NiMH batteries. After the charge cycle has ended, the charger continues monitoring the battery voltage. If the voltage drops below a fixed threshold level, due to an external load on the battery or self-discharge, a new charge cycle begins with the charge termination algorithms immediately enabled. 1.60 42 1.55 40 1.50 38 SINGLE CELL VOLTAGE 1.45 36 1.40 34 BATTERY TEMPERATURE 2A 32 1.35 CURRENT CHARGE 1.30 1A 30 TOP-OFF 1.25 0 20 60 40 TIME (MIN) BATTERY TEMPERATURE (°C) Complete 4-Cell NiMH Battery Charger Figure 1 shows a fast, 2A charger featuring the high efficiency LTC4011 550kHz synchronous buck converter. The LTC4011 simplifies charger design by integrating all of the features needed to charge Ni-based batteries, including constant current control circuitry, charge termination, automatic trickle and top off charge, automatic recharge, programmable timer, PowerPath control and multiple status outputs. Such a high level of integration lowers the component count, enabling a complete charger to occupy less than 4cm2 of board area. The –ΔV charge termination algorithm begins shortly after the full charge current starts flowing. A fixed delay time prevents false termination due to battery voltage fluctuations from batteries that are deeply discharged or haven’t been charged recently. For batteries that are near full charge, the –ΔV termination sequence begins immediately to prevent overcharging. CELL VOLTAGE (V) During a fast charge cycle, a constant current is applied to the battery while allowing the battery voltage to rise to the level required (within limits) to force this current. As the battery accepts charge, the battery voltage and temperature slowly rise. As the battery approaches full charge, the voltage rises faster, reaches a peak, then begins to drop (–ΔV); at the same time, the battery temperature begins to quickly rise (ΔT/Δt). Most fast or quick charge termination methods use one or both of these conditions to end the charge cycle. 80 28 100 DN380 • F02 Figure 2. Typical NiMH Fast Charge Profile Conclusion The LTC4010 and LTC4011 provide complete standalone solutions for reliable, robust and safe fast charging of NiCd and NiMH batteries. Proper charging is critical to not only obtain maximum battery capacity but to also avoid high temperatures, overcharge and other conditions which adversely affect battery life. Data Sheet Download http://www.linear.com For applications help, call (408) 432-1900, Ext. 2364 Linear Technology Corporation dn380 LT/TP 0106 409K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●