Lt1510 Charger With Av Termination

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by LTC Applications Staff

Any portable equipment that requires fast charge needs proper charge termination. Commonly, a LT1510 constant-voltage, constant-current type charger controlled by a microcontroller is used. Sometimes, however, a microcontroller is not available or is not suitable for fast-charge termination.

When fast charging NiCd batteries with constant current, the internal battery temperature rises toward the end of the charge. Since the temperature coefficient of NiCd is negative, the temperature rise causes the battery voltage to drop. The drop can be detected and used for termination (called -AV termination). The circuit in Figure 234 is a solution for a 3-cell (Panasonic P140-SCR) NiCd battery charger with -AV termination.

U1 in Figure 234 is programmed by resistor R2 for a conservative charge current of 0.8A, which is 0.57C. Typical fast-charge current is 1C. (The boldfaced C represents a normalization concept used in the battery industry. A C rate of 1 is equal to the capacity of the cell in ampere-hours, divided by 1 hour. Since the capacity of the P140-SCR is 1.4 ampere-hours, C is 1.4 amperes.)

To determine the voltage droop rate, the battery was connected to an LT1510 charger circuit programmed for a 0.8A constant-current. The data was plotted as voltage versus time and the results are shown in Figure 235. The voltage slope is calculated to be -0.6mV/s. After the battery voltage dropped 300mV from the peak of 4.93V (100mV per cell), the charger was disabled.

At the heart of the circuit in Figure 234 is U3, a sample-and-hold IC (LF398). For every clock pulse at pin 8, the output of U3 (pin 5) updates to the input level on pin 3. When the battery voltage drops, the input to U3 also drops. If the update step at the output of U3 is sufficiently negative, U2B latches in the high state and Q1 turns on. Q1 terminates the charge by pulling down the LT1510's Vc pin, and thereby disabling it.

U2A and the associated passive components smooth, amplify and level shift the battery voltage. The timer (U4) updates the hold capacitor (C8) every fifteen seconds. The timer signal stays high for 7ms, sufficient time for the hold capacitor to be charged to the input level. U2B and the associated parts form a latch that requires a momentary negative voltage at pin 6 to change state. R15 supplies the negative feedback and Q2, R16, R17 and C10 reset the latch on turn-on.

U3's output voltage droops at a rate proportional to the hold capacitor's internal leakage and the leakage current at pin 6 (10pA typical). This droop is very low and does not affect the operation of the circuit.

The minimum negative battery voltage slope required to trigger termination (-dV/dT) is 0.3mV/s. It can be calculated from:

VTRIG is the trigger voltage of U2B,

VTRIG = VREF x R12/(R11 + R12) = 5 x 1/101 = 49.5mV

Vref= 5V

TCLK is the clock period, 15 seconds,

GU2A is the gain of the first stage, = R8/(R4 || R5) = 11

Schematic Jbl Charge
Figure 234. Schematic Diagram: 3-Cell NiCd Charger with -AV Termination

The circuit in Figure 234 was built and connected to a the time of termination is very consistent because the system that discharges the battery to 3V after termination, discharge time at constant current is a better measure of at constant current of 0.8A. Once the battery drops to 3V, charge level than charge time. A secondary termination the system reenables charging, and thus the complete method, such as time, battery temperature, or the like, is system repeats charge/discharge cycles indefinitely. The duration of 70 charge discharge cycles was recorded. The following is condensed data from the test:

1. Average Charge Time: 2:00:55 Hours

2. Standard Deviation of Charge Time: 5:37 Minutes

3. Average Discharge Time: 1:59:14 Hours

4. Standard Deviation of Discharge Time: 48 Seconds.

The ratio of standard deviation of charge time to average charge time proves that the charger has good repeatability. However, the ratio of standard deviation of discharge time to average discharge time shows that the charge level at also recommended.

TIME (MIN)

Figure 235. Voltage-Droop Rate, 3-Cell NiCd Battery

TIME (MIN)

Figure 235. Voltage-Droop Rate, 3-Cell NiCd Battery

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