NiCd/NiMH Battery

Fast-Charge Controllers

6

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Getting Started

The MAX712/MAX713 are simple to use. A complete

linear-mode fast-charge circuit can be designed in a

few easy steps. A linear-mode design uses the fewest

components and supplies a load while charging.

1) Follow the battery manufacturer’s recommendations

on maximum charge currents and charge-termination

methods for the specific batteries in your application.

Table 1 provides general guidelines.

2) Decide on a charge rate (Tables 3 and 5). The slow-

est fast-charge rate for the MAX712/MAX713 is C/4,

because the maximum fast-charge timeout period is

264 minutes. A C/3 rate charges the battery in about

three hours. The current in mA required to charge at

this rate is calculated as follows:

–––––––––––––––––––––––––

(charge time in hours)

Depending on the battery, charging efficiency can be

as low as 80%, so a C/3 fast charge could take 3 hours

and 45 minutes. This reflects the efficiency with which

electrical energy is converted to chemical energy within

the battery, and is not the same as the power-

conversion efficiency of the MAX712/MAX713.

3) Decide on the number of cells to be charged (Table 2).

If your battery stack exceeds 11 cells, see the Linear-

Mode High Series Cell Count section. Whenever

changing the number of cells to be charged, PGM0

and PGM1 must be adjusted accordingly. Attempting

to charge more or fewer cells than the number pro-

grammed can disable the voltage-slope fast-charge

termination circuitry. The internal ADC’s input volt-

age range is limited to between 1.4V and 1.9V (see

the Electrical Characteristics), and is equal to the

voltage across the battery divided by the number of

cells programmed (using PGM0 and PGM1, as in

Table 2). When the ADC’s input voltage falls out of

its specified range, the voltage-slope termination cir-

cuitry can be disabled.

4) Choose an external DC power source (e.g., wall

cube). Its minimum output voltage (including ripple)

must be greater than 6V and at least 1.5V higher

than the maximum battery voltage while charging.

This specification is critical because normal fast-

charge termination is ensured only if this require-

ment

is

maintained

(see

Powering

the

MAX712/MAX713 section for more details).

5) For linear-mode designs, calculate the worst-case

power dissipation of the power PNP and diode (Q1

and D1 in the Typical Operating Circuit) in watts,

using the following formula:

load - minimum battery voltage) x (charge current

in amps)

6) Limit current into V+ to between 5mA and 20mA. For a

fixed or narrow-range input voltage, choose R1 in the

Typical Operation Circuit using the following formula:

R1 = (minimum wall-cube voltage - 5V)/5mA

8) Consult Tables 2 and 3 to set pin-straps before

applying power. For example, to fast charge at a

rate of C/2, set the timeout to between 1.5x or 2x the

charge period, three or four hours, respectively.

Table 1. Fast-Charge Termination Methods

Charge

Rate

NiMH Batteries

NiCd Batteries

ΔV/Δt and/or

temperature, MAX713

ΔV/Δt and

temperature,

MAX712 or MAX713

> 2C

2C to C/2

ΔV/Δt and/or

temperature,

MAX712 or MAX713

ΔV/Δt and/or

temperature, MAX713

ΔV/Δt and/or

temperature, MAX713

ΔV/Δt and/or

temperature, MAX712

< C/2