Login
Register

ENGLISH简体中文
(Chinese)日本語
(Japanese)РУССКИЙ
(Russian)

Products

Solutions

Design

Buy

Support

About Us

My Maxim

Maxim > Design
support > App notes
> Amplifier
and Comparator Circuits > APP 527
Maxim > Design support >
App notes
> Battery
Management > APP 527
Maxim > Design support >
App notes
> Measurement
Circuits > APP 527

Sep 30, 2002

Keywords: comparator, reference, low-power,
hysteresis, comparators
Related
parts

Download,
PDF format (26kB)

[?]

APPLICATION NOTE 527

Simple Circuit Measures Battery Drain

Abstract: Simple
test circuit using a comparator and reference to measure
battery drain.

Measuring
battery life for a portable system is a time-consuming task,
and the methods that accelerate battery discharge don't
provide reliable results. In the usual approach you simply
measure elapsed time while operating the product to the point
of battery discharge. Running several such systems in parallel
obviously gives more data, if you can afford to tie up the lab
equipment.

You can try to derive battery life from
data-sheet specifications associated with the circuit
components, but a calculated value is usually far short of the
actual operating time. Current-drain specs tend to be
conservative for low-power ICs, because they are tested with
high-speed equipment that cannot easily measure low supply
currents. Unlike many electrical parameters, battery life (in
most cases) is better specified as a realistic typical than as
a guaranteed minimum.

The movie "Chinatown" has
inspired a simple alternative to the expensive
data-acquisition systems and chart recorders normally required
in these efforts. (Jack Nicholson placed a cheap watch under
the tire of a parked car so he could return at his convenience
to check the time of departure.) A similar trick marries a
cheap (but low-power) clock to a low-power
comparator/reference circuit (Figure 1).

Figure
1. This inexpensive clock tracks the operating time for a
battery-powered portable system. When the battery voltage (or
a selected output) drops below the discharge threshold set by
R1, the stopped clock retains the elapsed operating
time.

The clock can be a "Spartus quartz alarm" at
$9.95, or any other drug-store style, non-digital,
battery-powered analog
clock. IC1 is a CMOS
comparator/reference circuit that gates power to the clock.
The IC's low current
drain
(4µA) lets it steal power directly from the circuit under
test. Why not power the clock from the input terminals?
Because it doesn't run properly that way-the clock has a
stepper motor that draws its current in brief surges, with
amplitudes as high as 100ma. For the circuit shown, a large
filter capacitor at the clock's input terminal did not solve
the problem.

When the test circuit's battery voltage
(or output voltage, if desired) falls below a selected
threshold, the comparator
output swings low and turns off Q1, removing power to the
clock. The inactive clock then reads the running time,
provided you set it to 12:00 before the test.

To set
the operating threshold voltage, connect a power supply to the
input terminals and adjust it to the minimum voltage for which
the circuit will just operate. Adjust R1 so the clock just
stops running. Then remove the power supply, set the clock to
12:00, connect the test circuit, and go
home.

Related parts

MAX921

Ultra Low-Power,
Single/Dual-Supply Comparators

Free
samples

Automatic updates
Would you like to be
automatically notified when new application notes are
published in your areas of interest? Sign
up for EE-Mail™.

Download,
PDF format (26kB)
APP 527: Sep 30, 2002

AN527, AN 527, APP527,
Appnote527, Appnote 527

|

|

|     Privacy Policy
|     Legal Notices

Copyright © 2011 by Maxim Integrated
Products