Low-Battery Detection

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coolcat
 
Posts: 9
Joined: Mon Sep 10, 2007 12:58 am

Low-Battery Detection

Post by coolcat »

Excerpt from data sheet
Low-Battery Detection
The MAX756/MAX757 contain on-chip circuitry for lowbattery
detection. If the voltage at LBI falls below the regulator’s
internal reference voltage (1.25V), LBO (an opendrain
output) sinks current to GND. The low-battery monitor's
threshold is set by two resistors, R3 and R4 (Figure
1), which forms a voltage divider between the input voltage
and the LBI pin. The threshold voltage is set by R3
and R4 using the following equation:

R3 = [(VIN / VREF) - 1] (R4)


To check what is used in the minty boost currently:
R3 = 100K
R4 = 110 K

Input voltage needs to be lesser than this value for Low battery out LED to glow:
Vin = ((100K/110K) + 1) * 1.25
= 2.38 volts.

This triggers the LED to light up on supplying 5 volts at 100 ma. This is not what is desired.

NiMh battery has a voltage of 1.2 + 1.2 = 2.4 volts.

So if we desire that the low battery is triggered at 1.9 volts [Normally the voltage of battery is fairly linear through the discharge of battery]. use the following:


r4 = r3 / (vin/vref)-1

r4 = r3/0.52


if R4 = 120K
select R3 = 62.4K
or similar combination (R3 + R4 should not exceed 200K).


R3 goes towards battery + side, R4 goes to the ground. Junction goes to LBI.

connect the LED via a 1K resistor to the output (USB). The output still gets 5 volts but the battery may not be able to drive the LED. About 5ma should be good to drive the SMD LED.

Can you please update the schematic of minty boost to incorporate these values.

rokii
 
Posts: 1
Joined: Tue Sep 11, 2007 6:22 pm

Post by rokii »

Actually the values are fine for nimh batteries. Nimh battery voltages does not fall linearly.

Check out this and look at the discharge curve (200mA discharge rate)

http://data.energizer.com/PDFs/nh15-2000.pdf

By the time the battery falls to 1.19v/cell most of the charge has been used up.

Cheers

coolcat
 
Posts: 9
Joined: Mon Sep 10, 2007 12:58 am

Post by coolcat »

Data specs look good.

Real life experience states following.

I selected the resistor for low voltage detectiion of 1.85 volts. After setting ipod to charge through mintyboost, noticed the low battery led glowing.

Checking voltages:
battery1 : 1.2 volts.
battery2 : 0.72 volts

One battery discharged faster than the other.

So having a lower voltage is better. :)

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rhymeswithdreidel
 
Posts: 1
Joined: Mon Sep 24, 2007 10:34 am

Post by rhymeswithdreidel »

so... do you guys have suggestions for part numbers from Mouser for completing the project?

i just ordered the PCBs from adafruit today, but want to make sure i have all the parts from Mouser by the time they get here. ;)

-john

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