battery monitoring question

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#1
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Hi guys

I was wondering, for a voltage monitor circuit, simply using a voltage divider with a couple of resistors and feeding it to an ADC input of an AVR, is it sufficient for a normal circuit? say for monitoring the voltage of a battery when it is being charged.

I do know that resistors value change non-linearly depending on the ambient temperature, but is the change significant enough to be concern about for such an application like battery voltage monitoring?

or do I need to use a something like zero temperature coefficient resistor, or some other method to get an accurate reading?

thanks

Zhuhua Wu - Electronic Engineering Student

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To make such a decision, you need to figure out the accuracy that you require for your battery monitoring application, and compare that requirement with the measurement error caused by the resistor's tolerance, the resistor's temperature drift, the accuracy of the voltage reference, any errors in the ADC, and possibly noise in the measuring circuit.

Since you haven't told us the specifics of your battery monitoring application, it's pretty difficult to be more definitive.

- S

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Note that Atmel has a load of material about battery charging using AVRs...

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Quote:
or do I need to use a something like zero temperature coefficient resistor, or some other method to get an accurate reading?
I think you need not bother about this at all.
Common metal resistors have coefficient 50 ppm/degreeC (which is 5 Ohm for 100kOhm resistor).
Also note that in a voltage divider the temperature change applies to both resistors.
So the divide ratio remains (nearly) the same.

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So divide down the highest voltage expected to a smidge less than 5V and yer golden.

Imagecraft compiler user

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To answer your questions:

    I am not sure the accuracy that required for my application, I still need to do some googling for that (thanks for pointing that out) The battery is going to be monitored is normal SLA battery, likely between 3Ah to 5Ah, but a deep cycle one might consider.
    The purpose of monitoring is prevent the battery to be overly discharged and damage the battery permanently.

mnehpets wrote:
To make such a decision, you need to figure out the accuracy...

Zhuhua Wu - Electronic Engineering Student

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awneil wrote:
Note that Atmel has a load of material about battery charging using AVRs...

Thanks you awneil, I think I was googling avr battery monitoring, but didn't came up something useful, I will try searching "avr battery charging" instead. :-)

Zhuhua Wu - Electronic Engineering Student

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Thanks Visovian, that's something very good to know, thanks a lot

Visovian wrote:
Quote:
or do I need to use a something like zero temperature coefficient resistor, or some other method to get an accurate reading?
I think you need not bother about this at all.
Common metal resistors have coefficient 50 ppm/degreeC (which is 5 Ohm for 100kOhm resistor).
Also note that in a voltage divider the temperature change applies to both resistors.
So the divide ratio remains (nearly) the same.

Zhuhua Wu - Electronic Engineering Student

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Thanks bobgardner, an simply and straight forward is always appreciated :)

bobgardner wrote:
So divide down the highest voltage expected to a smidge less than 5V and yer golden.

Zhuhua Wu - Electronic Engineering Student

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bug13avr wrote:
The battery is going to be monitored is normal SLA battery, likely between 3Ah to 5Ah, but a deep cycle one might consider.
The purpose of monitoring is prevent the battery to be overly discharged and damage the battery permanently.

SLA batteries are reasonably tolerant of charge/discharge voltage. You should be OK with metal film resistors of 1% tolerance or better (as suggested by Visovian).

- S