Detecting "negative delta V"

[Suraj143]

Hi I need to terminate the charging of NIMH batteries.I'm going to charge two NIMH batteries (2600mAh, 1.2V).

Tell me what value do I have to measure to terminate the charge?

 

[ericgibbs]

Hi I need to terminate the charging of NIMH batteries.I'm going to charge two NIMH batteries (2600mAh, 1.2V).

Tell me what value do I have to measure to terminate the charge?

hi Suraj,

Look at this pdf.

 

[Suraj143]

Hi Eric thanks for that.

It says......

there is a very slight drop in voltage (5-10 mV/ Cell) at 
the end of charging which is very difficult to detect 
using a 10-bit ADC.Also there is a chance of detecting 
the wrong end of charging due to noise. So instead of the 
negative delta voltage, we use a dV = 0 condition for a 
certain time duration. This gives very good approximations 
for detecting the end of charging.

So how do I make dV = 0?What do I have to do?

 

[ericgibbs]

hi,
IIUC they use a simple integration method for detecting 'deltva V', rather than instantaneous values of 'delta V'.

I would have to study the subject in order to give a more meaningful answer.

 

[Suraj143]

Hi eric gibbs I got what it says.

It says when its charging the NIMH curve will rise to its maximum voltage & after drop around 5-10mV.This is the termination point.

But with 10bit ADC due to noise & etc...detecting a 5-10mV is risky.So instead of negative delta V, they make dv=0 which I don't know.

Hope you will get that secret sooner

 

[Suraj143]

I think they mean this.

When the battery is charging if you sample AD you need to compare the new AD result with the previous AD result.
While charging these two values never be comes zero.likewise when its reaches to the maximum value those new & previous values will be same.In this case the difference is = 0,After detecting this point just continue AD sampling for a specific time & if it is still zero then terminate the charge.

 

[ericgibbs]

I think they mean this.

When the battery is charging if you sample AD you need to compare the new AD result with the previous AD result.
While charging these two values never be comes zero.likewise when its reaches to the maximum value those new & previous values will be same.In this case the difference is = 0,After detecting this point just continue AD sampling for a specific time & if it is still zero then terminate the charge.

hi,
Thats also the way I read it, ie: testing for a zero change of Vb between samples.

 

[audioguru]

Instead of using a complicated old circuit, why not use a new battery charger IC that can charge a Ni-MH battery?