info needed on nimh/nicad charging

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Hello there,

    I understand that one of the common methods for termination of fast
charge is the delta V technique. Kindly clarify the following :

1. Is the voltage to be monitored while charging current is flowing or after
switching it off briefly?
2. If the latter, how soon ( or how much later ) should the voltage be
measured after the current is switched off?
3. How often should the voltage be monitered if the charging rate is around
0.5C?
4. What should be the optimum maximum voltage for nimh cells?
5. If zero delta is used, should the charging be stopped immediately on
finding that two consecutive readings are the same , or should we wait for a
few consecutive readings?

Thanks.
Prasad.




Re: info needed on nimh/nicad charging


> Hello there,
>
>     I understand that one of the common methods for termination of fast
> charge is the delta V technique. Kindly clarify the following :
>
> 1. Is the voltage to be monitored while charging current is flowing or after
> switching it off briefly?
> 2. If the latter, how soon ( or how much later ) should the voltage be
> measured after the current is switched off?

This is a complex issue.
If you measure it with the current on, resistive effects in the path
come into play.
IF you turn the current off, the voltage starts to drop, so you must
measure at EXACTLY the same delay every time.

I once built a charger that used a 555 as the timing and voltage
detector.  The voltage dropped for the entire off time, but since the
off time was constant, the thing worked great.
I tried to duplicate the operation using the PC parallel port for
control.  This FAILED miserably because the sample time latency was not
constant.  I leave the current on.

> 3. How often should the voltage be monitered if the charging rate is around
> 0.5C?

This is another tradeoff area.  First consider how much current you
want.  If the current is too low, the zero slope may not happen.  Too
much and temperature effects cloud the issue.  I've been using C as the
charging current with reasonable success.

I monitor the current every minute with the current left on for the
reason above.  I terminate after five consecutive readings of the same
value Or at the first lower value.

Here are some of the reasons why.

Noise is an issue if you try to measure small changes.

My voltage measurement is just the voltage readback from a programmable
supply.  It has only 10mV resolution.  It really should be better, but
since I'm charging at least 4 cells most of the time, it works ok.

For series cells, the zero slope happens at different times for each
cell.  In aggregate, it smears out.
The problem gets much worse for NiCd cause the pack may never go
negative.  Most of my charging is for NiCd.
The 5 minute compromise seems to work for me.


> 4. What should be the optimum maximum voltage for nimh cells?

You're gonna find this varies all over the map depending on the age and
abuse of the cells.  I use 1.6V/cell as an overvoltage termination, but
sometimes, even that is not high enough for OLD computer packs.
Don't forget to compensate for the resistance in the test setup.
My wire and circuit breaker are 0.3 ohm total.  Charging one cell at
4A, the error term is almost as large as the cell voltage.

> 5. If zero delta is used, should the charging be stopped immediately on
> finding that two consecutive readings are the same , or should we wait for a
> few consecutive readings?
Depends on how far apart the readings, see above.

Don't forget to terminate on time.  Once you've put 1.2X C into the
pack, you might as well quit.  Don't forget to terminate on temperature.
If you start with a full pack or if something goes wrong, you want to
shut it down before it explodes or catches fire.


Cell life will be better if you're conservative.

mike

>
> Thanks.
> Prasad.
>
>



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Re: info needed on nimh/nicad charging


>
>> Hello there,
>>
>>     I understand that one of the common methods for termination of fast
>> charge is the delta V technique. Kindly clarify the following :
>>
>> 1. Is the voltage to be monitored while charging current is flowing or
>> after
>> switching it off briefly?
>> 2. If the latter, how soon ( or how much later ) should the voltage be
>> measured after the current is switched off?
>
>
> This is a complex issue.
> If you measure it with the current on, resistive effects in the path
> come into play.
> IF you turn the current off, the voltage starts to drop, so you must
> measure at EXACTLY the same delay every time.
>
> I once built a charger that used a 555 as the timing and voltage
> detector.  The voltage dropped for the entire off time, but since the
> off time was constant, the thing worked great.
> I tried to duplicate the operation using the PC parallel port for
> control.  This FAILED miserably because the sample time latency was not
> constant.  I leave the current on.
>
>> 3. How often should the voltage be monitered if the charging rate is
>> around
>> 0.5C?
>
>
> This is another tradeoff area.  First consider how much current you
> want.  If the current is too low, the zero slope may not happen.  Too
> much and temperature effects cloud the issue.  I've been using C as the
> charging current with reasonable success.
>
> I monitor the current every minute with the current left on for the
> reason above.  I terminate after five consecutive readings of the same
> value Or at the first lower value.
>
> Here are some of the reasons why.
>
> Noise is an issue if you try to measure small changes.
>
> My voltage measurement is just the voltage readback from a programmable
> supply.  It has only 10mV resolution.  It really should be better, but
> since I'm charging at least 4 cells most of the time, it works ok.
>
> For series cells, the zero slope happens at different times for each
> cell.  In aggregate, it smears out.
> The problem gets much worse for NiCd cause the pack may never go
> negative.  Most of my charging is for NiCd.
> The 5 minute compromise seems to work for me.
>
>
>> 4. What should be the optimum maximum voltage for nimh cells?
>
>
> You're gonna find this varies all over the map depending on the age and
> abuse of the cells.  I use 1.6V/cell as an overvoltage termination, but
> sometimes, even that is not high enough for OLD computer packs.
> Don't forget to compensate for the resistance in the test setup.
> My wire and circuit breaker are 0.3 ohm total.  Charging one cell at
> 4A, the error term is almost as large as the cell voltage.
>
>> 5. If zero delta is used, should the charging be stopped immediately on
>> finding that two consecutive readings are the same , or should we wait
>> for a
>> few consecutive readings?
>
> Depends on how far apart the readings, see above.
>
> Don't forget to terminate on time.  Once you've put 1.2X C into the
> pack, you might as well quit.  Don't forget to terminate on temperature.
> If you start with a full pack or if something goes wrong, you want to
> shut it down before it explodes or catches fire.
>
>
> Cell life will be better if you're conservative.
>
> mike
>
>>
>> Thanks.
>> Prasad.
>>
>>
>
>
>

Yea Mike, thanks from me too.  I've been tinkering with a custom charger
for the large assortment of NiMH batteries I use lately.

--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
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Re: info needed on nimh/nicad charging


On Sun, 27 Mar 2005 11:11:26 -0800, in sci.electronics.design mike

snip
>Cell life will be better if you're conservative.
>
>mike
>
>>


I'd never considered the aggregate problem before

Excellent summary Mike.



martin


Opinions are like assholes -- everyone has one


Re: info needed on nimh/nicad charging


charging', on Sun, 27 Mar 2005:

>Cell life will be better if you're conservative.

So all the cons should join the Republican party?
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
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Re: info needed on nimh/nicad charging



> charging', on Sun, 27 Mar 2005:
>
>>Cell life will be better if you're conservative.
>
> So all the cons should join the Republican party?

1) Nah, the Republicans want everyone to show pivture ID when voting.
Post office pictures don't count.  

2) Since when have Republicans been conservative?

--
  Keith




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