NiMH charger design

[Steve Robertson]

Hi. I just bought a Jaycar(PowerTech) AA charger.
It detects full charge by detecting dV (drop in voltage at full charge)
so it wont overcharge(in theory)
However when charging 4 AA's it connects them in series
Now trying to detect dV across 4cells must be a dodgy idea??
If one cell decides not to charge(or takes longer than the others)
then the other 3 will keep getting the 700mA charging current: risking frying them??
Is this something to worry about or does it not really matter for normal use
(My old charger did fry batts, it used constant current for 5hour's time, plus the heat of the
charger heated up the batts)
The thing is, I could get a better charger with 4 separate charging circuits but for the extra cost
I could buy another set of 4 batts (or maybee even more)

Any suggestionstions??

I may even build a charger, the IC's are quite cheap (MC33340, about $3)

 

[Phil Allison]

"Steve Robertson" <nospam@nospam.com> wrote in message
news:3f989490@clear.net.nz...

Hi. I just bought a Jaycar(PowerTech) AA charger.
It detects full charge by detecting dV (drop in voltage at full charge)
so it wont overcharge(in theory)

** That "theory " works reliably for Ni-Cds only.

However when charging 4 AA's it connects them in series
Now trying to detect dV across 4cells must be a dodgy idea??

** Yes, one cell may charge first, dip voltage and trip the cut out.

The scheme depends on the cells being well matched.

If one cell decides not to charge(or takes longer than the others)
then the other 3 will keep getting the 700mA charging current: risking
frying them??

** Not likely - the charged Ni-Cd cells are dropping volts all the
time and so the dv/dt is negative.




............ Phil

 

[Steve Robertson]

However when charging 4 AA's it connects them in series
Now trying to detect dV across 4cells must be a dodgy idea??


** Yes, one cell may charge first, dip voltage and trip the cut out.

The scheme depends on the cells being well matched.
Yes it probhably works very well when the cells are reasonably new.

But is the follwing correct??
With 4 cells in series, is the dV trip voltage is 4x the trip voltage of one cell
(say 1v for 4AA or .25v for 1AAbatt). This is why there is a switch to 'tell'
the circuit how many cells are being charged. If one cell takes longer to charge than the others
then it wont trip at the right time. Not good if charging at 700mA

 

[Phil Allison]

"Steve Robertson" <nospam@nospam.com> wrote in message
news:3f98bec6@clear.net.nz...

However when charging 4 AA's it connects them in series
Now trying to detect dV across 4cells must be a dodgy idea??


** Yes, one cell may charge first, dip voltage and trip the cut out.
The scheme depends on the cells being well matched.

Yes it probably works very well when the cells are reasonably new.
But is the following correct??

With 4 cells in series, is the dV trip voltage is 4x the trip voltage of
one cell
(say 1v for 4AA or .25v for 1AAbatt).

** Dv/dt means what it says. It is not a voltage difference. When
the charging voltage gradient goes negative, it trips - after some pre-set
time delay to eliminate any "noise".

This is why there is a switch to 'tell' the circuit how many cells are
being charged.

** Could be any number of reasons for that switch.

If one cell takes longer to charge than the others then it wont trip at
the right time. Not good if charging at 700mA

** Huh ?

If one or more cells charges early - then there is no "right time."




.............. Phil

 

[Steve Robertson]

** Dv/dt means what it says. It is not a voltage difference. When
the charging voltage gradient goes negative, it trips - after some pre-set
time delay to eliminate any "noise".

by volt differnce I meant a drop in voltage between 2 measurements
Dv/dt = change in volt / change in time ?

From a controller spec sheet
MAX713 terminates when a conversion shows the battery
voltage has fallen by at least 2.5mV per cell.
=====
So when all 4 batts have -ve volt gradient then fast charge stops.
So if 4 batts are in series & one batt doesnt fall by 2.5v for whatever
reason then the charger stays on high charge (untill a safety timeout)
So the 3 good batts will be over charged
(assuming the IC wont cut out when 1 batt goes -ve dv/dt, it 'waits' for all 4)
???

Probhably a non issue but my last charger damaged the batts,
the ends poped out a bit. I now have corrosion on the batt contacts on the camera
probhably from the batts as the charger has a bit of corrosion also. The camera will have to
be stripped to clean the contacts. Dont want this to happen again.

 

[Phil Allison]

"Steve Robertson" <nospam@nospam.com> wrote in message
news:3f98dbd2@clear.net.nz...

** Dv/dt means what it says. It is not a voltage difference. When
the charging voltage gradient goes negative, it trips - after some
pre-set
time delay to eliminate any "noise".


by volt differnce I meant a drop in voltage between 2 measurements
Dv/dt = change in volt / change in time ?

** Depends on the method - there is such a thing as analogue.

So when all 4 batts have -ve volt gradient then fast charge stops.

** Depends on the gradient setting.

Probhably a non issue but my last charger damaged the batts,
the ends poped out a bit. I now have corrosion on the batt contacts on the
camera
probhably from the batts as the charger has a bit of corrosion also. The
camera will have to
be stripped to clean the contacts. Dont want this to happen again.

** Failure to detect the peak and cut off is not uncommon - my Sony 8
mm camera battery charger was "engineered" to destroy the batteries ASAP.



.............. Phil

 

[Alex Gibson]

"Steve Robertson" <nospam@nospam.com> wrote in message news:3f989490@clear.net.nz...

Hi. I just bought a Jaycar(PowerTech) AA charger.
It detects full charge by detecting dV (drop in voltage at full charge)
so it wont overcharge(in theory)
However when charging 4 AA's it connects them in series
Now trying to detect dV across 4cells must be a dodgy idea??
If one cell decides not to charge(or takes longer than the others)
then the other 3 will keep getting the 700mA charging current: risking frying them??
Is this something to worry about or does it not really matter for normal use
(My old charger did fry batts, it used constant current for 5hour's time, plus the heat of the
charger heated up the batts)
The thing is, I could get a better charger with 4 separate charging circuits but for the extra cost
I could buy another set of 4 batts (or maybee even more)

Any suggestionstions??

I may even build a charger, the IC's are quite cheap (MC33340, about $3)

For a charger look at the microchip app notes
http://www.microchip.com/1010/suppdoc/design/lowpwr/index.htm

have app notes for li-ion, nicad and lead acid


another here
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1060/ln/en

other useful looking app notes
http://www.maxim-ic.com/appnotes10.cfm/ac_pk/5/ln/en