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charging 10 cell nimh pack 6 cells at a time?

I need to charge a 10 cell nimh pack, but my input voltage is only
12VDC. I dont want to use a switching power supply.

I'm thinking that if I charge the 10 cells in two banks, i.e., 5 at a
time, and I switch between the banks very quickly (100 times per
second say), that I will be able to charge the entire 10 cells okay.
And since I am switching between the packs so quickly I could still
use dt/dt termination (change in temperature rise) since its a
relatively slow phenomenon and switching between packs is essentially
like using a pulsed-current charge method which is pretty standard.

Anyone ever do this? Comments?
 
R

Richard Henry

Jan 1, 1970
0
I need to charge a 10 cell nimh pack, but my input voltage is only
12VDC. I dont want to use a switching power supply.

I'm thinking that if I charge the 10 cells in two banks, i.e., 5 at a
time, and I switch between the banks very quickly (100 times per
second say), that I will be able to charge the entire 10 cells okay.
And since I am switching between the packs so quickly I could still
use dt/dt termination (change in temperature rise) since its a
relatively slow phenomenon and switching between packs is essentially
like using a pulsed-current charge method which is pretty standard.

Anyone ever do this? Comments?

Use a switching power supply.
 
Use a switching power supply.

Whats the point of you even responding? I'm NOT going to use a
switching power supply. Thats the whole point of the post.

Haven't you people ever heard of experimenting for experimentings
sake? How did any of you learn electronics with your boring can't-do,
shouldn't-try, attitudes?
 
P

PeterD

Jan 1, 1970
0
Whats the point of you even responding? I'm NOT going to use a
switching power supply. Thats the whole point of the post.

Haven't you people ever heard of experimenting for experimentings
sake? How did any of you learn electronics with your boring can't-do,
shouldn't-try, attitudes?

Well you could...

Oh, wait you don't want the best solution, only one that you've
already determined is what you want.

Sure you can do it that way.

But WHY? Design your charger to charge the first five cells then the
other five, why 'high speed' switch between the two banks. Do one
bank, then the other.
 
Well you could...

Oh, wait you don't want the best solution, only one that you've
already determined is what you want.

Sure you can do it that way.

But WHY? Design your charger to charge the first five cells then the
other five, why 'high speed' switch between the two banks. Do one
bank, then the other.- Hide quoted text -

- Show quoted text -

I understand there might be a better way to do things, but I _want_ to
do it this way to see if it can be done! Plus it could be a lot
cheaper than a switching version.

Because this pack will be used in a device that needs to be available
at any time, and if someone decides to stop the charge at some random
point during the charge cycle, you end up with a battery pack where
half the cells are dead and the other half are partially charged. So
switching between them at a relatively high speed ensures the cells
all get a roughly equal amount of charge per unit time. (human time
anyway)

All you need is a 555 timer switching the current source between the
banks 60 times a second or so.
 
M

Martin Riddle

Jan 1, 1970
0
I understand there might be a better way to do things, but I _want_ to
do it this way to see if it can be done! Plus it could be a lot
cheaper than a switching version.

Because this pack will be used in a device that needs to be available
at any time, and if someone decides to stop the charge at some random
point during the charge cycle, you end up with a battery pack where
half the cells are dead and the other half are partially charged. So
switching between them at a relatively high speed ensures the cells
all get a roughly equal amount of charge per unit time. (human time
anyway)

All you need is a 555 timer switching the current source between the
banks 60 times a second or so.


NIMH batteris really dont like to be pulsed charged. Lead Acid maybe
but not NIMH.

Cheers
 
NIMH batteris really dont like to be pulsed charged. Lead Acid maybe
but not NIMH.

Cheers- Hide quoted text -

- Show quoted text -

Okay then two seperate LM317's charging the banks seperately, one
charge controller that uses dt/dt (BQ2002T, ~$1 @ 1k qty), a relay for
connecting it all up, and there you have a way to charge 10 nimh cells
with only 12V and no switching power supply, and you do it by charging
5 cell banks simulatenously. Should be cheap.

Comments?
 
Okay then two seperate LM317's charging the banks seperately, one
charge controller that uses dt/dt (BQ2002T, ~$1 @ 1k qty), a relay for
connecting it all up, and there you have a way to charge 10 nimh cells
with only 12V and no switching power supply, and you do it by charging
5 cell banks simulatenously. Should be cheap.

Comments?

It seems if you are not careful, you can get the two different groups
of batteries at different potentials. As you discharge the batteries,
you only have one pack. I think this can lead to a scenario where you
get cell reversal. Take a look at this:
http://www.batteryuniversity.com/print-partone-16.htm

The nice thing about charging the pack as one group is if the
batteries are well matched, they see the same charge and discharge
current, so there is less of a likelihood of cell reversal.
 
R

Richard Henry

Jan 1, 1970
0
Okay then two seperate LM317's charging the banks seperately, one
charge controller that uses dt/dt (BQ2002T, ~$1 @ 1k qty), a relay for
connecting it all up, and there you have a way to charge 10 nimh cells
with only 12V and no switching power supply, and you do it by charging
5 cell banks simulatenously. Should be cheap.

Comments?

Use the switching power supply.
 
I need to charge a 10 cell nimh pack, but my input voltage is only
12VDC. I dont want to use a switching power supply.

I'm thinking that if I charge the 10 cells in two banks, i.e., 5 at a
time, and I switch between the banks very quickly (100 times per
second say), that I will be able to charge the entire 10 cells okay.
And since I am switching between the packs so quickly I could still
use dt/dt termination (change in temperature rise) since its a
relatively slow phenomenon and switching between packs is essentially
like using a pulsed-current charge method which is pretty standard.

Anyone ever do this? Comments?

Overkill, put 10 batteries in series it equals 12v
 
It seems if you are not careful, you can get the two different groups
of batteries at different potentials. As you discharge the batteries,
you only have one pack. I think this can lead to a scenario where you
get cell reversal. Take a look at this:http://www.batteryuniversity.com/print-partone-16.htm

The nice thing about charging the pack as one group is if the
batteries are well matched, they see the same charge and discharge
current, so there is less of a likelihood of cell reversal.- Hide quoted text -

- Show quoted text -

Remember though, the charge termination comes from one thermistor,
which is in thermal contact with both "banks", so the charge
termination will occur at the same point as if you were charging all
the cells as one pack. I'm not sure there is any way, from the cells
perspective, that a difference between charging all the cells at once
and charging two banks at once could be perceived.
 
Remember though, the charge termination comes from one thermistor,
which is in thermal contact with both "banks", so the charge
termination will occur at the same point as if you were charging all
the cells as one pack. I'm not sure there is any way, from the cells
perspective, that a difference between charging all the cells at once
and charging two banks at once could be perceived.

dT/dt isn't all that precise. In any event, you would have to
implement a discharge before charge if you really want to trust that
both sides got charged to the same level.

dT/dt is really flaky. Remember, you use multiple end of charge
detection schemes, so how do you know voltage depression didn't
terminate the charge?

I had a customer problem one time that ended up being due to an extra
layer of kapton tape between the batteries and the thermistor.
 
E

ehsjr

Jan 1, 1970
0
[email protected] wrote:

Remember though, the charge termination comes from one thermistor,
which is in thermal contact with both "banks", so the charge
termination will occur at the same point as if you were charging all
the cells as one pack. I'm not sure there is any way, from the cells
perspective, that a difference between charging all the cells at once
and charging two banks at once could be perceived.

When one of the banks reaches the DT/dt set point, it shuts
down charging. The other bank does not get a full charge,
except by chance. You need individual control and sensing
of each bank.

If you have the capability to separate the thing into
2 banks as you mentioned in your earlier post:
"Okay then two seperate LM317's charging the banks seperately,"
then there is no issue. Run one supply into two separate
controllers, with equal termination set points. The
controllers can share some circuitry, but each bank needs
to be terminated depending on the signal from its own
sensor.

There's drawbacks/compromises/etc to whichever way you do
it. I like your idea separate LM317's, provided you can
separate the pack into two individual banks of 5 cells each,
and you terminate the charge individually.

Actually, since anything you do is a compromise when you
split the pack, I'd set each 317 for constant current with
a taper charge when terminal voltage is reached. The easy
way is to set a TL431 to the terminal voltage, and have it
shunt current from the source when that voltage is reached.
That way, you can continue to pump constant current from
the 317, but divert some portion of it through the 431, and
provide the NiMh with whatever C/X you decide.

A nice side benefit is that you can light a red led and
extinguish a green led with that approach, so that you see
that the pack is fully charged, and use the current through
the led as part of what you divert. You can also put a
resistor in parallel with the led to divert more current.

The TL431 has a max of 100 mA AIRC - if you need to shunt
more than say 75 mA, you can control a transistor with the
431.

Is that the best possible approach for charging an NiMh.
No. But it is simple, it works, and you are forced to
compromise in your situation, so it seems a reasonable
way.

Ed
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
If you want switching for charging try this use one lm555 and connect 5 batteries in series one bank going to gnd the other 5 bank going to 12v lm555 can source and sink current just adjust the current to batteries specs.
 
[email protected] wrote:

<snip>









When one of thebanksreaches the DT/dt set point, it shuts
downcharging. The other bank does not get a full charge,
except by chance.  You need individual control and sensing
of each bank.

If you have the capability to separate the thing into
2banksas you mentioned in your earlier post:
"Okay then two seperate LM317'schargingthebanksseperately,"
then there is no issue.  Run one supply into two separate
controllers, with equal termination set points.  The
controllers can share some circuitry, but each bank needs
to be terminated depending on the signal from its own
sensor.

There's drawbacks/compromises/etc to whichever way you do
it.  I like your idea separate LM317's, provided you can
separate the pack into two individualbanksof 5 cells each,
and you terminate the charge individually.

Actually, since anything you do is a compromise when you
split the pack, I'd set each 317 for constant current with
a taper charge when terminal voltage is reached. The easy
way is to set a TL431 to the terminal voltage, and have it
shunt current from the source when that voltage is reached.
That way, you can continue to pump constant current from
the 317, but divert some portion of it through the 431, and
provide the NiMh with whatever C/X you decide.

A nice side benefit is that you can light a red led and
extinguish a green led with that approach, so that you see
that the pack is fully charged, and use the current through
the led as part of what you divert.  You can also put a
resistor in parallel with the led to divert more current.

The TL431 has a max of 100 mA AIRC - if you need to shunt
more than say 75 mA, you can control a transistor with the
431.

Is that the best possible approach forchargingan NiMh.
No.  But it is simple, it works, and you are forced to
compromise in your situation, so it seems a reasonable
way.

Ed- Hide quoted text -

- Show quoted text -

Very interesting Ed.

I'm thinking that charging the 10 cells in two banks, or all at once,
are equivalent as far as charging all the cells equally and fully. My
logic is:

- in both cases, the same current flows through the cells to charge
them
- therefore the temperature curve during charging is identical between
the two methods
- and therefore, the charge termination point is the same between the
two methods

So really, if the concern is how equally the cells get charged, there
is no difference between the two methods.

What do you think?
 
E

ehsjr

Jan 1, 1970
0
Very interesting Ed.

I'm thinking that charging the 10 cells in two banks, or all at once,
are equivalent as far as charging all the cells equally and fully. My
logic is:

- in both cases, the same current flows through the cells to charge
them
- therefore the temperature curve during charging is identical between
the two methods
- and therefore, the charge termination point is the same between the
two methods

So really, if the concern is how equally the cells get charged, there
is no difference between the two methods.

What do you think?



Why not experiment with all your ideas? Then you can
decide which you like best. :)

Ed
 
J

JosephKK

Jan 1, 1970
0
Whats the point of you even responding? I'm NOT going to use a
switching power supply. Thats the whole point of the post.

Haven't you people ever heard of experimenting for experimentings
sake? How did any of you learn electronics with your boring can't-do,
shouldn't-try, attitudes?

Whiner. Actually the switching rate should be about 1 per second, the
battery makers learned this from the plating folks.
 
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