Re: charging 10 cell nimh pack 6 cells at a time?

On Nov 18, 12:29 pm, "acann...@xxxxxxx" <acann...@xxxxxxx> wrote:
On Nov 17, 10:11 pm, m...@xxxxxxxxx wrote:



On Nov 17, 4:59 pm, "acann...@xxxxxxx" <acann...@xxxxxxx> wrote:

On Nov 17, 4:29 pm, Martin Riddle <martin_...@xxxxxxxxxxx> wrote:

On Mon, 17 Nov 2008 13:06:43 -0800 (PST), "acann...@xxxxxxx"

<acann...@xxxxxxx> wrote:
On Nov 17, 12:37 pm, PeterD <pet...@xxxxxxxxxx> wrote:
On Mon, 17 Nov 2008 11:41:51 -0800 (PST), "acann...@xxxxxxx"

<acann...@xxxxxxx> wrote:
On Nov 17, 11:21 am, Richard Henry <pomer...@xxxxxxxxxxx> wrote:
On Nov 17, 9:50 am, "acann...@xxxxxxx" <acann...@xxxxxxx> wrote:

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.

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.- 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.

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?

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.

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.
.

Relevant Pages

  • Re: charging 10 cell nimh pack 6 cells at a time?
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  • charging 10 cell nimh pack 6 cells at a time?
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  • Re: charging 10 cell nimh pack 6 cells at a time?
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  • Re: charging 10 cell nimh pack 6 cells at a time?
    ... I dont want to use a switching power supply. ... and I switch between the banks very quickly (100 times per ... that I will be able to charge the entire 10 cells okay. ...
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