rechargeables(nicad-nimh) question

rechargeables(nicad-nimh) question

Post by patrick mitche » Thu, 04 Jan 2007 15:08:27



Assume you have a pack made of cells of equal capacity and discharge rate.
What happens if you make the pack where some of the cells were charged  and
others dead- no charge. You charge that pack with a smart charger that
quickly charges, then goes to trickle. When you pull the pack to use it,
should you expect the cells in the pack to discharge equally or will the
pack discharge unevenly due to the different charge state prior to the
charging cycle. Further, if they arent evenly charged, with the individual
adjusting of the cells to the same charge state, will they ever be equally
charged?       Thanks Pat
 
 
 

rechargeables(nicad-nimh) question

Post by fhhuber50677 » Thu, 04 Jan 2007 18:04:42


I would expect the charged cells to get overcharged and the dead cells
to not be fully charged when the smart charger detects peak.

If its NiCd or NiMh... the heat of overcharging the high cells will
give a false peak indication in the low cells because of how the cells
react to being hot.  The smart charger will shut down at that point.

The designers of the chargers assume that no one is dumb enough to make
up a pack from a combination of cells in assorted states of charge.

Eventually over several (unacceptable performance) charge/discharge
cycles, a NiCd pack would just about equalize itself if slow charged.
NiMh would probably have a cell failure due to the abuse before they
self-equalize.

*********

If you try it with LiPo... get video... should be a fire.

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rechargeables(nicad-nimh) question

Post by Doug McLare » Fri, 05 Jan 2007 00:26:42



| Assume you have a pack made of cells of equal capacity and discharge rate.
| What happens if you make the pack where some of the cells were charged  and
| others dead- no charge. You charge that pack with a smart charger that
| quickly charges, then goes to trickle.

It would probably charge until several of the cells peaked (the ones
that were charged already), and then decide it was done and stop.  So
some cells would be fully charged, and some almost empty.

| When you pull the pack to use it, should you expect the cells in the
| pack to discharge equally or will the pack discharge unevenly due to
| the different charge state prior to the charging cycle.

Well, it would work somewhat (but not well) for a bit, at least until
the cells that were almost empty became completely empty.  Then rather
than adding 1.2-1.4 volts to the total, they'd put in zero (or less)
and your pack would seem nearly dead very suddenly.

| Further, if they arent evenly charged, with the individual adjusting
| of the cells to the same charge state, will they ever be equally
| charged?

If you put them on a slow charger (C/10 or so) and left them on the
charger for at 13 hours or so, yes.  You should do this anyway with a
new NiCd or NiMH pack, once when new and once whenever completely
discharged (like if you left your TX on overnight) just to `form' and
`equalize' the pack, so it shouldn't be a catastrophe if you made a
pack like you were talking about, as long as you prepare it properly
afterwards.

The problem is that if you never do a formative charge, and never let
your peak charger trickle charge once done for a long time, then the
problem will never correct itself (or will only do so a little bit
each charge.)  And when those not-originally-charged cells get fully
discharged, then they get reverse charged a little, which weakens
them, and it'll happen over and over.  Your pack will never work well,
and will probably get worse each time you use it.

Fortunately, NiCd and NiMH cells handle over-charging at a low rate
just fine (as long as it's not overdone) so that formative charge will
take care of it for you pretty easily.  As fhuber said, LiPos aren't
so forgiving, though of course most mistakes result in puffed (ruined)
cells rather than an actual fire.  (But a fire is always a possiblity.)

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