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On Wed, 31 Aug 2005 16:48:11 GMT, (Doug McLaren)
wrote:

In article ,
H. Dziardziel wrote:

| On 27 Aug 2005 05:04:45 GMT, Jim Yanik . wrote:
|
| For say, 1500mAh cells
|
| A typo, "1500mAh cells" should read "1500mA."

Are you sure? 1500 mAh seems right to me. You're talking about the
capacity of the cell itself, not the discharge rate, right?

The 1500mA was an estimated 3C charge rate for the original
NiCads, hence the following words "..that would take about one
hour.." (to begin to fully charge the new high capacity NiMh etc).

Capacity is given in mAh. Charge/Discharge rates in mA.

Well, I do believe that strikes a note but thanks for the reminder
to be more alert. Sigh.

| Unlike NiCads, NiMh are exothermic while charging so will get
| warm, especially at this rather high charge rate..

NiCds get warm during charging too. Just not as warm, due to their
generally lower internal resistance.

| Once fully charged however the temperature (T) rises sharply after
| the very small voltage drop that indicate full charge.

Same with NiCd and NiMH cells -- it's just that with NiMH cells, the
voltage drop is smaller.

| NiCads, being endothermic are more reliably temperature end of
| charge detected since the T change is sharper. In either case,
| _hot_ cells indicate overcharging

Endothermic? No way.

Apparantly there is an endothermic component to the chemical charging
process (and I see references to that in many places)

Yes.and it's significant but I don't claim to understand the
electrochemical.theory behind it,. Good NiCads stay pretty cool
even at high rates. By the way, Li-ion are also somewhat
endothermic until nearly fully charged.

but the current
going through the internal resistance will generate heat as well, and
from what I've seen, the overall temperature _increases_ as you
charge, even for NiCds, during the entire charge cycle, even when
you're not charging at an extremely high rate. (Of course, if you're
charging at a low rate, any temperature changes are hard to detect, at
least until the cell is fully charged, where it becomes much easier as
all energy is going into heat, not just some.)

Perhaps at low charge rates the endothermic component is slightly
stronger than the exothermic heating, but if you're charging a NiCd
battery in 20 minutes -- at a 3C rate -- no way. It's going to get
warm

I'm warming to your point. Regards.

..

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