Trickle charging NiCads

capacity

I would check into dendrites. Overcharging of NiCd cells, especially long term constant trickle charging, contributes to those. Once a dendrite punctures the separator, the cell has a foot in the grave.

It comes down to what amount of trickle charging while fully charged does not cause enough dendrite growth to make cells die too much earlier than they would be declared dead for some other reason.

Another thing - charging when already fully charged makes the cells warmer and that makes them age faster. My experience so far is that C/10 warms cells a little noticeably, and that C/4 warms cells enough for reasonably reliable detection of full charge via either direct detection of warming (temperature sensor) or indirect detection of warming (voltage decreases after increasing). It appears to me that C/20 *usually* does not warm cells enough after they are fully charged to accelerate their aging much, though I would worry about the dendrite issue if the cells are cooking 24/7/365 at C/20.

Should you find it practical to reliably detect full charge and then switch to a lower "trickle charge" current, such as with usage of one of those "smart charger" ICs that detects reversal of increasing voltage that results from C/4 or faster charge causing cells to heat up after achieving "completion of charging", it appears to me good to have such a "smart charger" reduce the charging current to something close to or slightly outpacing self-discharge of "topped-off" cells. I suspect that C/100 and C/200 will work well, C/200 may keep cells only

80-90% "full", though I also suspect that C/50 may be, maybe "usually is", "not that big a deal" for NiCd cells being cooked at such rate 24/7/365.

NiMH appears to me to have less tolerance than NiCd for trickle charging

24/7/365 at rate exceeding self-discharge that occurs at "full charge". Not that I ventured major testing efforts, but some NiMH cell datasheets and some web-search-able items along those lines makes it appear to me that a NiMH cell has ususlly zero to sometimes very minimal, likely-well-tolerated overcharging when trickle-charged at C/200 while fully charged. (NiCd appears to me to "usually not die much earlier than life expectancy" if trickle-charged 24/7/365 at C/100, fair-to-good-chance at C/50, some-to-fair chance even at C/20.)

- Don Klipstein ( snipped-for-privacy@misty.com)

"pimpom"

** No way for you to conclude that unless YOU are a expert yourself.

It is easy to find links that COMPLETELY contradict the ones given.

Google " NiCd trickle charge dendrites " for some sobering info.

** No way for you to conclude that unless YOU are a expert yourself.

Which of course you ain't.

...... Phil

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Thanks for that Don, I was buying a pair of RC battery powered cars for my son and I a few years ago and after a bit of research concluded that NiCd's were more robust and less susceptable to abuse than the NiMH. Even though the NiMH were lighter and held more charge, I bought the NiCd's. I also read that the NiMH only store about 70% of the charge that is pumped through them. Whereas the NiCd's keep 90 to

95% of the charge. I wonder if anyone has tested this?

George Herold

Thanks for that Don, I was buying a pair of RC battery powered cars for my son and I a few years ago and after a bit of research concluded that NiCd's were more robust and less susceptable to abuse than the NiMH. Even though the NiMH were lighter and held more charge, I bought the NiCd's.

** Fool.

I also read that the NiMH only store about 70% of the charge that is pumped through them. Whereas the NiCd's keep 90 to

95% of the charge. I wonder if anyone has tested this?

** Even if true ( and it is not) how would it matter ??

..... Phil

Why? Cost per A-hr was less for NiCds. Also thought my 9 year old son could leave them plugged in for a day with out destroying them.

Oh, have you measured this? It certainly makes no difference for an RC car powered from my wall socket, but I heard that Hybrid cars also use NiMH, If so that's certainly a factor in all the efficiency talk.

Thanks for that Don, I was buying a pair of RC battery powered cars

Why? Cost per A-hr was less for NiCds.

** Barely - and having more AH is the important thing.

Also thought my 9 year old son could leave them plugged in for a day with out destroying them.

** Same goes for NiMH - god knows what crap you have been reading.

Oh, have you measured this? It certainly makes no difference for an RC car powered from my wall socket,

** Which was the only point at issue.

.... Phil

How about something like this:

RogerN

"> ** Same goes for NiMH - god knows what crap you have been reading."

Hmm I have no idea either. I was more than a year ago and I'm lucky if I can remember what I was doing last week. I also recall that the NiMH's were more easily damaged by overheating. So you had to let them cool off more before recharging and also charge them slower. For RC car racing I bought three 1.5 A-hr NiCd's for the price of one 4.0 A-hr NiMH... and by cycling them, one in car, one cooling off and one recharging, we could race cars all day.

Ignoring the RC car racing issue. Do you have any data or information on the charging effeiciency of NiCds vs NiMH? Perhaps if you charge the NiMH slowly they do a better job of hanging on to all the electrons?

George Herold