# Rate of charge of NiMH AAA cells at home

I have a reasonably modern charger (a few months old) and some old
rechargeable cells (about 5 years old). Is it wise to use the two
together?
-----
The charger is specified to give a charge rate of 350 mA.
I think the charge voltage is 2.8 V per pair of cells.
The charger has a "negative-delta-V" sensor.
The AAA cells are NiMH and are each rated as holding 0.185 mAh.
(Q.1) Will this 350 mA *RATE* of charge be too much for such
cells to take resulting in noticeable adverse effects?
------
Assuming the charge in the cells is fully depleted and assuming
that the charge is taken up in a linear way, then the figures
suggest a maximum charge time of 30 minutes.
(Q.2) Is it highly likely that the "negative-delta-V" will be
sensed only after the cells have gone into overcharge (venting gas
and messing up who-knows-what internally)?
-- Tzortzakakis Dimitrios major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr Ï "Alex Coleman" Ýãñáøå óôï ìÞíõìá news:96FC7478CB82931E75@204.153.244.170...
All cells have printed on them "suggested charging current is X:mA"Check if that's under 350 mA, which should be.
Only with a 100% efficiency, which is impossible.It would be more realistic if you assumed a 50% efficiency.
What do you mean venting gas?Modern NiCd cells are supposed to be sealed, aren't they?I've never heard before of a negative delta V sensor.
No.
The 2.8V is a nominal number that is loosely related to actual. A more useful number would be the overvoltage termination number.
negative-delta-V will overcharge NiMH.
You sure about that? VERY old NiCd cells may have been that low. Assuming you really meant 185mAH. I don't recall seeing a AAA NiMH much below 500mAH.
Probably, but more important is the charge termination method. You can buy 8.5 minute charge cells these days.
This is a risky assumption. Cells are rarely fully depleted in actual use. Sooner or later, you're gonna stuff mostly charged cells in the charger. The 2.8V number implies that two cells are charged in series. Things get really bursty when you stuff in one fully charged and one depleted cell. No matter how determined you are, you'll do it eventually. Ask me how I know ;-)
and assuming
Need to be certain of the cell capacity numbers before drawing any conclusions. It certainly is possible to charge cells so fast that they vent before they even get warm.
mike
Your cells wil live MUCH longer when you charge them at a low rate.
P.
| > The charger is specified to give a charge rate of 350 mA. | > I think the charge voltage is 2.8 V per pair of cells. | | The 2.8V is a nominal number that is loosely related to actual. | A more useful number would be the overvoltage termination number.
What is this `overvoltage termination number' you speak of?
You can't determine that a NiCd or NiMH cell is fully charged or not only based on a single instantaneous voltage reading -- you just can't say `oh, it reads 1.42 volts -- it's fully charged now!'
(That does work on LiPo and Pb cells, however.)
| > The charger has a "negative-delta-V" sensor. | | negative-delta-V will overcharge NiMH.
If it does, it's only by a small amount, if done properly. NiMH cells do have a negative voltage peak just like NiCd cells do, but it's smaller, so older chargers that look for a larger peak can often miss it, and then THAT will overcharge your NiMH cell. But `negative-delta-V' _does_ work properly on NiMH if it's sensitive enough.
(And as a rule of thumb, the older a charger is, the less likely it is to be sensitive enough, since they started making them more sensitive to handle NiMH cells when NiMH cells started becoming popular. Of course, being _too_ sensitive is a bad thing too, as it can cause false peaks.)
| > The AAA cells are NiMH and are each rated as holding 0.185 mAh. | | You sure about that? VERY old NiCd cells may have been that low. | Assuming you really meant 185mAH. I don't recall seeing a AAA NiMH | much below 500mAH.
First of all, his units are obviously wrong. It might be 185 mAh or 0.185 Ah, but not 0.185 mAh (which would be off by a factor of 1000.)
But you're right -- I've never seen AAA NiMH cells that small, and the smallest AAA NiCd cells I've seen in a while have been about 250 mAh.
Note that in most cases you should not charge NiMH cells at over 1 C. So if your cells really are 185 mAh, they should not be charged at over 185 mA.
(Exceptions include sub-C cells, which can be charged at 2C safely, and then there's the new NiMH cells which say they can be charged in 15 minutes or even faster, which would be 4C or more, but I've never used those new cells myself.)
Is it possible that you've got AA (not AAA) cells, 1850 mAh capacity, and the charge rate suggested on the package is 0.185 mA for 12-14 hours or so? That would make a lot more sense.
| > (Q.1) Will this 350 mA *RATE* of charge be too much for such | > cells to take resulting in noticeable adverse effects? | | Probably, but more important is the charge termination method.
Both are pretty important, actually. 350 mA would cook 180 mAh NiMH cells pretty quickly, for example. They might tolerate it for a few charges, but they wouldn't last long. (And overcharging at 350 mA ... ouch.)
The overvoltage termination is somewhere around 1.6V/cell. It's intended as a fail-safe to reduce the number of exploded cells when something goes horribly wrong in the charging process.
Same reason there's also a timer to shut off charging after 1.5C or so.

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