# Rate of charge of NiMH AAA cells at home

Discussion in 'Electronic Equipment' started by Alex Coleman, Oct 27, 2005.

1. ### Alex ColemanGuest

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)?

2. ### mikeGuest

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

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3. ### ehsjrGuest

That - .185 mAh - is impossibly low. Aside from that, don't
use your charger on those cells.

The cells are 5 years old, and I assume you don't have a charger
for them. So it seems that it's not worth investing any time/energy
in buying or building a charger. However, if you want to do it as
an experiment, build a trickle charger as follows:

-----
Vcc ----|LM317|---+
----- |
| |
+------+--- To nicd +

Gnd ------------------ To nicd -

Vcc can be anything from +6 to +30. Worst case, the
LM317 will need to dissipate about 1/2 watt. With 12
volts Vcc, it will need to dissipate 200 mW.
The circuit limits the charge current to ~18 mA
You can leave the cells in the charger for > 24
hours with no ill effect. Without knowing the specifics
of your cells, assume that they require a full 24 hour
charge with this circuit. You can charge them in series
or one at atime with this circuit - it makes no difference.

Ed

4. ### Watson A.Name - \Watt Sun, the Dark Remover\Guest

You forgot the reverse protection diode. When the cell(s) is/are
connected without power, they will force current back into the circuit,
with who knows what results.

5. ### ehsjrGuest

It's not needed. The datasheet says no protection diode is needed
for output voltage of 25 volts or less, and shows the circuit
without that protection.

Ed

6. ### IkkeGuest

Your cells wil live MUCH longer when you charge them at a low rate.

P.

7. ### Watson A.Name - \Watt Sun, the Dark Remover\Guest

I thought that most analog chips cannot tolerate a reverse voltage of
more than negative .3V - at least that's what the datasheets say. And
that would be especially true for regulators because they have such a
low internal resistance and high current capability. I just looked at
the datasheet for Nat'l LM317 and that's what it says: -0.3V
_absolute_maximum_ rating. That would be far exceeded by the -1.5V or
more of the cell(s).

8. ### Doug McLarenGuest

| Alex Coleman wrote:

| > 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.)

9. ### mikeGuest

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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10. ### mikeGuest

There was a well-known manufacturer who mounted BIG electrolyics on
a metal plate with a hole in the plate where the cap vent was located.
Somebody put a bunch of 'em in backwards, covering the vent hole.
Rumor was that they had to replace a LOT of ceiling in the cycle room.
mike

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11. ### Watson A.Name - \Watt Sun, the Dark Remover\Guest

One other thought that might be a concern. The battery's reverse
voltage is put across the filter caps in the PS. It would be more of a
concern if there were two cells in series with negative 3V or so across
the charger. Putting reverse voltage across an electrolytic can cause
it to "unform", and if the voltage is high enough, it might explode.

A guy at work was telling me last week about his shenanigans when he
worked for Xerox. He said for kicks he would put a 'lytic across a PS
and cause it to explode like a firecracker. Sometimes the rubber bung
would shoot up into the ceiling and imbed itself into the acoustic
ceiling! )

I think that if I did that, I'd go to someone else's workbench, so
there'd be on tell-tale evidence above _my_ workbench! :-O

12. ### ehsjrGuest

The battery voltage is the same polarity as the capacitor. The
"reverse current" scenario you described was power loss on
the input side, not a battery installed backwards. Are you now
talking about a scenario where the battery is installed backwards?
That would require a diode to protect things.

Regarding diode protection and the -.3 volt differential you
*any* time you had a source connected to the output of *any*
value, a protection diode would be mandatory. Yet the datasheet
states it is not needed below 25 volts (see page 8 & 9 for the
discussion of protection diodes) and it shows two battery charger
circuits (including the one I drew) without protection diodes on
page 19. That suggests that the internal backwards polarity path
resistance is low enough to keep the differential within .3v unless
the voltage on the output pin exceeds 25 volts. The discussion on
diode protection indicates that the diode protects Vout against
*short circuit* on the input, and the diode to protect the adj pin
protects against either an input or an output *short*. The condition
of loss of power on the input side is simply not a factor when
Vout < 25 volts. With no short on the input, if current flows from
Vout to Vin, it will be small, and will simply keep the power supply
cap charged through the low internal resistance of the chip.

Ed