Rate of charge of NiMH AAA cells at home

[Alex Coleman]

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)?

 

[mike]

Alex Coleman wrote:

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?

No.

-----

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.

The charger has a "negative-delta-V" sensor.

negative-delta-V will overcharge NiMH.

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.

(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.
You can buy 8.5 minute charge cells these days.

------

Assuming the charge in the cells is fully depleted

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

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)?

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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[ehsjr]

Alex Coleman wrote:

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.

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|---+
----- |
Adj [68R]
| |
+------+--- 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

 

[Watson A.Name - \"Watt Su]

"ehsjr" <ehsjr@bellatlantic.net> wrote in message
news:SU58f.13788$%A1.13439@trndny01...

Alex Coleman wrote:
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.

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|---+
----- |
Adj [68R]
| |
+------+--- 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

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.

 

[Watson A.Name - \"Watt Su]

"ehsjr" <ehsjr@bellatlantic.net> wrote in message
news:5LA8f.2331$iE5.303@trndny08...

Watson A.Name - "Watt Sun, the Dark Remover" wrote:
"ehsjr" <ehsjr@bellatlantic.net> wrote in message
news:SU58f.13788$%A1.13439@trndny01...

Alex Coleman wrote:

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.

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|---+
----- |
Adj [68R]
| |
+------+--- 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


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.



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

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).