# How long for an initial NiMH charge?

A

#### Andy

Jan 1, 1970
0
I have a standalone charger for some AA (and AAA) cells when AA cells

The charge rate is a steady 150 mA (50 mA for AAA) and there is no cut-
off mechanism. The charger leaflet recommends 5 hours for cells between
uses and that means for cells which have a reasonable amount of charge

So when I get a brand new set of NimH cells (presumably with almost no
charge) the initial charge needs to:

(1) fully charge the cells
(2) sightly overcharge some cells to equalize their charges

QUESTION: As a rule of thumb what extra percentage of charge time
should I add on for this first charge?

I was guessing wildly at 80 extra: 18 hours at 150 mA for 550 mAH cells.
What do you reckon?

E

#### ehsjr

Jan 1, 1970
0
Andy said:
I have a standalone charger for some AA (and AAA) cells when AA cells

The charge rate is a steady 150 mA (50 mA for AAA) and there is no cut-
off mechanism. The charger leaflet recommends 5 hours for cells between
uses and that means for cells which have a reasonable amount of charge

So when I get a brand new set of NimH cells (presumably with almost no
charge) the initial charge needs to:

(1) fully charge the cells
(2) sightly overcharge some cells to equalize their charges

QUESTION: As a rule of thumb what extra percentage of charge time
should I add on for this first charge?

I was guessing wildly at 80 extra: 18 hours at 150 mA for 550 mAH cells.
What do you reckon?

18 hours at 150 mA for 550 mAH cells is *way* too much.
The "overnight" or 14 hour rate is c/10, which means
55 mA for 14 hours for those cells.

To use your charger for this, modify it. Otherwise,
you take the risk of overcharging. The modification
is simple - requires three parts: an LM317 and a 22 ohm
resistor in series with a 1 ohm resistor to make 23 ohms.

-----
Charger + ---Vin|LM317|Vout---+
----- |
| |
+----------+---> To NiCd +

Gnd ------------------------------> To Nicd -

That will give you a charge current of a bit over 54 mA
and you can put your batteries in the charge for 14 hours
(or more) without damage.

Ed

A

#### Alex Coleman

Jan 1, 1970
0
Andy said:
I have a standalone charger for some AA (and AAA) cells when AA
cells held about 500 to 600mAH.

The charge rate is a steady 150 mA (50 mA for AAA) and there is no
cut- off mechanism. The charger leaflet recommends 5 hours for
cells between uses and that means for cells which have a

So when I get a brand new set of NimH cells (presumably with
almost no charge) the initial charge needs to:

(1) fully charge the cells
(2) sightly overcharge some cells to equalize their charges

QUESTION: As a rule of thumb what extra percentage of charge time
should I add on for this first charge?

I was guessing wildly at 80 extra: 18 hours at 150 mA for 550 mAH
cells. What do you reckon?

18 hours at 150 mA for 550 mAH cells is *way* too much.
The "overnight" or 14 hour rate is c/10, which means
55 mA for 14 hours for those cells.

To use your charger for this, modify it. Otherwise,
you take the risk of overcharging. The modification
is simple - requires three parts: an LM317 and a 22 ohm
resistor in series with a 1 ohm resistor to make 23 ohms.

-----
Charger + ---Vin|LM317|Vout---+
----- |
| |
+----------+---> To NiCd +

Gnd ------------------------------> To Nicd -

That will give you a charge current of a bit over 54 mA
and you can put your batteries in the charge for 14 hours
(or more) without damage.

Ed

I may not have expressed myself clearly but I am not looking to recharge
within a particular fixed time. I am looking to see what OVERCHARGE I
should give the cells in order to fully top them all up.

My charger is 150 mAh and is recommended that it be used for 5 hours to
TOP UP "standard" cells. When it was bought, standard meant about 550
mAh.

I take your point that the starting recommendation may need revising
although the current assumes a certain type of cell with a certain
impedance and I think that as cells have increased in capacity then so
has their impedance which in turn will possibly (?) affect the nonimal
current. Is this right?

Assuming that the charge current is right (we can assume a change in the
150 mA to whatever value we wish) then ...

.... my QUESTION is what amount of extra charge would you guess is needed
for the inital charge?

This is where a group of cells are primed to work together and run flat
together and which need all topping up (to 100%) together even though
some will have started with more or less charge than others.

E

#### ehsjr

Jan 1, 1970
0
Alex said:
Andy said:
I have a standalone charger for some AA (and AAA) cells when AA
cells held about 500 to 600mAH.

The charge rate is a steady 150 mA (50 mA for AAA) and there is no
cut- off mechanism. The charger leaflet recommends 5 hours for
cells between uses and that means for cells which have a

So when I get a brand new set of NimH cells (presumably with
almost no charge) the initial charge needs to:

(1) fully charge the cells
(2) sightly overcharge some cells to equalize their charges

QUESTION: As a rule of thumb what extra percentage of charge time
should I add on for this first charge?

I was guessing wildly at 80 extra: 18 hours at 150 mA for 550 mAH
cells. What do you reckon?

18 hours at 150 mA for 550 mAH cells is *way* too much.
The "overnight" or 14 hour rate is c/10, which means
55 mA for 14 hours for those cells.

To use your charger for this, modify it. Otherwise,
you take the risk of overcharging. The modification
is simple - requires three parts: an LM317 and a 22 ohm
resistor in series with a 1 ohm resistor to make 23 ohms.

-----
Charger + ---Vin|LM317|Vout---+
----- |
| |
+----------+---> To NiCd +

Gnd ------------------------------> To Nicd -

That will give you a charge current of a bit over 54 mA
and you can put your batteries in the charge for 14 hours
(or more) without damage.

Ed

I may not have expressed myself clearly but I am not looking to recharge
within a particular fixed time. I am looking to see what OVERCHARGE I
should give the cells in order to fully top them all up.

My charger is 150 mAh and is recommended that it be used for 5 hours to
TOP UP "standard" cells. When it was bought, standard meant about 550
mAh.

I take your point that the starting recommendation may need revising
although the current assumes a certain type of cell with a certain
impedance and I think that as cells have increased in capacity then so
has their impedance which in turn will possibly (?) affect the nonimal
current. Is this right?

Assuming that the charge current is right (we can assume a change in the
150 mA to whatever value we wish) then ...

... my QUESTION is what amount of extra charge would you guess is needed
for the inital charge?

This is where a group of cells are primed to work together and run flat
together and which need all topping up (to 100%) together even though
some will have started with more or less charge than others.

The "overcharge" you should give your cells is C/10 for 14 hours,
but that's not really overcharge as the word is normally used.

It is not really overcharge, per se. Cells do not charge
at 100% efficiency. So if you apply a 55 mA charging current
to a 550 mAH cell, it takes more than 10 hours of charge
time to get 550 mAH of energy into the battery - thus the
14 hour charge period. The "overcharge" is the extra 4 hours
over the 10 hour period during which the charger provides a
total of 550 mAH. Since the battery does not absorb 100%
of the energy supplied by the charge during the 10 hours, you
need to "overcharge" for another 4 hours to give the battery
enough time to reach full charge.

The value of the "14 hour approach" is that you do *not*
overcharge the batteries. It applies equally to fully or
partly discharged packs. You provide sufficient energy
over time for the pack to fully charge, but it is delivered
at a rate that won't damage the cells. Mind you, if you
are going to leave the pack on the charger for extended periods
well beyond 14 hours, damage will result. For extended period
charging, the rate needs to be reduced significantly after
the pack reaches full charge.

Overcharging NiCds is not recommended, where you use the
word overcharge to indicate raising the cell's voltage
higher than it should be. The example you gave of charging
a 550 mAH pack at 150 mA for 18 hours will damage the
pack. It would be fine for a 1500 mAH pack.

Ed

A

#### Alex Coleman

Jan 1, 1970
0
Alex said:
Andy wrote:

I have a standalone charger for some AA (and AAA) cells when AA
cells held about 500 to 600mAH.

The charge rate is a steady 150 mA (50 mA for AAA) and there is
no cut- off mechanism. The charger leaflet recommends 5 hours
for cells between uses and that means for cells which have a

So when I get a brand new set of NimH cells (presumably with
almost no charge) the initial charge needs to:

(1) fully charge the cells
(2) sightly overcharge some cells to equalize their charges

QUESTION: As a rule of thumb what extra percentage of charge
time should I add on for this first charge?

I was guessing wildly at 80 extra: 18 hours at 150 mA for 550 mAH
cells. What do you reckon?

18 hours at 150 mA for 550 mAH cells is *way* too much.
The "overnight" or 14 hour rate is c/10, which means
55 mA for 14 hours for those cells.

To use your charger for this, modify it. Otherwise,
you take the risk of overcharging. The modification
is simple - requires three parts: an LM317 and a 22 ohm
resistor in series with a 1 ohm resistor to make 23 ohms.

-----
Charger + ---Vin|LM317|Vout---+
----- |
| |
+----------+---> To NiCd +

Gnd ------------------------------> To Nicd -

That will give you a charge current of a bit over 54 mA
and you can put your batteries in the charge for 14 hours
(or more) without damage.

Ed

I may not have expressed myself clearly but I am not looking to
recharge within a particular fixed time. I am looking to see what
OVERCHARGE I should give the cells in order to fully top them all
up.

My charger is 150 mAh and is recommended that it be used for 5
hours to TOP UP "standard" cells. When it was bought, standard

I take your point that the starting recommendation may need
revising although the current assumes a certain type of cell with
a certain impedance and I think that as cells have increased in
capacity then so has their impedance which in turn will possibly
(?) affect the nonimal current. Is this right?

Assuming that the charge current is right (we can assume a change
in the 150 mA to whatever value we wish) then ...

... my QUESTION is what amount of extra charge would you guess is
needed for the inital charge?

This is where a group of cells are primed to work together and run
flat together and which need all topping up (to 100%) together
even though some will have started with more or less charge than
others.

The "overcharge" you should give your cells is C/10 for 14 hours,
but that's not really overcharge as the word is normally used.

It is not really overcharge, per se. Cells do not charge
at 100% efficiency. So if you apply a 55 mA charging current
to a 550 mAH cell, it takes more than 10 hours of charge
time to get 550 mAH of energy into the battery - thus the
14 hour charge period. The "overcharge" is the extra 4 hours
over the 10 hour period during which the charger provides a
total of 550 mAH. Since the battery does not absorb 100%
of the energy supplied by the charge during the 10 hours, you
need to "overcharge" for another 4 hours to give the battery
enough time to reach full charge.

The value of the "14 hour approach" is that you do *not*
overcharge the batteries. It applies equally to fully or
partly discharged packs. You provide sufficient energy
over time for the pack to fully charge, but it is delivered
at a rate that won't damage the cells. Mind you, if you
are going to leave the pack on the charger for extended periods
well beyond 14 hours, damage will result. For extended period
charging, the rate needs to be reduced significantly after
the pack reaches full charge.

Overcharging NiCds is not recommended, where you use the
word overcharge to indicate raising the cell's voltage
higher than it should be. The example you gave of charging
a 550 mAH pack at 150 mA for 18 hours will damage the
pack. It would be fine for a 1500 mAH pack.

Ed

Hi Ed.

Interesting reply. Well, it was too obvious and I missed it. I had
completely overlooked efficiency. 700 mAh in does not equal 750 mAh
stored. Damn you. Heh!

OK, now I get what you are saying. It is interesting that you mention
packing charge in.

Is it a general truism that a slow charge packs in more energy in total
than a fast charge? (I.E. the energy density per unit of storage is
greater for a slow charge?)

Also, is there perhaps a "knee" on the speed versus density chart which
is the sweet spot between fast/slow charging and improved energy
density? If so, then what charge rate does it indicate is a good rate
for denisty.

Finally, I think I do mean "overcharge". Let me explain. If one cells
starts with 30% charge and another with 45% charge then to get both
cells fully charged (in a charger which takes the two in series) then
the cell starting with a 45% charge will get overcharged.

If the charge rate is gentle then I guess this may not lead to as much
damage as a brutally fast rate and maybe there is no harm done but I
would still call it "overcharged".

I have the hunch that many modern cells are ok with very rapid charging
rates such as C or C/2 and do not suffer as much as the old cells did at
a charge rate of C/5. Do you know if this is actually true?

E

#### ehsjr

Jan 1, 1970
0
Alex said:
Alex said:
Andy wrote:

I have a standalone charger for some AA (and AAA) cells when AA
cells held about 500 to 600mAH.

The charge rate is a steady 150 mA (50 mA for AAA) and there is
no cut- off mechanism. The charger leaflet recommends 5 hours
for cells between uses and that means for cells which have a

So when I get a brand new set of NimH cells (presumably with
almost no charge) the initial charge needs to:

(1) fully charge the cells
(2) sightly overcharge some cells to equalize their charges

QUESTION: As a rule of thumb what extra percentage of charge
time should I add on for this first charge?

I was guessing wildly at 80 extra: 18 hours at 150 mA for 550 mAH
cells. What do you reckon?

18 hours at 150 mA for 550 mAH cells is *way* too much.
The "overnight" or 14 hour rate is c/10, which means
55 mA for 14 hours for those cells.

To use your charger for this, modify it. Otherwise,
you take the risk of overcharging. The modification
is simple - requires three parts: an LM317 and a 22 ohm
resistor in series with a 1 ohm resistor to make 23 ohms.

-----
Charger + ---Vin|LM317|Vout---+
----- |
| |
+----------+---> To NiCd +

Gnd ------------------------------> To Nicd -

That will give you a charge current of a bit over 54 mA
and you can put your batteries in the charge for 14 hours
(or more) without damage.

Ed

I may not have expressed myself clearly but I am not looking to
recharge within a particular fixed time. I am looking to see what
OVERCHARGE I should give the cells in order to fully top them all
up.

My charger is 150 mAh and is recommended that it be used for 5
hours to TOP UP "standard" cells. When it was bought, standard

I take your point that the starting recommendation may need
revising although the current assumes a certain type of cell with
a certain impedance and I think that as cells have increased in
capacity then so has their impedance which in turn will possibly
(?) affect the nonimal current. Is this right?

Assuming that the charge current is right (we can assume a change
in the 150 mA to whatever value we wish) then ...

... my QUESTION is what amount of extra charge would you guess is
needed for the inital charge?

This is where a group of cells are primed to work together and run
flat together and which need all topping up (to 100%) together
even though some will have started with more or less charge than
others.

The "overcharge" you should give your cells is C/10 for 14 hours,
but that's not really overcharge as the word is normally used.

It is not really overcharge, per se. Cells do not charge
at 100% efficiency. So if you apply a 55 mA charging current
to a 550 mAH cell, it takes more than 10 hours of charge
time to get 550 mAH of energy into the battery - thus the
14 hour charge period. The "overcharge" is the extra 4 hours
over the 10 hour period during which the charger provides a
total of 550 mAH. Since the battery does not absorb 100%
of the energy supplied by the charge during the 10 hours, you
need to "overcharge" for another 4 hours to give the battery
enough time to reach full charge.

The value of the "14 hour approach" is that you do *not*
overcharge the batteries. It applies equally to fully or
partly discharged packs. You provide sufficient energy
over time for the pack to fully charge, but it is delivered
at a rate that won't damage the cells. Mind you, if you
are going to leave the pack on the charger for extended periods
well beyond 14 hours, damage will result. For extended period
charging, the rate needs to be reduced significantly after
the pack reaches full charge.

Overcharging NiCds is not recommended, where you use the
word overcharge to indicate raising the cell's voltage
higher than it should be. The example you gave of charging
a 550 mAH pack at 150 mA for 18 hours will damage the
pack. It would be fine for a 1500 mAH pack.

Ed

Hi Ed.

Interesting reply. Well, it was too obvious and I missed it. I had
completely overlooked efficiency. 700 mAh in does not equal 750 mAh
stored. Damn you. Heh!

OK, now I get what you are saying. It is interesting that you mention
packing charge in.

Is it a general truism that a slow charge packs in more energy in total
than a fast charge? (I.E. the energy density per unit of storage is
greater for a slow charge?)

I don't know. I wasn't trying to compare a fast,
automatically terminated charge with a slow charge.
I was comparing a fast charge that goes *WELL*
beyond where it should terminate to a slow charge.
Lets say you need to put in 700 mAH, due to charging
inefficiency in the cell, to fully charge your 550 mAH
cells. With 550 mAH cells being charged at 150 mA for
18 hours, that's a total of 2700 mAH, or 2000 more
mAH than the cells can use for charging. That extra
energy is converted 100% to heat starting ~ 4 hours and
20 minutes into the cycle - you are cooking the cells
for 13 + hours at 150 mA * (Vcharger - Vbattery). This
is likely to damage the battery.

With the C/10 14 hour charger (55 mA) it takes about 12
hours and 45 minutes to fully charge the battery. You
cook the battery for about an hour and 15 minutes at
a much lower heat 55 mA * (Vcharger - Vbattery).
Vcharger will be lower as well, since it needs to be
at a level above Vbattery that will drive only 55 mA,
not 150 mA. The net effect is the the heat in the cells
is much lower, and the cells can dissipate it without
venting.

Also, is there perhaps a "knee" on the speed versus density chart which
is the sweet spot between fast/slow charging and improved energy
density? If so, then what charge rate does it indicate is a good rate
for denisty.

The above is not in any way applicable to an 18 hour charge
at ~ C/4 (150 mA with a 550 mAH pack). At issue is getting
a full charge, without damaging the pack, with a charger that
has no automatic control. Knees of the curve and sweet spots
are 100% irrelevant to the issue. If you had a full featured
charger that could sense the condition of the pack and adjust
and terminate the charge as appropriate, it might be germaine
if one stretches logic into some bizzare shape. But for a
battery *pack*, knees of curves are irrelevant for the reason
you mention below: the cells may differ. And if the cells
differ in charge, the knees would have to differ.
Finally, I think I do mean "overcharge". Let me explain. If one cells
starts with 30% charge and another with 45% charge then to get both
cells fully charged (in a charger which takes the two in series) then
the cell starting with a 45% charge will get overcharged.

Ok, then you need to stop meaning it. It is BAD for NiCds.
There is unavoidable overcharging, and designers have used
a number of techniques to minimize it - taper charge, float
charge, multi-stage charge, terminated charge and so forth.
As you used it, you seek to overcharge. Not good. When
charging is placed in your hands, as it is with your charger
that has no auto shut off and a single (relatively) high
charge rate, *you* need to be gentle to the pack.

With an automatic charger designed for the cells you are
charging, you plug it in and let the charger do its thing.
The *charger* decides what is a charge and what is an
overcharge, and adjusts its output accordingly.

If the charge rate is gentle then I guess this may not lead to as much
damage as a brutally fast rate and maybe there is no harm done but I
would still call it "overcharged".

It's leaving the pack on the high rate charger
*WAY TOO LONG* that is the problem.
I have the hunch that many modern cells are ok with very rapid charging
rates such as C or C/2 and do not suffer as much as the old cells did at
a charge rate of C/5. Do you know if this is actually true?

Yes, there are cells that can be charged quickly with a
charger designed for that. None of the fast chargers charge
the pack for anything remotely resembling 18 hours, and they
all operate automatically, either terminating the charge
cycle or reducing the charge rate dramatically.

Ed

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