How bad is this super simple Ni-MH Charger (transformer, diode, and a resistor)

[Russ]

How bad a solution would this super simple Ni-MH charger be?

Say I have a transformer putting out 5v AC. I pass it through a diode
then through a current limiting resistor then though the 4 X 1.2v
charging AA Ni-MH batteries. The resistor would limit the current to
C/10 (C being the amp hour capacity of the batteries) so that it would
be able to trickle safely without the need to ever turn off...

The DC would still be oscilating from 0v to +5v and the current would
peek at C/10 at 5V. The resistor value would be calculated assuming
zero resistance in the batteries....

I am thinking of using this in a cradling hand held device that would
sit in the cradle most of the time.

Does this type of pulsing DC charging current hurt the Ni-MH
batteries?

Would adding a smoothing capacitor make a big difference?

Russ

 

[mike]

How bad a solution would this super simple Ni-MH charger be?

Say I have a transformer putting out 5v AC. I pass it through a diode
then through a current limiting resistor then though the 4 X 1.2v
charging AA Ni-MH batteries. The resistor would limit the current to
C/10 (C being the amp hour capacity of the batteries) so that it would
be able to trickle safely without the need to ever turn off...

The DC would still be oscilating from 0v to +5v and the current would
peek at C/10 at 5V. The resistor value would be calculated assuming
zero resistance in the batteries....

I am thinking of using this in a cradling hand held device that would
sit in the cradle most of the time.

Does this type of pulsing DC charging current hurt the Ni-MH
batteries?

It's been argued that pulsed DC charging is better for batteries.
But, consult the battery vendor about long-term overcharge. Older
NiMH batteries didn't like this at all. Might be happier with NiCD.

You have some interesting problems when the battery is fully discharged
or worse, has a shorted cell. If you start out with higher voltage and
bigger resistor, the effects are mitigated. An incandescent light bulb
makes a good PTC resistor to supply (more) constant current vs voltage.
and is fail-safe in the event of a shorted battery.

Some think that the life of a cell is dependent more on how long it's
charged rather than the amount of charge. That suggest intermittent
charging at higher level is better.

Would adding a smoothing capacitor make a big difference?

Yes, it will make the average charging current higher for a given
resistance.

Another alternative is a charge pump. Put a cap in series with diode
clamp to ground and a series diode. This gives you a defined
charge/cycle. Still critically
dependent on the difference between battery voltage and peak source voltage.
mike

Russ

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Russ]

Thanks Mike. Your note helped a lot.

I have decided to change my design needs. The new design would need
the DC to be regulated as I will be running the device of the
batteries while cradled (charging).

So the charging circuit I am thinking of using would use a LM7809 to
get a regulated 9V then pass that through a 27 ohm resistor (using the
higher voltage higher resistor idea of yours) to the 4 AA batteries.

This would give a I(max) of 200mA with the V(delta) being 9v-4.4v =
5.6v

4.4v is 1.1 each for the AA batteries that are close to discharged.
The are Ni-MH rated at 2100mAH.

As they charge the V(delta) would lower to about 9v - 5v = 4v (maybe a
little less)

this would give leave the batteries with a constant trickle charge of
about 150mA (or slightly less).

Anyone see any problems with my logic?

Thanks!

 

[Jan Panteltje]

Thanks Mike. Your note helped a lot.

I have decided to change my design needs. The new design would need
the DC to be regulated as I will be running the device of the
batteries while cradled (charging).

So the charging circuit I am thinking of using would use a LM7809 to
get a regulated 9V then pass that through a 27 ohm resistor (using the
higher voltage higher resistor idea of yours) to the 4 AA batteries.

This would give a I(max) of 200mA with the V(delta) being 9v-4.4v =
5.6v

4.4v is 1.1 each for the AA batteries that are close to discharged.
The are Ni-MH rated at 2100mAH.

As they charge the V(delta) would lower to about 9v - 5v = 4v (maybe a
little less)

this would give leave the batteries with a constant trickle charge of
about 150mA (or slightly less).

Anyone see any problems with my logic?

Yes

 

[mike]

Thanks Mike. Your note helped a lot.

I have decided to change my design needs. The new design would need
the DC to be regulated as I will be running the device of the
batteries while cradled (charging).

So the charging circuit I am thinking of using would use a LM7809 to
get a regulated 9V then pass that through a 27 ohm resistor (using the
higher voltage higher resistor idea of yours) to the 4 AA batteries.

This would give a I(max) of 200mA with the V(delta) being 9v-4.4v =
5.6v

4.4v is 1.1 each for the AA batteries that are close to discharged.
The are Ni-MH rated at 2100mAH.

As they charge the V(delta) would lower to about 9v - 5v = 4v (maybe a
little less)

this would give leave the batteries with a constant trickle charge of
about 150mA (or slightly less).

Anyone see any problems with my logic?

Thanks!

Yes, you're gonna get up to 1/3 amp into a shorted battery. And if you
trickle charge it at 150mA, it will short eventually.
Running the device while charging is problematic unless the drain is
constant under all conditions. Usually this is not the case and you
need separate paths for charge and operation to keep the charge current
manageably low.

To get an appreciation of the problem, spend a Saturday going to garage
sales. You'll find a dead cordless phone at approximately every other
sale. The handheld battery is always shot. And they don't even try to
run the device while it's charging.
mike

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Russ]

ok. no running other circuits of the battery while charging.

I have read many different opinions on long-term trickle charging.
Some say C/10 - C/20 is OK. Others say there is no safe long-term
trickle for Ni-MH batteries.

Long-term for me is 3 months - it is for an installation that would be
on display for 3 months.

From what I could gather Ni-Cd batteries can handle a C/10 charge
quite well long-term.


Here is my problem:

I am building an installation that has a circuit inside a cradled
device that draws 300mA. The device turns on when lifted from the
cradle then charges (and turns off) when cradled. The device circuit
takes 4 AA batteries. It would be lifted and drawing 300mA from the
batteries on and off for max 6 hours total a day (1800mAH). However it
may go a whole day (or two) without being lifted.

So a constant trickle charge of C/15 would seem to work IF the
batteries could handle the trickle once fully charged...

Would switching to Ni-Cd batteries help? how do they handle repeated
stop/use/charge cycles. Any memory effect problems?

Thanks again for the help.

 

[Russ]

here is a link to a spec sheet from Gold Peek batteries.

They state the following regarding Continous Overcharge at C/10 for
their 1800mAh Ni-MH AA:

"180mA maximum current for 1 year
No conspicuous deformation and/or leakage"

http://www.gpina.com/pdf/180AAHC.PDF

Do these specs differ very much from one Ni-MH manufacturer to the
next?

 

[Dave VanHorn]

"180mA maximum current for 1 year
No conspicuous deformation and/or leakage"

http://www.gpina.com/pdf/180AAHC.PDF

Do these specs differ very much from one Ni-MH manufacturer to the next?

They certainly do.
"No conspicuous deformation and/or leakage"
Note that this says nothing about any electrical capabilities...

Nicads are much more tolerant of long term overcharge.
Sanyo "C" type (not size) NIMH cells are pretty tolerant.

 

[Russ]

Another data point on Ni-MH long-term trickle charging, this time from
Energizer:

"a rate of 0.1C appears to balance adequate charge input with minimum
adverse effects in overcharge."

data.energizer.com

So it seems that newer Ni-MH batteries can handle a constant trickle
charge up to C/10. I'm sure this is not good for the battery and won't
let you get up into the 900-1000 recharges range, but will probably
get you up to the 500 point.

So a constant C/15 charge would seem to be OK.