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Trickle-charge NiMH

R

Robert Scott

Jan 1, 1970
0
I would like to design a product that uses a NiMH battery (3.6v, 700mAH). But I
do not want to include the proper NiMH fast-charging circuitry, partly because
of cost, but mostly because of board space. I am willing to give up fast and
medium speed charging in order to be able to use a simple series resistor. The
question is, what charging rate is safe for a full-time trickle charge? I was
thinking that about C/15.
 
S

Spehro Pefhany

Jan 1, 1970
0
I would like to design a product that uses a NiMH battery (3.6v, 700mAH). But I
do not want to include the proper NiMH fast-charging circuitry, partly because
of cost, but mostly because of board space. I am willing to give up fast and
medium speed charging in order to be able to use a simple series resistor. The
question is, what charging rate is safe for a full-time trickle charge? I was
thinking that about C/15.

You should refer to the battery handbook from your selected
manufacturer for this information. Sanyo has some pretty good info.
The good news is that you're unlikely to cause the battery to go into
thermal runaway spraying caustic all over the place with C/15. ;-)



Best regards,
Spehro Pefhany
 
N

Nico Coesel

Jan 1, 1970
0
I would like to design a product that uses a NiMH battery (3.6v, 700mAH). But I
do not want to include the proper NiMH fast-charging circuitry, partly because
of cost, but mostly because of board space. I am willing to give up fast and
medium speed charging in order to be able to use a simple series resistor. The
question is, what charging rate is safe for a full-time trickle charge? I was
thinking that about C/15.

AFAIK NiMH batteries like to be trickle charged in pulses. Panasonic
has some excellent information on charging NiMH.
 
R

Robert Scott

Jan 1, 1970
0
You should refer to the battery handbook from your selected
manufacturer for this information. Sanyo has some pretty good info.
The good news is that you're unlikely to cause the battery to go into
thermal runaway spraying caustic all over the place with C/15. ;-)

Even if it does not go into thermal runaway, is it likely to shorten the life of
the battery to use full-time C/15?
 
E

ehsjr

Jan 1, 1970
0
Robert said:
Even if it does not go into thermal runaway, is it likely to shorten the life of
the battery to use full-time C/15?

According to Powerstream, full-time (i.e. standby) charging should be
around .03 to .05 C, so C/15 is a little high.
http://www.powerstream.com/NiMH.htm

I have charged _NiCd_ successfully at .031 C. It takes a lot
of time, but it works. That does not mean it is the best way
to charge them, but they do eventually come up to terminal
voltage which is 1.43 volts per cell. That said, I have not
done that with NiMh. So, try it and see if it works for you.
If you can get a good charge at between .03 and .05 C,
you can leave your cells on the charger full time per Powerstream.

Ed
 
S

Spehro Pefhany

Jan 1, 1970
0
Even if it does not go into thermal runaway, is it likely to shorten the life of
the battery to use full-time C/15?

IIRC, neither Sanyo nor Panasonic recommend it, period. Panasonic
says:

The overcharging of nickel-metal hydride
batteries, even by trickle charging, causes a
deterioration in the characteristics of the
batteries. To prevent overcharging by trickle
charging or any other charging method, the
provision of a timer to regulate the total charging
time is recommended.


Best regards,
Spehro Pefhany
 
R

Robert Scott

Jan 1, 1970
0
..To prevent overcharging by trickle
charging or any other charging method, the
provision of a timer to regulate the total charging
time is recommended.

OK, I'll see what I can do with a timer. Thanks.
 
M

Martin Brown

Jan 1, 1970
0
Spehro said:
IIRC, neither Sanyo nor Panasonic recommend it, period. Panasonic
says:

The overcharging of nickel-metal hydride
batteries, even by trickle charging, causes a
deterioration in the characteristics of the
batteries. To prevent overcharging by trickle
charging or any other charging method, the
provision of a timer to regulate the total charging
time is recommended.

I know that is what they say, but the shelf life of charged NiMH is not
exactly stellar and my camera is a bit tetchy about batteries that are
even slightly off their peak fully charged voltage. I have been giving
some thought to having a solar powered trickle charger that keeps a set
of 4 NiMH cells properly charged countering their comparatively high
self discharge rate. The recommended method of low duty cycle high
current top up is not so attractive on a solar powered device.

ISTR you can leave them almost forever on a C/300 charge rate.
(though the manufacturers do not approve)

Regards,
Martin Brown
 
S

Spehro Pefhany

Jan 1, 1970
0
I know that is what they say, but the shelf life of charged NiMH is not
exactly stellar and my camera is a bit tetchy about batteries that are
even slightly off their peak fully charged voltage. I have been giving
some thought to having a solar powered trickle charger that keeps a set
of 4 NiMH cells properly charged countering their comparatively high
self discharge rate. The recommended method of low duty cycle high
current top up is not so attractive on a solar powered device.

Why? Can't you use a capacitor to supply the peak current?

ISTR you can leave them almost forever on a C/300 charge rate.
(though the manufacturers do not approve)

Regards,
Martin Brown


Best regards,
Spehro Pefhany
 
J

JosephKK

Jan 1, 1970
0
AFAIK NiMH batteries like to be trickle charged in pulses. Panasonic
has some excellent information on charging NiMH.

Hmmm. Makes me wonder how they would respond to a C/10 "burp" 1 second
discharge once every 5 minutes with a C/30 trickle charge. Might be an
interesting experiment.
 
S

Spehro Pefhany

Jan 1, 1970
0
Do the calc's. Q, delta Q, Delta V for batt and cap.

There's no numbers provided, but assuming a 286us pulse (bq2002) and
C/2 on a 2.2Ah per cell pack, and 1V delta V, you'd need 315uF. So a
470uF cap would be more than enough. No big deal. What "cal's" did you
have in mind?


Best regards,
Spehro Pefhany
 
J

JosephKK

Jan 1, 1970
0
There's no numbers provided, but assuming a 286us pulse (bq2002) and
C/2 on a 2.2Ah per cell pack, and 1V delta V, you'd need 315uF. So a
470uF cap would be more than enough. No big deal. What "cal's" did you
have in mind?


Best regards,
Spehro Pefhany

OP specified 3.6 V, 700 mAh and asked about C/15 charge rate. It is
still there.

For low duty cycle C/2 pulse float charging from solar source, would you
fill in this table? Please assume the battery is at half charge.

Batt. Cap.
Q initial

Q final

Delta Q

Delta V
 
S

Spehro Pefhany

Jan 1, 1970
0
OP specified 3.6 V, 700 mAh and asked about C/15 charge rate. It is
still there.

For low duty cycle C/2 pulse float charging from solar source, would you
fill in this table? Please assume the battery is at half charge.

Why don't you do it for me, based on the 286us pulse width I found..
 
J

JosephKK

Jan 1, 1970
0
Why don't you do it for me, based on the 286us pulse width I found..

Cause i don't think that short of pulse will do much charging, won't be
anywhere near square enough (i), and i have no idea of what rep rate you
expect to achieve. Nor what final charge rate you expect.
 
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