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Guest
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 Posted:
Thu Feb 17, 2005 11:50 pm Post subject:
Charge rate with my adjustable regulator and a NiMH pack |
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A while back I built an adjustable power supply with a 12V 1.2A power
supply, a full wave bridge rectifier, and an LM117 adjustable
regulator. I was thinking about using this adjustable power supply to
quick charge a small 5-cell 6V NiMH battery pack I use in a R/C car.
If I connect the battery pack to my adjustable supply (set @ 6V) what
current will the power supply put out? Will the power supply put out
the 1.2A rating of the transformer? I do plan to carefully time the
charge so that I do not overcharge the pack.
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mike
Guest
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| Posted:
Fri Feb 18, 2005 12:47 am Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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computer_prog@hotmail.com wrote:
| Quote: | A while back I built an adjustable power supply with a 12V 1.2A power
supply, a full wave bridge rectifier, and an LM117 adjustable
regulator. I was thinking about using this adjustable power supply to
quick charge a small 5-cell 6V NiMH battery pack I use in a R/C car.
If I connect the battery pack to my adjustable supply (set @ 6V) what
current will the power supply put out? Will the power supply put out
the 1.2A rating of the transformer? I do plan to carefully time the
charge so that I do not overcharge the pack.
|
You have a BUNCH of issues.
6V won't charge the pack. If you don't know what number to use, you're
doomed from the start.
Depending on your heat sink, the LM117 will get VERY hot. Never rely
on the thermal shutdown for anything but a secondary safety mechanism.
You need a controlled, known current limit.
YOu need a controlled, known voltage.
You need to monitor voltage, current, time and temperature until you get
the setup calibrated. Once you get it set up and calibrated to the
particular pack you're using, you'll just need the heavy metal container
that contains the flying pieces of sharp metal if something goes wrong.
Been There Done That!
Timing the charge is easy. Deciding what that time should be, not so
much.
In the general case, homemade battery chargers by people who don't know
what they're doing are a BAD idea. Designing a (non lithium) battery
charger is 99%
trivial and 1% emergency room. Don't take that 1% too lightly.
Any advice you get here is likely to be on how to safely recharge batteries.
Post this in one of the many RC newsgroups to learn how to WIN RACES.
If you want to WIN, you'll do things to batteries that will make a
battery engineer hide behind a heavy object...far away.
WEAR SAFETY GLASSES!!!
mike
--
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with links. Delete this sig when replying.
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Luhan Monat
Guest
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| Posted:
Fri Feb 18, 2005 2:37 am Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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computer_prog@hotmail.com wrote:
| Quote: | A while back I built an adjustable power supply with a 12V 1.2A power
supply, a full wave bridge rectifier, and an LM117 adjustable
regulator. I was thinking about using this adjustable power supply to
quick charge a small 5-cell 6V NiMH battery pack I use in a R/C car.
If I connect the battery pack to my adjustable supply (set @ 6V) what
current will the power supply put out? Will the power supply put out
the 1.2A rating of the transformer? I do plan to carefully time the
charge so that I do not overcharge the pack.
|
Don't use a regulator at all. Divide the amper-hours rating of the
battery by 10 hours to get the safe continuous charge rate. Since R=E/I
, divide (12-6)volts by the charge rate to get a suitable resitance.
Then multiply the charge rate by 6 to get the wattage for the resistor.
This is a low-tech approach; but is usually safe, any easy.
--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm
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Guest
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| Posted:
Fri Feb 18, 2005 4:13 am Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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| Quote: | You need a controlled, known current limit.
YOu need a controlled, known voltage.
|
The pack is a 6V 1200mah pack. The sheet said that the pack can be
slow charged @ 6V 100ma for 12 hours. The pack came with a small and
simple 6V 100ma wall transformer for slow charging. I have been using
this wall charger for a few months now.
The sheet that came with the battery pack also said that it can be
"quick charged" at 6V, 1A, for 60 minutes. I was looking to see if I
could speed up the charge time by using the 1.2A Transformer based
power supply I built. |
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A.Nonny Mouse
Guest
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| Posted:
Fri Feb 18, 2005 4:31 am Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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Luhan Monat wrote:
| Quote: | computer_prog@hotmail.com wrote:
A while back I built an adjustable power supply with a 12V 1.2A power
supply, a full wave bridge rectifier, and an LM117 adjustable
regulator. I was thinking about using this adjustable power supply to
quick charge a small 5-cell 6V NiMH battery pack I use in a R/C car.
If I connect the battery pack to my adjustable supply (set @ 6V) what
current will the power supply put out? Will the power supply put out
the 1.2A rating of the transformer? I do plan to carefully time the
charge so that I do not overcharge the pack.
Don't use a regulator at all. Divide the amper-hours rating of the
battery by 10 hours to get the safe continuous charge rate. Since R=E/I
, divide (12-6)volts by the charge rate to get a suitable resitance.
Then multiply the charge rate by 6 to get the wattage for the resistor.
This is a low-tech approach; but is usually safe, any easy.
|
C/10 charging (charge current = amp-hour rating / 10) isn't recommended
for NiMH so it's probably safer for the battery to use C/15. Charging
efficiency is about 65% so you need to supply a total charge of C/.65 to
fully charge the cell. This means you need to charge for a total of
15/.65=23 hours to ensure a full charge at C/15 rate. Continuous
overcharge is not recommended for NiMH so don't leave cells in the
charger for much longer than this or you'll degrade them.
If you check the National web page, you'll find an app note for using
the LM117 as a constant current source so you shouldn't have to make too
many changes to your circuit. |
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mike
Guest
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| Posted:
Fri Feb 18, 2005 6:11 am Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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computer_prog@hotmail.com wrote:
| Quote: | You need a controlled, known current limit.
YOu need a controlled, known voltage.
The pack is a 6V 1200mah pack. The sheet said that the pack can be
slow charged @ 6V 100ma for 12 hours. The pack came with a small and
simple 6V 100ma wall transformer for slow charging. I have been using
this wall charger for a few months now.
The sheet that came with the battery pack also said that it can be
"quick charged" at 6V, 1A, for 60 minutes. I was looking to see if I
could speed up the charge time by using the 1.2A Transformer based
power supply I built.
|
Problem is that you're reading the nominal/marketing numbers off the
components. Those are NOT the actual numbers you'll measure when you
plug it all together.
Yes, a fully discharged 1200mAH pack can be charged at 1 amp for 60
minutes. A half discharged 1200mAH pack charged at 1 amp for 60 minutes
will get VERY hot. Notice that I haven't mentioned the voltage because
it changes over the charge time and as the pack ages. Unless you know
the EXACT state of charge, timed current charge is very unreliable and
can be dangerous at the 1 hr. rate. And you propose setting the voltage
and letting the current go where it will. That's even more dangerous.
One reasonable way to fast charge NiMH is to monitor the voltage and
terminate when the rate of change in voltage goes to zero. You'll also
want to monitor temperature.
While I'm on a rant...
Cell reversal is very bad for rechargeables. In high current
applications like RC cars, how do you know when the battery is
discharged? I expect most people notice the speed slowing down
and run it some more. This will almost definitely reverse the weakest
cell, weakening it further. A few cycles of this and you'll end up with
a shorted cell. Put your fixed voltage charger on that and you'll
charge the crap out of the cells that were still good before you tried
to charge 'em.
It ain't a simple problem.
mike
--
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
..
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/ |
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Ian Stirling
Guest
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| Posted:
Fri Feb 18, 2005 6:11 am Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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computer_prog@hotmail.com wrote:
| Quote: | You need a controlled, known current limit.
YOu need a controlled, known voltage.
The pack is a 6V 1200mah pack. The sheet said that the pack can be
slow charged @ 6V 100ma for 12 hours. The pack came with a small and
simple 6V 100ma wall transformer for slow charging. I have been using
this wall charger for a few months now.
The sheet that came with the battery pack also said that it can be
"quick charged" at 6V, 1A, for 60 minutes. I was looking to see if I
could speed up the charge time by using the 1.2A Transformer based
power supply I built.
|
LM117 datasheet.
http://www.st.com/stonline/books/pdf/docs/2154.pdf
Look at "current limited 6V charger".
When current through R3 exceeds about 0.6A, the transistor turns on, and
pulls the voltage set at the junction of R1/R2 down until the current falls to
0.6A.
Maximum voltage is set as it usually is, by the ratio of R1/(R2+R3) |
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Rich Grise
Guest
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| Posted:
Fri Feb 18, 2005 6:11 am Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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On Fri, 18 Feb 2005 01:18:04 +0000, Ian Stirling wrote:
| Quote: | computer_prog@hotmail.com wrote:
You need a controlled, known current limit.
YOu need a controlled, known voltage.
The pack is a 6V 1200mah pack. The sheet said that the pack can be
slow charged @ 6V 100ma for 12 hours. The pack came with a small and
simple 6V 100ma wall transformer for slow charging. I have been using
this wall charger for a few months now.
The sheet that came with the battery pack also said that it can be
"quick charged" at 6V, 1A, for 60 minutes. I was looking to see if I
could speed up the charge time by using the 1.2A Transformer based
power supply I built.
LM117 datasheet.
http://www.st.com/stonline/books/pdf/docs/2154.pdf
Look at "current limited 6V charger".
When current through R3 exceeds about 0.6A, the transistor turns on, and
pulls the voltage set at the junction of R1/R2 down until the current falls to
0.6A.
Maximum voltage is set as it usually is, by the ratio of R1/(R2+R3)
|
But he's not interested in maximum voltage. He's interested in 1A constant
current, for 60 minutes.
Although, the LM117 datasheet _does_ show a simplified circuit for a
current regulator.
Good Luck!
Rich |
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Ian Stirling
Guest
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| Posted:
Fri Feb 18, 2005 5:19 pm Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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Rich Grise <richgrise@example.net> wrote:
| Quote: | On Fri, 18 Feb 2005 01:18:04 +0000, Ian Stirling wrote:
computer_prog@hotmail.com wrote:
You need a controlled, known current limit.
YOu need a controlled, known voltage.
The pack is a 6V 1200mah pack. The sheet said that the pack can be
slow charged @ 6V 100ma for 12 hours. The pack came with a small and
simple 6V 100ma wall transformer for slow charging. I have been using
this wall charger for a few months now.
The sheet that came with the battery pack also said that it can be
"quick charged" at 6V, 1A, for 60 minutes. I was looking to see if I
could speed up the charge time by using the 1.2A Transformer based
power supply I built.
LM117 datasheet.
http://www.st.com/stonline/books/pdf/docs/2154.pdf
Look at "current limited 6V charger".
When current through R3 exceeds about 0.6A, the transistor turns on, and
pulls the voltage set at the junction of R1/R2 down until the current falls to
0.6A.
Maximum voltage is set as it usually is, by the ratio of R1/(R2+R3)
But he's not interested in maximum voltage. He's interested in 1A constant
current, for 60 minutes.
Although, the LM117 datasheet _does_ show a simplified circuit for a
current regulator.
|
I meant to expand on that a bit.
I was assuming that he'd used the standard "variable resistor connected
between adj, gnd, and out" circuit.
In which case, he could lift out the ground of the pot, stick either
one resistor in, or one of several selected by a switch, to add current
limiting to the supply.
I then meant to add.
If you also connect another NPN transistor with its emitter grounded,
and its NPN collector wired to the base a PNP transistor, and the PNP
transistors emitter connected to the ADJ pin,
with the PNP transistors collector connected to the NPN transistors base.
(The NPN transistor turns on as its base voltage increases.
This then causes current to flow in base of the PNP transistor, turning
it on, causing current to flow to the base of the NPN transistor from the
ADJ pin/resistors, and the NPN transistor is then latched on.)
Now, you add a negative temperature coefficient thermocouple in series
with a variable resistor, (you want the thermistor to have a resistance
of about 2K ohms at the temperature).
You connect the thermistor to the positive rail, and a variable resistor
to the negative rail, and connect through a 100 ohm resistor to the base
of the NPN transistor (the one connected to the PNP).
This gives you a current limited supply, that switches off when the
thermisor (applied closely to the battery) warms to a preset threshold,
which should be 5-10C over room temperature.
(this has problems in chargers that will not be in a very constant temperature
environment).
If it's in a fairly constant environment, so you can set it to a reasonable
temperature (say 30C in an office environment, then the battery will be
charged at a constant current until it gets too hot, at which time it
turns off. |
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Fred Bloggs
Guest
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| Posted:
Fri Feb 18, 2005 7:19 pm Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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computer_prog@hotmail.com wrote:
| Quote: | You need a controlled, known current limit.
YOu need a controlled, known voltage.
The pack is a 6V 1200mah pack. The sheet said that the pack can be
slow charged @ 6V 100ma for 12 hours. The pack came with a small and
simple 6V 100ma wall transformer for slow charging. I have been using
this wall charger for a few months now.
The sheet that came with the battery pack also said that it can be
"quick charged" at 6V, 1A, for 60 minutes. I was looking to see if I
could speed up the charge time by using the 1.2A Transformer based
power supply I built.
|
You can speed up the charge time considerably with fast charge, and
"duh" that's why it's called "fast" charge. The drawback is that the
charging circuit becomes more complicated. The 60 minute duration, cell
temperature of 40oC above ambient, or cell cutoff voltage of
1500mV/cell, whichever occurs *first*, are backup termination criteria
to prevent damage or rupture and are not invoked in normal fast charge
operation- only when something has gone wrong. The actual normal and
proper charge termination methods are *either* 0.5oC/minute temperature
rise or delta Vbatt/sec < 0 Volts/sec. As you can see these are *rates*-
involve long time constants and precision measurements. You will not be
able to build this yourself. Since you're not interested in electronics,
and only how it is used, you should buy a NiMH fast charger. |
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Mark
Guest
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| Posted:
Fri Feb 18, 2005 11:51 pm Post subject:
Re: Charge rate with my adjustable regulator and a NiMH pack |
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This man is correct.
Also get yourself a current meter and put it in series. Then you can
see g=how much current is actually going in to the battery.
Charge current = (VS - VB) / R
where VS is the ACTUAL voltage out of the supply
VB is the actual battery volrage
R is the resistor in series between them |
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