Charge rate with my adjustable regulator and a NiMH pack

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 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.

 

[Mark]

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

 

[Ian Stirling]

Rich Grise <richgrise@example.net> wrote:

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.