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.
Well, you can't set it at 6V or else it will never charge because a 5
cell NiMH has 1.45V/cell under charge.
Therefore your pack voltage under charge will be
5 x 1.45 = 7.25VDC
You don't say what the capacity of your pack is but IIRC from my days
in R/C racing they used 1.4AHr packs so that is what I'll base all the
following calculations on.
If you wanted a slow charge you could charge at C/10 rate which would
be 140mA.
If you set your power supply to 12VDC and then hooked a current limit
resistor between the power supply and the battery you would calculate
as follows
R = (12-7.25)/.14
which equals 33.9 ohms.
Figuring your power dissipated in the current limit resistor
P = .14x.14x33.9
which equals .664 Watts
so for a C/10 charge you would be fine with a 33 ohm, 1 watt resistor.
To summarize set the power supply to 12VDC and put a 33 ohm, 1 watt
between the power supply and the battery pack.
It should take about 15 hours to get your pack charged completely at
this rate but at C/10 you will not cause any damage if it goes longer.
If you want a faster charge C/5 (.28 amps) then half the resistor
value to 16 ohms and double the power rating to 2 watts. This should
have you charged in about 7 to 8 hours.
Not knowing anything about the internals of your power it's hard to
say whether either of these current rates would cause damage to your
supply but I doubt that .28 amps out would cause any grief unless you
have some kind of low current fuses or really low power devices in
your internal power supply circuity. Really it's not possible to know
without more info.
If you want a really fast charge, on the order of 2 and 1/2 hours you
can put a 6.8 ohm, >= 3.5W between the 12VDC power supply and the
battery and charge the pack at C/2. If you do this, I would get the
battery off charge pretty soon after the 2.5 hours. I would also
watch my power supply carefully to make sure it can take it.
-wlb