Super User is a question and answer site for computer enthusiasts and power users. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I've got a battery pack for one of my peripherals that recently died, so i took it apart to find half the (nicad) cells were dead (I tested each cell's voltage both with and without a load - the dead ones dropped off to zero with load).

So what i wish to do is replace them with nimh's. Apparently the easiest way to charge nimh's is at 10% of C (C being 2500mAh) for 15 hours at 1.5V per cell (i will have 10 cells in series).

Now the question: I have a 15v power adaptor (correct voltage) but it is 400mA. I want to 'throttle' it to 250mA somehow. Is there some way i can do this with a diode or resistor or transistor or something?


share|improve this question

closed as off topic by Bob, Journeyman Geek, Sathya Jun 7 '12 at 12:19

Questions on Super User are expected to relate to computer software or computer hardware within the scope defined by the community. Consider editing the question or leaving comments for improvement if you believe the question can be reworded to fit within the scope. Read more about reopening questions here.If this question can be reworded to fit the rules in the help center, please edit the question.

up vote 2 down vote accepted

There are IC chips like the LM317 that vary the voltage to keep the current constant. But you can't do it with out building at least a small circuit. An alternative would be to put a potentiometer is series with the battery, but you would have to periodically adjust it to keep the current constant.

share|improve this answer
Is that similar to this? – Chris Aug 28 '09 at 0:15
Found a good example of that circuit: – Chris Aug 28 '09 at 0:48
Chris, The second one's a good example. I might replace the fixed resistor with a pot or better yet a pot in series with a fixed resistor to provide an absolute limit. With the pot you can adjust the current. – Jim C Aug 28 '09 at 11:55

Simplest method to limit current is with a series resistor. Be careful with the power rating of the resistor, though.

My understanding of battery charging is that the best method is to use a control circuit. Here are some examples of battery charging circuits that would perform better than a series resistor:

Linear Tech LTC4060 NiMH/NiCd Battery Charging Circuit

Maxim MAX712 NiCd/NiMH Battery Charging Circuit

National Semiconductor LM317 (as mentioned in another answer)

share|improve this answer
If i use a series resistor, would the resistor soak up part of the voltage, leaving less voltage for the batteries? – Chris Aug 27 '09 at 23:18
Yep. There would be a measurable voltage drop across the resistor. – hanleyp Aug 28 '09 at 2:57
Are we talking a lot of the voltage (eg 3 volts) being used by the resistor? Or only a little bit (eg < 1volt) ? – Chris Aug 28 '09 at 6:11
Oh, yeah. To answer your question, V=IR, so the voltage drop across a 60-ohm resistor at 250mA would be ~15V. :-) A resistor limits current, but is not best for a battery charging application. – hanleyp Aug 29 '09 at 14:08
I don't recommend using only a series resistor for a constant current supply for use when changing batteries. The cost of a correct circuit is too cheap to accept unnecessary risk of damaging the batteries (which can have dangerous failure modes from over-charging or over-voltage while charging). – mctylr May 14 '10 at 21:12

Not the answer you're looking for? Browse other questions tagged .