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500mah to 180mah, which resistor?

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500mah to 180mah, which resistor?

Old 05-14-2012, 05:08 PM
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IronZ
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Default 500mah to 180mah, which resistor?

Hello RCU!

I have an old Anderson Racing multi-charger. One of the sets of jacks charges at 500mah and I want to drop it down to 180mah(ish). How do I go about determining what resistor I need? I've looked around a bit and discovered that I am not an electronics engineer, haha. Does anyone have some input?

Thanks!
Z
Old 05-14-2012, 09:57 PM
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Goldenduff
 
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Default RE: 500mah to 180mah, which resistor?

What voltage is it currently at?
Old 05-14-2012, 10:14 PM
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Cronniss
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Default RE: 500mah to 180mah, which resistor?

Disclaimer: I am used to dealing with electronics at a constant output and not a measure over time, which is what charging a batter requires. The information I am giving you below is based on my knowledge of electronics and may need some tweaking to accommodate current over time as opposed to a constant output. (I have never studied a battery charger, so...) Basically this disclaimer is to inform you to make alterations to your charger at your own risk.


Well, Ohm's Law dictates that.

For your concerns, you're wanting to find R (resistance); so you would look at it as:

R = V / I

Since you already know thatyou desirea current (I) of ~180mah, it is going to be the output voltage (V) of your charger that is going to determine what size resistor you want too use.

With a constant voltage of 10V you would want a resistor of somewhere around 55.555 Ohms. Of course you're not going to find a resistor of that precision at Radio Shack, but give or take an Ohm or two will incease or decrease the current by a few miliamps. Going to 60 Ohms will give you a current of 166.666 miliamps, whereas using a resistor of 55 Ohms will give you 181.1818 miliamps.

Now, with a constant voltage of 12V you would want a resistor of around 66.666 Ohms. Again, going to 66 or 65 Ohms will increase the current...but not by a substantial amount. And, of course, going to 67 or 70 Ohms will decrease the current...and since it would be easier to find a resistor of 70 Ohms than 67, it would probably decrease the current more than you would like.

If your charger uses a different voltage level, then plug that value into the equation (V) and divide by 0.180 to find out how many Ohms you need.

I hope this helps!
Old 05-15-2012, 06:53 AM
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Rodney
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Default RE: 500mah to 180mah, which resistor?

The basics as stated above are true but has left out the internal impedance of the charger itself. That resistance value you get when you divide the open circuit voltage (the output voltage of the charger with nothing connected to it) minus the voltage of the battery being charged by the desired current you have the value of the total resistance required to limit the current to the value desired. Now you have to subtract the internal impedance from that calculated value of total resistance to get the value needed for the external resistor.
Old 05-15-2012, 07:38 AM
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Cronniss
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Default RE: 500mah to 180mah, which resistor?


ORIGINAL: Rodney

The basics as stated above are true but has left out the internal impedance of the charger itself. That resistance value you get when you divide the open circuit voltage (the output voltage of the charger with nothing connected to it) minus the voltage of the battery being charged by the desired current you have the value of the total resistance required to limit the current to the value desired. Now you have to subtract the internal impedance from that calculated value of total resistance to get the value needed for the external resistor.
See, I knew I was missing something! (Old to computers, but new to R/C and the electronics involved with R/C.)

Thanks Rodney!

Old 05-15-2012, 07:48 AM
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bgosselin
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Default RE: 500mah to 180mah, which resistor?

Personally, the use of a simple resistor can be tricky to figure once you try to take into account existing output impedance, voltages, etc. The way to go IMO for that small of a current is to use a LM317T linear regulator (available at pretty much any electronic components shop like Radio Shack) configured for constant current (easy to do with one resistor: 7ohms @ 1/2 watt for 180mA). This should work if the charger is just a "wall-wart" type of device and the loaded output voltage is high enough to charge the battery AND compensate for the regulator dropout (~2v) and program (1.25v) voltages. But, if this is a true charger with peak-detect and so on, any added components (even a simple resistor inline) may confuse the logic circuits.

But, if this is for a lithium battery, you also need something that would terminate the charge once cell voltage reaches 4.2v/cell. Even if this is for NiXX, you'd want a circuit that would either detect delta peak (requires logic circuits) or terminate using temperature (easier method that only requires a thermistor and a few other support components).
Old 05-15-2012, 12:21 PM
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mscic-RCU
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Default RE: 500mah to 180mah, which resistor?

This stuff gives me a headache. I would just buy a new charger!
Old 05-15-2012, 05:08 PM
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IronZ
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Default RE: 500mah to 180mah, which resistor?

Thanks for all the help! With nothing attached, it's putting out 25 volts. What I didn't check was if attaching a battery to another set of jacks affects the voltage at all. I don't think it does, could be wrong though. That being said...

R=V/I

(I'm using 200mah now, my batteries are 2000mah and I was using 180 just to be cautious)

125=25\.200

So I need a 125 ohm resistor! That's actually easier than I thought. I'm not real sure why I was struggling with it, lol.

My picture won't post, but the charger is basically 6 wallwarts in 1, nothing fancy.

Thanks!
Z




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