# Current limiting resistor for battery charger?

Discussion in 'The Projects Forum' started by imbaine13, Jan 8, 2014.

1. ### imbaine13 Thread Starter Member

Oct 6, 2013
62
0
Hello guys,

I'm really confused as to what value of a resister to use to control charging current into a charging battery.
Here are some details, Battery is a 6V 4.5AH, the charger (wall adapter) unloaded voltage is 10v but it says 9.5v in the casing, and the loaded output current is 1.5A. There is circuitry meant to stop the battery charging when the voltage reaches 7.25 volts.

The whole thing is designed this way, power from the adapter goes through the charge regulator before getting to the battery, with a mosfet switch. The biggest problem is that I need to limit the current right down to 0.4 amps. I've seen several articles on the internet that say: R=(9.5v-6v)/0.4A, which would give an 8.75Ω resistor, or roughly a 10Ω one. This would mean I get 0.4 amps only when the battery voltage is 6 volts (BATTERY DEAD!) and about 0.225A as the battery gets full!!!
Would someone please verify if this is accurate?

Have a nice day.

2. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507

Yes, that's why using a voltage source and a resistor is such a crummy way to build a battery charger.

3. ### imbaine13 Thread Starter Member

Oct 6, 2013
62
0
I made the charge controller myself, and I was considering using a lower power source to charge the battery, but it did not perform to my expectations, so I have decided to use an alternative. The circuit for the charge controller has no current limiting resistor because the power source I was planning on using was putting out exactly 0.4A. Now that I have substituted the power source for a much more powerful one, I need the resistor because the charge controller is already soldered, and I can't make modifications to it. I thought I would just link the resitor between the power adpter (power source) and the charge controller. The illustations are below.

4. ### 4pyros New Member

Jan 3, 2014
22
3
Your charge controller should be limiting the current.
You can make a current limiter that is independent of the voltage.

5. ### wayneh Expert

Sep 9, 2010
12,405
3,256
A low tech solution is a lightbulb in series, instead of a fixed resistor. The lightbulb will have a higher resistance when it's hot (higher current, when the battery is dead) and a lower resistance when it's cool (low current, battery approaching full charge).

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Even better is a low-wattage 120V light bulb in-series with the transformer primary. You can experiment with different lamp wattages until you find one that limits the max charging current to your liking. The resistance change that happens with the lamp's self-heating filament makes it almost a current-source.