# Limiting The Current Of A Battery Charger

Discussion in 'General Electronics Chat' started by uponone, Jul 12, 2011.

1. ### uponone Thread Starter New Member

May 18, 2010
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0
Hi,

I was recently given a 6A manual automotive battery charger. I would like to modify it to use with my motorcycle battery. The battery manufacture recommends that the battery is charged 1.4A. Is there a way of reducing the amps from 6amps down to 1.4? Perhaps a wirewound resistor?

Any help is much appreciated.

Chris

2. ### #12 Expert

Nov 30, 2010
16,655
7,293
I used an LM338K

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3. ### iONic AAC Fanatic!

Nov 16, 2007
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The question arises, will the charger sense that the battery is charged with this additional circuitry?

4. ### uponone Thread Starter New Member

May 18, 2010
10
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This is a manual battery charger. As far as I can tell it does not contain any circuitry to determine when the battery is fully charged. Batteries must be visually monitored and charged for the correct amount of time. The charger cannot be left on the battery. Basically what i am saying is it is a CHEAP model.

The LM338 looks very promising. Using the attached schematic I see I can create a 5A current regulator. I think I am missing something on the math though. How to I calculate the correct resistor value to get the current down to 1.5A?

Thanks again for the help,
Chris

• ###### LM338.gif
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5. ### #12 Expert

Nov 30, 2010
16,655
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R=E/I
R=1.24volts/1.5amps
R= .82666 ohms.

The 1.24 volt reference in the chip might be a little off. That would cause you to adjust the resistance a little bit.

Oops. You said 1.4 amps. That's .8857 ohms. Still about 4 watts. You're going to wind up using more than one resistor to get really accurate results. Like one ohm in parallel with 8 ohms (7.75 ohms if everything is exactly what the spec sheet says.)

Last edited: Jul 12, 2011
6. ### uponone Thread Starter New Member

May 18, 2010
10
0
I get it now! Thanks #12! I think this solution is going to work just fine. Thanks for turning me on to the LM338K. I can see a few other projects this will come in useful!

Chris

7. ### pistnbroke Member

May 9, 2011
32
1
problem is all you lot know nothing about battery charging ... The manufacturer specifies the Maximum charge current for the battery to avoid overheating so connecting your so called 6A battery charger may give a current in the right range...a 6A battery dcharger does not charge at 6A !!! thats its maximum...Most important is to check the open circuit voltage of the charger as that is what your battery will go up to ..sealed battery probably 14.2v ...gasses over 14.4v .... ....A 21 w 12v bulb is probably your best bet as it will limit current to about 1.7A and as the current drops its resistance will go down so that the voltage drop will be minimal.. A Voltage regulator set to 14.2 along with the bulb would be ideal.

8. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,838
961

Parallel resistors will always have a resistance close to the SMALLEST resistor of the group.

In the case of 8 ohms and 1 0hm in parallel, the result is 8/9 of an ohm.(close to 1 ohm)

9. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Here's a very handy online calculator for determining series/parallel resistor combinations:
http://www.qsl.net/in3otd/parallr.html

One problem you'll likely experience with the LM338 idea is the regulator dropout voltage. You'll lose at least 1.7v from IN to OUT, and another nominal 1.25v from OUT to ADJ, so that's roughly a 3v drop. Your charger will have to be putting out at least 17v to charge the battery up to 14v.

10. ### russ_hensel Distinguished Member

Jan 11, 2009
820
47
A good circuit for battery charging is a constant voltage circuit with current limiting. A few op amps and power transistors can do the whole thing.

11. ### pistnbroke Member

May 9, 2011
32
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non so deaf as those who cannot hear ...KISS

12. ### uponone Thread Starter New Member

May 18, 2010
10
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Good point on the voltage drop, I completely forgot to take this into consideration. The charger is definitely not putting out 17v! I will give pistnbroke's light bulb/voltage regulator solution a go.

Thank you everyone for taking the time to help me with this. It is very much appreciated!

Chris

13. ### pistnbroke Member

May 9, 2011
32
1
Well what does it put out open circuit and what current do you get if you just connet it to the battery .without this informaton you and I are wastiing there time ..they have been charging batteries for 100 years without chips and regulators ...KISS

14. ### SgtWookie Expert

Jul 17, 2007
22,183
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I've attached a couple of LM317-based battery charger circuits. The LM338 can be used instead if desired with no modifications to the circuit necessary.

The first schematic includes temperature compensation, and also a diode (D6) which keeps the charge circuit from draining the battery in case the mains power fails or the charge circuit is otherwise unpowered.

Other diodes can be substituted for D5, but D6 really needs to be a Schottky diode rated for at least as much current as you set the charger up to provide.

As shown in the schematic, R4 sets the charging current.
As the battery voltage nears fully charged, current will decrease. If you adjust potentiometer R2 so that the output voltage is 13.6v-13.7v at room temp (25°C/77°F), you can leave the charger on the battery indefinitely.

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15. ### #12 Expert

Nov 30, 2010
16,655
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Since this is dragging on like, forever, I might say that I chose the 338 because it has a bigger die and thus is easier to keep cool (theta Jc=1.4). The 317 can do 1.4 amps (as the OP requested) (theta Jc=2), but the 338 is easier to keep cool.

ps, I can't believe someone hasn't suggested a switching regulator by now. I just used the 338 because it's simple and I had one laying around.

Last edited: Jul 14, 2011
16. ### uponone Thread Starter New Member

May 18, 2010
10
0
Thank you for the schematic. That is very helpful, I think I will have to build one.

I will look into the switching regulator. I think at this point I have enough to go on. I will let you all know how it goes.

Thanks again for all the help everyone. I have definitely learned a bunch regarding battery charging.

17. ### SgtWookie Expert

Jul 17, 2007
22,183
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#12's suggestion on the LM338 is very valid (larger die) - but of course you will need a large heat sink whichever linear regulator that you might use.

I didn't want to get into switching regulators, as they are very difficult for novices to understand, and troubleshooting them usually requires more than a DMM (digital multimeter).

18. ### #12 Expert

Nov 30, 2010
16,655
7,293
Watch out! The pinout of the 338K is different from the 317K!

19. ### uponone Thread Starter New Member

May 18, 2010
10
0
Thank you for pointing that out!