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# Resistor size to not fry resistor

Posts: 2
Hi
I want to put a 2 Ohm resister into a 12 volt battery chargers, 2 amp rated charging line to halve the current but i do not know what size resister I need that will handle the current flow without frying the resistor
I was thinking of using a 50 watt Wire Wound Aluminium Housed Resistor
would that handle the current
Any help is appreciated

• Posts: 22,930
edited 2020-03-20 - 15:49:57
Using Ohm's Law, a 2 Ohm resistor with 2 Amps current ... E = I x R = 2 x 2 = 4 Volts. Power = Voltage x Amperage = 4 x 2 = 8 Watts. I'd say any resistor rated for at least 10W would work. You'd generate about 8W of heat which needs to be dissipated ... You don't want to put the resistor in a closed wooden box for example.

It's not clear just what you're trying to do with this resistor. You mention halving current. Remember that the current will vary as the battery charges. Battery chargers these days tend to be "smart" about the actual charging voltage and current used as the battery charges. The voltage produced by a battery also varies as the battery becomes more and more charged up.
• Posts: 8,517
50W is a bit of overkill. At 2A the resistor would dissipate 8 Watts so 8-10 watts would be fine if that reduces the current to 1 amp. The actual current you get will also depend on the charger output voltage at 1A.
• Posts: 2
The web is a wonderful place
Thank you both Mike Green and Kwinn for your much appreciated assistance
To answer Mike Green, in order to properly desulphate a battery, my understanding is that you need to use a old style battery charger that is not smart and then allow the voltage to climb over 15 to 16 volts. This would normally damage the battery, but with a reduced current this will not be the case
I am having trouble finding a adequate 2 Ohm resistor, but can find a 1 Ohm resister.
If i put 2, 1 Ohm resistors in series, will that give me a 2 Ohm Resistance
• Posts: 1,079
MaverickCT wrote: »
The web is a wonderful place
Thank you both Mike Green and Kwinn for your much appreciated assistance

I am having trouble finding a adequate 2 Ohm resistor, but can find a 1 Ohm resister.
If i put 2, 1 Ohm resistors in series, will that give me a 2 Ohm Resistance

Yes. Resistors in series add. (Rx = R1 + R2)

• Posts: 3,362
Also if you are using two resistors each can have half the power dissipation, so only 5W is necessary. I have found it is considerably harder to find power resistors above 5W. But look what I found...

https://www.goldmine-elec-products.com/prodinfo.asp?number=G23074

They also have a couple of 4 ohm range 15-20 watt resistors which could be used in parallel to get 2 ohms and would run very cool.
• Posts: 670
If you pumped 12 volts directly into that 2 ohm resistor it would draw about 6 amps. So 12 volts x 6 amps would be 72 watts. So I would go with a 100 watt resistor, to handle the heat. In your case 12 volts x 2 amps is 24 watts. But are there any short circuit or spike load concerns. I'm thinking your using large battery such as a lead acid type.
• Posts: 44
edited 2020-03-22 - 07:31:44
Maybe that means that (16Volt-12Volt)/2Amp=2Ohm.
But if so, it causes the battery dangerously overcharged and generate hydrogen, I think.

(Edit)
Sorry I misread and misunderstood much.
Now I guess that you might think that an X ohm resister divides the current by X ?
• Posts: 8,517
DigitalBob wrote: »
If you pumped 12 volts directly into that 2 ohm resistor it would draw about 6 amps. So 12 volts x 6 amps would be 72 watts. So I would go with a 100 watt resistor, to handle the heat. In your case 12 volts x 2 amps is 24 watts. But are there any short circuit or spike load concerns. I'm thinking your using large battery such as a lead acid type.

The resistor will be connected between a 12V battery and a charger limited to 2 amps. No need for more than 8-10 ohms.
• Posts: 6,007
MaverickCT wrote: »
Hi
I want to put a 2 Ohm resister into a 12 volt battery chargers, 2 amp rated charging line to halve the current but i do not know what size resister I need that will handle the current flow without frying the resistor
If I understand correctly, the OP want to reduce the charger's current from 2 amps to 1 amp. I don't understand how a 2 Ohm resister will achieve that. MaverickCT, how are you going to connect the 2 Ohm resister? If you connect it in series to the load you will still get 2 amps if the load resistance is less than 4 Ohms. If the load is a battery, you will get 2 amps if the battery voltage is less than 8 Volts.

• Posts: 670
You really need a current regulator. But it sounds like your looking for something simple to rig up. Your chargers put out 16 volts now you want to reduce that with a resistor. Just get several 50 watt 1 ohm resistors add them in series until you get the drop you require.
I put a 10 ohm 5 watt resistor on my power supply set at 12 volts. The amp draw was about 1.2 amps. The 5 watt gets hot fast so you'll need a 50-100 watt to dissapate the heat if your chargeing for many hours.
This method is not the way to do it. But if you want simple, cheap, and a rig use the resistors.
• Posts: 670
Or just buy a variac, plug your charger into the variac and adjust it until you get the current. Cost you about \$120.00 . MPJA.com stock #15163TR
• Posts: 8,517
kwinn wrote: »
DigitalBob wrote: »
If you pumped 12 volts directly into that 2 ohm resistor it would draw about 6 amps. So 12 volts x 6 amps would be 72 watts. So I would go with a 100 watt resistor, to handle the heat. In your case 12 volts x 2 amps is 24 watts. But are there any short circuit or spike load concerns. I'm thinking your using large battery such as a lead acid type.

The resistor will be connected between a 12V battery and a charger limited to 2 amps. No need for more than 8-10 ohms.

Sorry, I meant 8-10 Watts, not ohms.
• Posts: 542
edited 2020-03-23 - 18:04:47
I can't figure out why some suggest 50-100 Watts when we talk about a 12V charger limited to 2A.

I'm not sure I understand what MaverickCT means in his second post, if he says he is actually using that not so smart old style battery charger. I also cant see what kind of battery technology he is using here.
I'm using a simple solution for charging some of my lead acid batteries (like 7,2Ah size); a cheap wall wart 15-17V or so, 1A max, and a PB137 regulator.

PB137 does probably not cost more then a big resistor and is rated for 1,5Amps at 13,7V output. Maybe two can be put in parallell to handle 2Amps but I'm not sure.

Forgot to mention but
Datasheet can be found here: http://www.farnell.com/datasheets/1767487.pdf