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Old 17-06-2013, 16:27   #1
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2 Ohm Resister

I was on a boat today that has a group 31 main battery, then there's a motorcycle size battery that runs a few things.There's a number 10 wire with a 2ohm resister between it from the group 31 to the smaller battery. What is the guy trying to do? Restrict the amount of charge going to the smaller battery because of the different sizes?
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Old 17-06-2013, 17:16   #2
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I was on a boat today that has a group 31 main battery, then there's a motorcycle size battery that runs a few things.There's a number 10 wire with a 2ohm resister between it from the group 31 to the smaller battery. What is the guy trying to do? Restrict the amount of charge going to the smaller battery because of the different sizes?
Hi Rider, It is not a good idea to mix different battery sizes. Sure you get extra amp hours available.
2 ohms resistor is protect the small battery against inrush current when battery is low on charge.
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Old 17-06-2013, 18:27   #3
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Re: 2 ohm resister

If it is meant to be an emergency supply for radios etc it should also have a diode as an isolator.
ps The voltage on the battery will be about the same but it will not be discharged by the main batteries being low. If it has been working OK for a long time leave it as is.
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Old 17-06-2013, 18:29   #4
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Re: 2 ohm resister

I checked the resister and it reads fine.The large battery was reading 13.6v but when i read the small battery is was 12.9 with the wire from the resister on or not.Does it need higher volts to work past the resister.
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Old 17-06-2013, 19:31   #5
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Re: 2 ohm resister

I don't think it is a resister
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Old 17-06-2013, 19:32   #6
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Re: 2 ohm resister

More likely a diode
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Old 17-06-2013, 19:36   #7
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I checked the resister and it reads fine.The large battery was reading 13.6v but when i read the small battery is was 12.9 with the wire from the resister on or not.Does it need higher volts to work past the resister.
13.6 - 12.9 = 0.7V implies it is a diode, too ???

A resistor in this application would be nonsense. But then it is a boat, right....
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Old 17-06-2013, 19:36   #8
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Re: 2 ohm resister

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Originally Posted by tuberider View Post
I checked the resister and it reads fine.The large battery was reading 13.6v but when i read the small battery is was 12.9 with the wire from the resister on or not.Does it need higher volts to work past the resister.
A resistor (resister?) will pass current whenever there is a potential difference (volts) of any size (micro volts though to mega volts) across it. If it really is a 2 ohm resister, then it must have a high power rating, thus will be physically big
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Old 17-06-2013, 19:44   #9
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Re: 2 ohm resister

Is that resistOr roundish with 3 or 4 colored bands on it, or squarish and about 2 inches long, or roundish and colored black with maybe a triangle designed on it? I'm just trying to figure out what is this component!

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Old 17-06-2013, 21:22   #10
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Re: 2 ohm resister

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13.6 - 12.9 = 0.7V implies it is a diode, too ???

A resistor in this application would be nonsense. But then it is a boat, right....
Exactly; I can't think of a single occasion where anyone would need a resistor (of any value) between the two batteries but I can think of many reasons why it would fail if it was a resistor.

As I (and others) have posted, I am certain the OP will find that it is (was?) an isolating diode.

But then it is a boat, right
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Old 17-06-2013, 22:06   #11
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Re: 2 ohm resister

Regardless it is resistor or silicon diode is's useless.
I set up many aux. batteries. I only use schotky diode,
reason beeing, shotky diode has 0.3 volts drop and allows
to aux. bat. to be well charge without using complex
electronic gadgets.
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Old 18-06-2013, 04:56   #12
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Re: 2 ohm resister

I called the manufacturer and it is a resister, and it is pretty big about 4" long half inch high.Another thing I can't seem to wrap my head around is how you can use a small wire to feed the small battery for charging. What always comes to mind is a boat trailer that has a small battery for the break away electric brakes.The car's alternator say puts out 60amps, the wire going to the trailer plug looks llike a number 14ga. How does that work when the alternator puts out 60amp to that small battery.
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Old 18-06-2013, 05:21   #13
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That 2 Ohm resistor will limit the current in that circuit to 2 Amps in typical charging situations. That would be when there is a 4 volt difference between the charge source and the small battery. The difference will be converted to heat at 24 Watts. Thus the large size. Over an extended charge period the small battery will reach the voltage if the charge source.

It a goofy, cheap, inefficient, shortcut. If it works for you it's okay. Most cruisers would shun the setup as wasteful. Depends.

There should be a current limiting device in series with the resistor to prevent it from becoming a flaming fireball if something goes wrong.
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Old 18-06-2013, 09:18   #14
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Re: 2 ohm resister

It is a logical choice, don't mess with it.

The purpose is to provide isolation for your instruments during engine starting. With a direct connection, when the starting battery drops to a low voltage the instruments would drop out and things like radar can take quite a while to restart, not something you want to happen when you had to start an engine in an emergency.

It works like this.

Under idle conditions it will limit the current charging the battery but as the battery charges the current gets less and less until both batteries are charged. As the current gets less the voltage drop in the resistor gets less and finally both batteries are at the same voltage (very nearly).

Under in-use conditions there will be power supplied from the starting battery, typically over 13 to 14 volts with an alternator running. If the AVERAGE current used by the instruments is 1/2 amp there would be a 1 volt drop in the resistor giving 12 to 13 volts for the instruments.

Under starting conditions when the instrument battery is 12.8 and the starting battery drops to 11 volts (which was killing the instrument), the 1.8 volts difference will be across the resistor giving about a 1 amp drain on the small battery instead of perhaps 50 amps without it. The 1 amp drain for a few seconds during starting will not diminish the instrument voltages.

A diode would give the same protection but is much more expensive and much less reliable. The choice of 2 ohms should have been based on the average current used by the instruments. Depending on what that is a resistor as low as 1/4 ohm would provide ample protection and allow larger average instrument load.
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Old 18-06-2013, 16:08   #15
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Re: 2 ohm resister

You have read a great and correct explanation.
The most important aspect was forgotten.
We talking about the boat emergency radio battery, don't we.
The whole purpose is oscillate, the house power and emergency
voltage source. System must guarantee, if anything happens
with your main, your emergency is up and fully charge.
Not killed ( discharged ) by the house.
Your resistor is back yard solution, someone who has no idea,
Simply wrong.
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