2 Ohm Resister

[Andina Marie]

Echo Charger $125 at Amazon, 2 ohm resistor $2.50 at Radio Shack.
It is a classical low pass RC filter using a small R and a very big C.
Although if it has colored bands round it, it is probably only 1 or 2 watt rating which should work OK but I would prefer to see a 10 watt rating.

[Lake-Effect]

Originally Posted by tuberider

Lake-Effect does the 6amps hold true for charging the battery thru the resister from the large battery.I looked at the small battery today and it's a 40ah battery.

The current through the resistor will be in proportion to the difference between the two batteries. So for example if the big battery is at 14.5v and the small battery is at 12 V, the charging current flowing from the big battery to the small battery will be (14.5 - 12)/2 = 2.5/2 = 1.25 A. From this you can see that as the small battery charges up and the voltage difference shrinks, the charging current also drops.

(Since there will never be 12v difference between the two batteries, you'll never see a 6 A charge current.)

As pointed out, this isn't optimal charging of the smaller battery, but it will always be getting some charge whenever the house battery is being charged. So, it will spend 99.99% of its life at just about full charge, and it only "earns its keep" by holding some instruments up while the house bank experiences a sharp drop caused by some intermittent heavy loads. The small battery could last several years in this arrangement, since it's not likely to ever be deeply discharged.

And others have also mentioned that if the two batteries are different types (eg flooded vs gel), you could end up overcharging and destroying the gel cell.

Since a 20w 2 ohm resistor is about $4, and a motorcycle battery is about $40 to $50, it's certainly a low-cost solution. Kind of clever, actually. Ohms' law for the win!

[smac999]

Originally Posted by goboatingnow

Suggesting a echo charge for this application is way of the mark. The resistor will do just fine


Dave

depends on the purpose of the small battery. if it's a radio battery for emg power. when the main drains it will drain the small one with it as well. since the resistor will bleed current in both directions. and a one way device should be used. like echo charger or diode.

[ryon]

Originally Posted by Lake-Effect

In the described system (big battery, 2 ohm resistor, small battery) the loads on the small battery will not see significant voltage fluctuations when the large battery has short-term high-current loads (eg windlass, starting, etc)

Regardless of the magnitude of a momentary load on the big battery, the highest possible current that can be drawn from the small battery through the resistor will be 6A... and that's only if the big battery's voltage drops to zero.

If the big battery experiences a load that drops its voltage by 1 volt, the additional load on the small battery is 1/2 = 0.5 A. Not enough to drop the small battery's voltage significantly. That's pretty good protection for the loads on the small battery.

Sure. But where does the current from your small battery go, once the large battery is at a lower potential? To your anchor light, or to the big battery? The answer is both -- with the one having the lowest resistance getting the most current. Limited by the resistor of course.

The current through the resistor will be in proportion to the difference between the two batteries. So for example if the big battery is at 14.5v and the small battery is at 12 V, the charging current flowing from the big battery to the small battery will be (14.5 - 12)/2 = 2.5/2 = 1.25 A. From this you can see that as the small battery charges up and the voltage difference shrinks, the charging current also drops.

(Since there will never be 12v difference between the two batteries, you'll never see a 6 A charge current.)

As pointed out, this isn't optimal charging of the smaller battery, but it will always be getting some charge whenever the house battery is being charged. So, it will spend 99.99% of its life at just about full charge, and it only "earns its keep" by holding some instruments up while the house bank experiences a sharp drop caused by some intermittent heavy loads. The small battery could last several years in this arrangement, since it's not likely to ever be deeply discharged.

And others have also mentioned that if the two batteries are different types (eg flooded vs gel), you could end up overcharging and destroying the gel cell.

Since a 20w 2 ohm resistor is about $4, and a motorcycle battery is about $40 to $50, it's certainly a low-cost solution. Kind of clever, actually. Ohms' law for the win!

Only IF the instruments are powered from the small battery side of the questioned resistor/diode AND assuming that the small battery has the moxie to power these instruments at all times, due to the current drop of the resistor that connects it to the main battery and charging circuits.

One small component, yet some large consequences depending on how it is used!

My point is still that you cannot just put a resistor in series with a battery and expect it to level out your voltage requirements. Resistors don't do that.

Now, what was the question, and what is the mystery component? I still think it should be a diode, and that the main battery should have one as well. But maybe there was some other problem they were trying to solve ...

[Lake-Effect]

Originally Posted by ryon

[it works] Only IF the instruments are powered from the small battery side of the questioned resistor/diode AND assuming that the small battery has the moxie to power these instruments at all times, due to the current drop of the resistor that connects it to the main battery and charging circuits.

All true.

My point is still that you cannot just put a resistor in series with a battery and expect it to level out your voltage requirements. Resistors don't do that.

Agreed. It's the resistor PLUS the small battery forming a RC low-pass function as Andina Marie pointed out.

[daddle]

A resistor in there is cheesy ... I used to fire EEs that tried that kind of shortcut. But it can work. As I think I said way above: put a fuse is series with that BS and keep the R away from flammable stuff. A resistor at over 10 watts is a dangerous thing. And don't think the E**2/R situation to start a fire won't occur.

[Wotname]

Originally Posted by goboatingnow

Why , it makes perfect electrical sense

Dave

Please enlighten me as to what this perfect electrical sense is...

[goboatingnow]

Originally Posted by Wotname

Please enlighten me as to what this perfect electrical sense is...

Th euse of teh 2 ohm resistor is to electrically decouple the small battery , whose function given its size is to power some instruments or a radio. The reisistor as pointed out by Andina does that isolation task quite admirably. It not only performs discharge isolation , but also regulates charge current ( which a diode would not).

Its an unusual setup , mainly because stable instrument power is an oddity on boats, but theres nothing nonsensical about it

dave

[tuberider]

Does the echo-charge have some kind of current limiting function where it won't let more then 15amps to pass thru.If that's the case it seems like the way to go. On the resister it also had 40/70w on it.

[btrayfors]

Originally Posted by tuberider

Does the echo-charge have some kind of current limiting function where it won't let more then 15amps to pass thru.If that's the case it seems like the way to go. On the resister it also had 40/70w on it.

Yes. The maximum pass-thru for the EchoCharge is 15A. This is more than sufficient for most boat installations, where the EchoCharge sits between the house batteries and the start battery.

On some larger boats where other devices (e.g., big cooling fans) are connected to the start battery and more than 15A is required, a device like the DuoCharge will do the trick. It can handle 30A and is programmable.

Understand that these are ONE-WAY charging devices. That is, when activated they allow current to pass in only one direction. This is unlike battery combiner devices (there are many) which essentially connect the batteries together allowing current to flow in large amounts and in both directions.

The idea is that you connect all charging sources (battery charger, alternator, generator, solar panels, wind generator, etc.) to the HOUSE battery bank. The EchoCharge or DuoCharge then sense whenever there is a charge on the house batteries and they bleed off a bit of current (max 15A or 30A) to top off the start battery. As has been noted many times, the start battery generally requires very little charging, since starting a small diesel only requires less than 0.5AH of power.

Bill

[rtbates]

Originally Posted by tuberider

Lake-Effect does the 6amps hold true for charging the battery thru the resister from the large battery.I looked at the small battery today and it's a 40ah battery.

A 2ohm resistor with 6 amps running through it will drop 12V!!

E(voltage)=I(current)xR(resistance)

12=6x2

not gonna work as a charger very well.

what really happens is the voltage across the 2ohm resistor is dependent on the voltage at the small battery and the voltage at the source, on the other side of the 2 ohm resistor. Maybe something like 12v at the small battery and 13.8 at the source, for a voltage across the 2 ohm of 1.8v

Then 1.8=Ix2 or an I of .9 amps, ie a trickle charge..

To properly use the equation E=IxR you must first know what is the known in order to figure out the unknown. In correct knowns like 6A through the 2ohm resistor make the whole process in-correct.

An yes I'm a retired EE..

[goboatingnow]

Originally Posted by rtbates

A 2ohm resistor with 6 amps running through it will drop 12V!!

E(voltage)=I(current)xR(resistance)

12=6x2

not gonna work as a charger very well.

what really happens is the voltage across the 2ohm resistor is dependent on the voltage at the small battery and the voltage at the source, on the other side of the 2 ohm resistor. Maybe something like 12v at the small battery and 13.8 at the source, for a voltage across the 2 ohm of 1.8v

Then 1.8=Ix2 or an I of .9 amps, ie a trickle charge..

To properly use the equation E=IxR you must first know what is the known in order to figure out the unknown. In correct knowns like 6A through the 2ohm resistor make the whole process in-correct.

An yes I'm a retired EE..

Formerly known ( or unknown) as Donald Rumsfeld

Dave

[Teknav]

I will not discuss the lack of merits for this modified battery circuit. One day, the boat catches an electrical fire. The insurance company investigates and finds out about this unconventional circuit; it was neither factory designed nor installed. You guess at what I am getting at... Is it really worth the trouble?

Mauritz
Going to behave now!

[Lake-Effect]

Originally Posted by Teknav

I will not discuss the lack of merits for this modified battery circuit. One day, the boat catches an electrical fire. The insurance company investigates and finds out about this unconventional circuit; it was neither factory designed nor installed. You guess at what I am getting at... Is it really worth the trouble?

Like all boat wiring, such a setup can be properly designed and fused.

I don't know where you sail, but I don't know of any boat outside of a showroom that is still 100% factory wiring.

[tuberider]

On boat trailers with the small battery mounted up front in case the trailer disconnects from the car it engages the electric brake. That small battery is charged by the car's battery. Is there something in the car that limits amps to the trailer battery.