Confused about calculating proper size of wire

Thread Starter

DBBCircuits

Joined Apr 22, 2015
10
I am trying to size a cable that will charge a boat battery that is being towed on a trailer from a truck battery. I have looked at a few calculators and they seem to be focused on getting the correct AWG given a length, voltage drop and supplied current. It seems to me, in my case, I do not have a current generator as a supply so isn't the correct question then "How much current will go through a wire of X AWG and Y length that will result in Z voltage drop"? Isn't it true that the alternator will not supply a steady current but will supply the amount of current the battery will accept at the voltage output of the alternator?

If I am correct, how do I calculate what size wire I need, given a length of wire required and an acceptable voltage drop?

Thanks for your help sorting out my confusion!
 

crutschow

Joined Mar 14, 2008
34,201
I would use a rough value of a 1V drop over the wire length at the maximum charging current you anticipate.
The alternator does indeed generate a constant voltage, with the battery drawing whatever charging current it takes to generate a battery terminal voltage equal to the generator voltage (minus any voltage drop in the wire).
This current will reduce to a trickle as the battery reaches full charge and its voltage rises.
 

Reloadron

Joined Jan 15, 2015
7,480
You may want to give this a read. My guess is you plan to basically place the boat battery in parallel with the truck battery during towing. The only difficult part is determining what the maximum current could ever reach. All of this assumes the boat battery will never be totally dead or drained. Finally I am not sure if doing this would be considered an automotive alteration the discussion of which is against forum rules, so I will let it go at that. :)

Ron
 

strantor

Joined Oct 3, 2010
6,781
I am not sure if doing this would be considered an automotive alteration the discussion of which is against forum rules, so I will let it go at that. :)

Ron
DBBCircuits, confirm that you will use the AUX of your truck's 7-pin trailer electrical connector to get 12V, so as to avoid this forum's ban on "automotive modifications".

Yes you are correct, the battery will pull whatever amps it wants to pull depending on charge level. Up to, and including enough amps to melt the wires of your trucks wiring harness if not fused properly. This is why jumper cables are so thick. I would recommend placing a resistor in series with the charge wire to limit the amps that the boat battery can pull. This will give you a slower charge, but if you're driving a couple hours to the coast then it should be fine.

If you're driving 5 miles to the lake and you want a full charge on dead flat deep cycle marine battery before you get there, you're going to need about 100lbs of welding cable connected directly to your truck battery, you're going to need a high dollar high performance alternator, and youre going to need to find another forum to discuss it.
 

shortbus

Joined Sep 30, 2009
10,045
Strantor, I disagree with your statements.

Jumper cables are the size they are because they are used to run the starter on the car with the dead battery. They are just a 'substitute' starter cable.

If the wire to charge the boat battery needs to be "welding cable" why is the charge wire coming from the alternator on the car only a 10 or 12 (depending on vehicle) gauge wire? Pretty sure that a 8 gauge wire running back to the boat battery would be more than enough. But then that puts into play the 'trailer connector', is it going to be able to handle the current? Being that the normal lighting wire is only in the 18 to 16 gauge range.
 

wayneh

Joined Sep 9, 2010
17,495
... so isn't the correct question then "How much current will go through a wire of X AWG and Y length that will result in Z voltage drop"?
The problem is that the current is not controlled and could go to a high level if the boat battery is dead. Then your wire becomes a fuse! So the online advice is right when it says to size a wire based on the maximum current it might see. You don't care so much about the ∆V across the wire as the power dissipation - heat - that ∆V will cause. Your challenge is that you don't know what this current might be.

They make cheap gizmos for charging one car's battery off the accessory jack (cigarette lighter) of another. That must limit current somehow? Seems like you need one of those in line.
 

strantor

Joined Oct 3, 2010
6,781
Strantor, I disagree with your statements.

Jumper cables are the size they are because they are used to run the starter on the car with the dead battery. They are just a 'substitute' starter cable.

If the wire to charge the boat battery needs to be "welding cable" why is the charge wire coming from the alternator on the car only a 10 or 12 (depending on vehicle) gauge wire? Pretty sure that a 8 gauge wire running back to the boat battery would be more than enough. But then that puts into play the 'trailer connector', is it going to be able to handle the current? Being that the normal lighting wire is only in the 18 to 16 gauge range.
Yep, you're right. I concede. Thank you for correcting me.
DBBCircuits, sorry for giving you bad intel.

I'll explain why I'm wrong...
I was thinking that a battery would sink as much current when dead, as it would source when charged.
I was thinking of the battery's internal resistance as if it were a fixed value, but it isn't.
To quote the AAC E-book,:
As a battery discharges, not only does it diminish its internal store of energy, but its internal resistance also increases (as the electrolyte becomes less and less conductive), and its open-circuit cell voltage decreases (as the chemicals become more and more dilute). The most deceptive change that a discharging battery exhibits is increased resistance.
So, a dead battery will draw less charging current than it will source as output current.
I never realized this since most chargers are constant-current chargers and not a voltage source. Never thought about it from this angle.
Now I know, and hopefully this helped anyone else like me who had their head in the wrong hole.


BUT, I don't think you're out of the woods just yet. This roll-around battery charger has the following charge selections:
Charge Rate Selector Switch:
Use the Charge Rate selector switch to select the charge rate or engine starting setting you require.

• 2A Slow Charge Rate – Intended for charging small batteries such as those commonly used in garden tractors, snow mobiles and motorcycles.

• 10A, 15A, 20A, 30A Fast and 35A, 40A, 44A, 50A, 60A, 80A Rapid Charge Rate – Use for charging automotive, marine and deep-cycle batteries. Not intended for industrial applications.

• 100A, 125A, 180A, 200A, 225A, 300A Engine Start – Provides 100, 125, 180, 200, 225 or 300 amps for cranking an engine with a weak or run down battery. Always use in combination with a battery.
So that tells me that you might expect to see as much as 80A (or more) pulled by a dead marine battery if charged by a voltage source and not a current source. I briefly looked and saw trucks with alternators rated @ 60A. You might want to check yours.
 

crutschow

Joined Mar 14, 2008
34,201
Wkipedia states that the charge current for a discharged battery is 20-40A from a vehicle alternator.
So assuming a maximum of 40A, you should use 12AWG wire to avoid exceeding the wire's maximum current rating.
100' of 12AWG wire has a resistance of 0.159Ω.
Assuming a (near) dead battery voltage of 10V and a 14V charging voltage, then the maximum charging current would be 4V / 0.159Ω = 25A with that wire size and length.
This current will decrease as the battery charges up and its voltage increases.
 

Thread Starter

DBBCircuits

Joined Apr 22, 2015
10
WOW, thanks for all the great comments.

From the info here, I think what I'll do is get an extra shore power cord (10 AWG) and wire in connectors that allow me to connect it to the vehicle and to the boat battery AND make sure there is an adequate fuse in the line. That way I have an extra shore power cord if needed plus it provides adequate wire size. Of course I will need to label the connectors appropriately such that there is no confusion about how to use it. It looks like those modifications would go against the policy of this forum, so I will not pursue it here.

Thanks for the thoughtful responses.
 

wayneh

Joined Sep 9, 2010
17,495
Of course I will need to label the connectors appropriately such that there is no confusion about how to use it.
Oooohhh. Relying on labels is not the greatest plan. They fall off, people fail to read them, you ignore them in a storm, and so on. A more active and foolproof system is better.

I know from my own boating experience that every imaginable mistake will be made at least once, and then a few unimagined ones will sneak in as well. Plan for safety.
 

shortbus

Joined Sep 30, 2009
10,045
As a side note in this, I replaced a 130A alternator last week in my sons work van. After doing this kind of stuff for most of my life, I read the instruction sheet that came in the alternator box. First thing it said and was repeated many times in it, "an alternator is NOT a battery charger! If the vehicle has a dead battery it MUST be charged before starting the vehicle. An alternator is a battery MAINTAINER." They went on to claim that using the alternator to charge a dead battery is the most common cause of alternator failure. This was a new not rebuilt GM/Delphi built alternator.


thanks wayneh I edited it. Sometimes I don't press keys hard enough. :)
 
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shortbus

Joined Sep 30, 2009
10,045
As a side note in this, I replaced a 130A alternator last week in my sons work van. After doing this kind of stuff for most of my life, I read the instruction sheet that came in the alternator box. First thing it said and was repeated many times in it, "an alternator is NOT a battery charger! If the vehicle has a dead battery it MUST be charged befor tarting the vehicle. An alternator is a battery MAINTAINER." They went on to claim that using the alternator to charge a dead battery is the most common cause of alternator failure. This was a new not rebuilt GM/Delphi built alternator.
 

Reloadron

Joined Jan 15, 2015
7,480
After doing this kind of stuff for most of my life, I read the instruction sheet that came in the alternator box.
OMG, you mean they actually have destructions err instructions? Right there in the box? I always thought those papers were light packing material, go figure. :)

Seriously, good point there shortbus.

Ron
 

crutschow

Joined Mar 14, 2008
34,201
I think the resistance of 100' of #12 wire, as noted in my previous post, would be sufficient to limit the current and avoid alternator failure.
Perhaps #14AWG wire would be better, particular if the round trip wire length is less than 100'.
#14AWG will carry 32A and has a resistance of 0.00253Ω/ft.
I think you can safely assume that a normally discharged battery will never be less than 9V.
 
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MaxHeadRoom

Joined Jul 18, 2013
28,576
Is that Brit-speak for whoring out your car?
N.A. equivalent to 'Pimp my Ride'.:p

Automotive alternator ratings are definitely misleading, e.g. such rating as 100 amp & 150 amp alternator, MAYBE they could sustain this for a few seconds, but for many minutes? just look at the windings and physical size.
Max.
 

bwilliams60

Joined Nov 18, 2012
1,442
Former alternator rebuilder and battery sales. Here is 2 cents worth for what it is worth. First of all, charging a battery on a boat as you travel is just a bad idea. All I see on here is what size wire you run for the main wire. What about ground? Most trailers provide ground through a rusty trailer hitch and ball and do not make the greatest connections at the best of times let alone the intermittent nature. Secondly, an alternator is made to maintain or replenish the battery and IS NOT a battery charger. So if you have a dead battery, be it car, truck or boat, the alternator's job is to bring it back to 100% as fast as it can so it can continue its job as a maintainer of charge. Therefore, if you have an alternator that puts out 130 amps at full charge, then you must take the length of the wire, go to your AWG chart and calculate the size wire you are going to need. If you go smaller, you now have a resistor wire that will get warm very quickly and cause you greater problems. Best bet is to get a small charger for the boat battery and maintain it so that it has a longer life. Max is right. An alternator will go to max output for a time, but only for a short time before varnish starts to melt, and wires start to short. Dead or low batteries are the number one cause of alternator failure. That much I do know for sure.
 

Thread Starter

DBBCircuits

Joined Apr 22, 2015
10
Much has been said about charging a dead battery with the alternator and how that is not the thing to do....For the record, I only want to maintain the charge in my boat battery while towing. I would like to leave my refrigerator in the boat on so that I can continue to use the boat as my 'Boaterhome' while traveling to the next cruise location. Sometimes that takes days, so it's just easier to keep the battery charged as I go rather than using a generator once I get to an overnight spot (Walmart frowns on the usage of generators in their parking lots :D).

Thanks for all your input.
 

shortbus

Joined Sep 30, 2009
10,045
N.A. equivalent to 'Pimp my Ride'.:p

Automotive alternator ratings are definitely misleading, e.g. such rating as 100 amp & 150 amp alternator, MAYBE they could sustain this for a few seconds, but for many minutes? just look at the windings and physical size.
Max.
Have you looked at a recent alternator? The new ones are nothing like the ones from ~15 years ago. The wire in the coils are much bigger now, and the diode/bridge technology has changed a lot.
 
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