# Confused about calculating proper size of wire

Discussion in 'The Projects Forum' started by DBBCircuits, Apr 22, 2015.

1. ### DBBCircuits Thread Starter New Member

Apr 22, 2015
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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!

2. ### crutschow Expert

Mar 14, 2008
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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.

3. ### Reloadron Well-Known Member

Jan 15, 2015
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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

4. ### strantor AAC Fanatic!

Oct 3, 2010
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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.

5. ### shortbus AAC Fanatic!

Sep 30, 2009
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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.

6. ### wayneh Expert

Sep 9, 2010
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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.

7. ### strantor AAC Fanatic!

Oct 3, 2010
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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,:
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:
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.

8. ### crutschow Expert

Mar 14, 2008
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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.

9. ### DBBCircuits Thread Starter New Member

Apr 22, 2015
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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.

10. ### crutschow Expert

Mar 14, 2008
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Yes, you should get an impressive result if you accidentally plug the AC mains to the battery charge connection.

11. ### wayneh Expert

Sep 9, 2010
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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.

12. ### shortbus AAC Fanatic!

Sep 30, 2009
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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.

Last edited: Apr 24, 2015
13. ### shortbus AAC Fanatic!

Sep 30, 2009
4,511
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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.

14. ### Reloadron Well-Known Member

Jan 15, 2015
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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

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15. ### crutschow Expert

Mar 14, 2008
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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.

Last edited: Apr 24, 2015
16. ### wayneh Expert

Sep 9, 2010
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Is that Brit-speak for whoring out your car?

17. ### MaxHeadRoom Expert

Jul 18, 2013
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N.A. equivalent to 'Pimp my Ride'.

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.

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18. ### bwilliams60 Active Member

Nov 18, 2012
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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.

19. ### DBBCircuits Thread Starter New Member

Apr 22, 2015
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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 ).

Thanks for all your input.

20. ### shortbus AAC Fanatic!

Sep 30, 2009
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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.