Camping/auxiliary battery wiring

[grandfatherjim]

I can hardly believe I'm starting a thread about this as there are so many out there, but there is a detail that I read once that I can't find.

I'm about to install a deep cycle battery for camping. I'll connect the two batteries with a relay which is energized by the famous blue wire at the regulator. I can do all this, no problem; I understand electricity.

Once a few years ago I read that the positive wire between the two batteries should be no bigger than (X, where X is the unknown) gauge, so that its resistance limits how fast the aux. battery charges in the event of it being deeply discharged. This makes sense to me and I want to do it.

Can anyone point me to something authoritative on the max gauge of that wire? It is the only thing holding me back from proceeding with this installation...and I want to head out camping on Thursday evening....

Thanks,
Jim

[Hippie]

See if there is info on this same discussion?
http://www.thesamba.com/vw/forum/viewtopic.php?t=451758
I think GoWesty sends a kit with 10 or 12 gauge wire, but using a smaller wire to limit charging rate--if that's what it's for--doesn't seem like a good idea to me as it would heat the wire up too much, wouldn't it?

[Amskeptic]

See if there is info on this same discussion?
http://www.thesamba.com/vw/forum/viewtopic.php?t=451758
I think GoWesty sends a kit with 10 or 12 gauge wire, but using a smaller wire to limit charging rate--if that's what it's for--doesn't seem like a good idea to me as it would heat the wire up too much, wouldn't it?

A wire used as a resistance, as Hippie noted, would cause heat.
But! the alternator is limited in its output, and if you look at the main output wire to the starter solenoid terminal, it is not particularly big. 70A max. If you have both batteries discharged and waiting at the starter solenoid post, *that" wire will get hot, it is not designed to carry two hungry batteries. I believe there is a charge amperage limitation in a smaller gauge wire to the secondary battery via the isolation relay, to more successfully split the total output of the alternator.

Hippie? Shoot this down if it is terminally wrong.
Colin

[grandfatherjim]

A wire used as a resistance, as Hippie noted, would cause heat.

I'm thinking just a bit, maybe more like warmth than heat. Let's make up an example: if the wire has .005 ohms resistance, and the battery is really down and would like 40 amps, and let's say the alternator is OK with sourcing that (neglecting the problem mentioned below for the moment), we'd have a .2 volt drop across the wire. The wire would then be dissipating 8 watts along its total length.

But! the alternator is limited in its output, and if you look at the main output wire to the starter solenoid terminal, it is not particularly big. 70A max. If you have both batteries discharged and waiting at the starter solenoid post, *that" wire will get hot, it is not designed to carry two hungry batteries. I believe there is a charge amperage limitation in a smaller gauge wire to the secondary battery via the isolation relay, to more successfully split the total output of the alternator.

Hippie? Shoot this down if it is terminally wrong.
Colin

The charge amperage limitation in a smaller gauge wire to the secondary battery is what I mean, exactly. I hadn't thought of it being to more successfully split the alternator's output but it seems sensible to me.
So my original concept is now enhanced: the wire from the alternator to the starter helps protect the alternator by being only X gauge, and the new wire helps to split the load between the two batteries better by being only Y gauge. Now to determine Y.

[Hippie]

[...]if the wire has .005 ohms resistance, and the battery is really down and would like 40 amps, and let's say the alternator is OK with sourcing that (neglecting the problem mentioned below for the moment), we'd have a .2 volt drop across the wire. The wire would then be dissipating 8 watts along its total length.[...]

But how is a 0.2 volt drop across the wire going to limit charging rate to any significant degree? 0.2 volts is only about 14% of the likely difference voltage of charging with a low battery voltage of, say, 12.6 volts and an alternator output of 14 volts. So instead of the battery seeing a 1.4V charge, it sees 1.2V.
Will that be enough difference?
I'm just thinking a high wattage resistor of a few ohms that cuts out during auxiliary battery use is one way to go, but I do not have personal experience with these...electricity and charging systems, yes...camping auxiliary battery systems, no.

Hippie? Shoot this down if it is terminally wrong.
Colin

I don't know enough about that relay circuit to shoot it down even if it's wrong.

[grandfatherjim]

I do not have personal experience with these...electricity and charging systems, yes...camping auxiliary battery systems, no.

Same here.

Yes; my made up example was not good. Better to start with the likely voltages as you say.
So if we use 1.4 volts and 40 amps - 56 watts, meaning a wire resistance of 0.035 ohms.

Hmmm.

[grandfatherjim]

OK here we go:
http://www.cirris.com/testing/resistance/wire.html

This site gives a handy calculator right at the top.

I have a run of about 6' of wire between the two batteries.
If 40 amps flows, this gives us:

Gauge Ohms Voltage drop Heat (watts)

8 .0038 0.152 6.08
10 .0060 0.240 9.60
12 .0095 0.380 15.20
14 .0152 0.608 24.32
16 .0241 0.964 38.56

[Amskeptic]

OK here we go:
http://www.cirris.com/testing/resistance/wire.html

This site gives a handy calculator right at the top.

I have a run of about 6' of wire between the two batteries.
If 40 amps flows, this gives us:

Gauge Ohms Voltage drop Heat (watts)

8 .0038 0.152 6.08
10 .0060 0.240 9.60
12 .0095 0.380 15.20
14 .0152 0.608 24.32
16 .0241 0.964 38.56

Your intellectual approach is giving me Math Tremors, but I was thinking only of the breaker points and test light parable, electricity prefers to flow through the path of least resistance. Make them the same, and the flow *potential* is equal. It looks like VW used the alternator wire to the starter as a resistance which we have all seen in wavy or bubbly insulation and toasted looking terminals. We might say, well, stupid engineers cheaping out, why didn't they use a bigger wire in the first place? But maybe they were giving the alternator a break. They did not consider that you would have big halogen headlamps/stereo rig/laptop docking/cool lighting schemes/refrigerators AND portable heat with forced air, etc.
Yes, the more discharged battery will draw a greater flow due to the higher potential between it and the alternator output, and if both batteries are down simultaneously, I think the alternator would not be happy, but I also think if there was a minor bottleneck in the wiring (to a degree), the alternator might not get too hot.
Colin

[grandfatherjim]

Yeah and in practical terms - the whole point of the aux battery is to not run down the starting battery too - so the alternator should only have to struggle for a short time while it replenishes the starting battery immediately after starting.
I'm going with 8 gauge. I have some around, and I'll keep an eye on that alternator-to-starter wire for a while.
Jim