2nd Battery - Wire size?

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2nd Battery - Wire size?

Postby elmo_4_vt » Mon Sep 19, 2005 3:15 pm

I've tried searching, but I haven't had any luck answering this question.
So it off to the experts on the site.

I have a 135A Alt. in my truck, it uses #4 or #2 wire to the vehicle battery.

In all the wireing diagrams that I've seen on this site, they really don't make much mention of what wire size is run between the alt. and the solenoid/trailer battery. The 7-pin plug on the truck, and trailer wiring both have a #10 set up for the power circuit, but I don't see how the current won't go above this wire rating and blow the fuse I'll put in-line near the alt. What will keep the current down? Do people use some sort of current-limiting device? I've never heard of people frying these wires, so there must be something. And I think that's how they're done on standard camping trailers.

Is it just that the battery will only take a small amount of juice (under the 35A rating of the wire) to recharge itself? Or is there something I'm missing here?

Thanks for any help.

Don

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Postby GeorgeTelford » Mon Sep 19, 2005 3:29 pm

Hi Don

Right first off, if the leisure battery is low it will easily exceed the wire rating and definately go over the rating of a 35 Amp fuse, even if it wacks out 70 Amps it will not blow the fuse or melt the wire either.

Reason, the alternator will only supply that for a very short while, as the terminal voltage rises quickly the ampage drops rapidly, that why it always makes me smile when people add huge 200 Amp alternators and are then suprised that it dont charge the batteries any better. A 35 Amp fuse will probably take 3-4 times more to blow instantly.

Ampage is drawn btw not pushed, so dont worry about trying to limit it.

The wires that are used as standard are in my experience always to small, if you measure the voltage at the starter battery end and then the leisure battery on a professional job you will no doubt find to much voltage drop, everyone I have measured as been useless.


Using the alternator as a "charger" is generally a waste of time, unless you fit a smart regulator to turn an alternator into a charger. An alternator is a power supply, it was never desuigned to charge batteries.
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Postby elmo_4_vt » Tue Sep 20, 2005 7:16 am

George,
I guess I see your point about the fuse, and it's instanious rating, but a 35A fuse will eventually (in less that 30-60 seconds) blow if the current stays above it's rating. So your saying that the charge should come down below the 35A in that time frame, even on a very discharged battery? Or is it best to only use an AC Powered Charger if you discharge the battery too much (say more than 50%)?

I don't understand your comment about the alternator not being meant to charge batteries? I'll give you that it might not be the most efficient at doing so, but I think that it's always been one of it's design functions. Also, they DO, of course, have a regulator... What is the difference between what you call a "smart" regulator, and the voltage regulator that all alternators have?

Thanks again for any help,
Don

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Truck: '96 Ford Bronco, Modified for off-road stuff

Trailer in progress: Not a tear drop exactly, sort of a mut of a couple different designs.
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Postby GeorgeTelford » Wed Sep 21, 2005 8:15 am

Hi Don

The ampage via a standard alternator (more on this later) only stays high for a very short while, it only stays high for under a minute, rapidly falling as the terminal voltage rises and the internal battery resistance drops.

The main reason the fuses do blow is on heavily discharged house batteries when the relay cuts in massive ampage can flow (dont forget that even if the alternator is only 65 Amps say the Starter battery is also in circuit and this can easily supply 200 Amps (think starting !!) again unless the house battery is really wrecked it will be very brief, if the fuse as blown it means the leisure is either unservicable or as been really over drained

Alternators are power supplies and not chargers, the starter battery only has to start the vehicle, after that it is a capacitor only and does not supply the power for running the vehicle, A starter battery rarely if ever gets charged to more than 70% (unless you bench charge it), but as long as it fullfills its primary task which is starting the vehicle no problem.

Why only 70%? well the standard voltage regulator reacts to terminal voltage, its set to keep the voltage around 13.8 to 14.2 Volts, this is not enough to keep the ampage flowing into the battery, hence it will never fully charge, what happens is that while the regulator is reacting the battery smooths, the regulator catches up the drops back.

They could set the voltage higher, but it starts the vehicle so why change? If you look into the charging regime needed for leisure batteries even the weakling Gell and AGM Types want a minimum 14.5 Volts and a decent lead acid wants 15v, if an alternator was designed to charge it would be set higher. The standard regulator is a compromise which works perfectly well for the starter battery but is pretty useless for charging a house battery to a decent useable level.

The only problem for us is that we want to get as much use as possible from our leisure (house) battery, to fully charge we need something that stage charges to a decent level, hence a smart regulator this takes control of the alternator keeps the voltage high and keeps the charging at right level until the battery is fully charged.

This means a 100 ah house battery will be good for 50 ah use, rather than 20 ah from standard alternator charge, a side benefit is that the cold starting ability of a fully charged starter is way better than the 70% achieved previously.
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Postby bdosborn » Sun Sep 25, 2005 12:50 pm

Don,

Batteries have an internal resistance the limits the amount of current they will accept. Thats why you usually only see a #10 wire to the trailer battery. The main wiring to the start battery has to carry the entire output of the alternator so that's why its so much bigger.
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Postby GeorgeTelford » Sun Sep 25, 2005 1:09 pm

Hi Bruce

10 awg will take 35 amps, it will also cause voltage drop.

Discharged batteries have a low resistance, which can draw huge ampage, without a fuse this could easily melt the insulation and short causing a fire.


The thicker wire is to transfer the Alternator output safely, that same output and MORE can be transfered to the house battery, rememeber ampage is drawn low house will draw high current from alternator and starter battery.
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