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Old 09-04-2009, 01:38 PM   #1
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charge from tow vehicle?

How much charge is really put into the batteries from the tow vehicle while driving?

Someone suggested that to get a better charge a pair of #10 wires can be put into the trailer batteries directly????

When the alternator is satisfied on the tow battery all the rest will go to trailer batteries very fast?


???????????
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Old 09-04-2009, 04:31 PM   #2
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Quote:
Originally Posted by LI Pets View Post
How much charge is really put into the batteries from the tow vehicle while driving?
The answer to that question is a function of how much amperage and voltage your alternator will put out, the size, length, and condition of the wiring, and the state of charge of the auto and trailer batteries. So...there is no simple answer.

Quote:
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Someone suggested that to get a better charge a pair of #10 wires can be put into the trailer batteries directly?
A #10 will charge better than a #12, but a 4/0 cable will charge just about as good as anything. The table below gives the recommended wire gauge at limiting allowable lengths of wire (at 12 volts) for various desired amperages AND limiting the voltage drop to 3% for the given length/amperage. Remember, an average length between, say, an alternator on a 3/4 ton truck and the trailer battery is probably just a bit shy of 30'. The up and downs and in and outs of a properly secured wiring job add a bunch of footage between points "A" (alternator) and "B" (battery).

As an example, if you had a typical 30 foot hookup, and you wished the trailer batteries to charge at a 30 amp rate, the table below suggests a #4 wire. #4 wire is what I use to wire my personal tow vehicles - the wire sizes larger than that are a bit harder to work with and more expensive. I use 2/0 for the wiring between the batteries, the battery system to ground, and the batteries to the panel. On the 345 Motor Home I think I put a 1/0 wire between the battery and the compressor - the (aftermarket) air compressor in the MoHo is huge, and is installed all of the way to the rear of the unit (a really, really long distance).

Voltage loss is even more important on Solar Charging Systems, and most installers will recommend a line sized for only a 1% loss.

Do a search on the internet, there are many sites which allow you to plug in all but one of the voltage, amperage, percent voltage loss desired, wire gauge, and line length as inputs and will spit out the remaining one variable after crunching the numbers.

Complicating the whole scenario is the fact that a full charged battery will hold something like 12.1 to 12.6 volts (maybe a bit more or less, depnding on the age, condition, and type of battery) and a "normal" alternator ouput may be limited to 13.1 to 13.6 volts. A 3% voltage loss gets critical in a hurry.





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Originally Posted by LI Pets View Post
When the alternator is satisfied on the tow battery all the rest will go to trailer batteries very fast?
It does not just switch from auto to trailer battery charge, as the charge on the auto battery is gradually brought to 100% more and more volt/amps are available for the trailer. The trailer batteries will always lag the auto battery due to the line and connection losses between the alternator and the trailer batteries.

If you want to rely on the Tow Vehicle to charge the trailer batteries, it would be way better to go with the next size larger wire than the table indicates (this would lower the line loss ot something less than 3%).

After all of the above, there are still connection losses and corrosion allowances to contend with.

The typical 12 gauge (or less) wiring on the +12 volt system (factory wiring) to the trailer connector at the rear of the Tow Vehicle really does little more than to recharge the batteries for the amount of volt/amps used by the braking system.
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Old 09-04-2009, 07:56 PM   #3
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Oops -

Forgot to point out that if you upgrade the positive voltage line from the tow vehicle you will also have to upgrade the negative (ground) line.

I think the voltage/line table loss in the previous post is for a one way circuit only.

If you have to run separate lines (pos AND neg) the voltage drop/line length table is additive. In other words, if you have 30' of positive #4 cable, and have to add 10' of negative #4 cable to complete the circuit, then you will have 40' of effective line loss (% voltage drop) in that circuit.

Best to actually use one of the more sophisticated voltage loss calculators found on the internet, then you can "customize" your actual installation and decide for yourself as to what (if anything) needs upgrading. Don't forget to add equivalent length of lines for each connection and termination when doing the overall calculation - some types of connections are much more inefficient than others.
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Old 09-04-2009, 09:24 PM   #4
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Most vehicles operate with a batt that is pretty much at full charge most of the time.
Your starter motor might draw 250>400 amps from the batt during the starting sequence, but that only lasts for a couple of seconds. As a result the amount of currant that is actually used is very small.
The alternator in the vehicle sences the vehicles batt, and adjusts its charge accordingly.
This means, that even with large wires, the amount of charge, that will go to the trailer batt is quite small.
A low batt, in the trailer, might take all day, of driving, to charge up, if at all.
Without a more sophisticated charging set-up, ( like many boats use), you really can't depend on the driving of the tow vehicle, to charge-up your trailer batts.
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Old 09-05-2009, 06:46 AM   #5
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I think it's not looking like a great idea.

Think I'll get back on the solar idea
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