Quote:
Originally Posted by LI Pets
How much charge is really put into the batteries from the tow vehicle while driving?
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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:
Originally Posted by LI Pets
Someone suggested that to get a better charge a pair of #10 wires can be put into the trailer batteries directly?
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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.
Quote:
Originally Posted by LI Pets
When the alternator is satisfied on the tow battery all the rest will go to trailer batteries very fast?
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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.