Voltage drop is certainly a concern with too thin a wire, but thicker wire also allows more amps to flow not just because of less voltage drop, but because the voltage regulator will better be able to 'see' the depleted house battery, instead of the fully charged engine battery. The VR then should allow hogher voltages to be held for longer, and not drop to 13.7 so quickly, where few amps flow.
If the 25 feet is one way distance, consider 2awg cable, and definitely go for more than 100 watts of solar with 232Ah of capacity. If you are only running one (+) cable and using the frame for ground, then run another cable from nearby frame to alternator mounting bolt and make sure the frame connection is pristine, tight, and slather it with thick grease after tightening it.
10 gauge and 100 watts and your batteries will be chronically undercharged, if you ever discharge them below 85%.
The quality of the solenoid can also make or break a system. The better solenoids have silver alloy main contacts. When solenoids fail they tend to fuse the contacts, and then there is no separation of batteries with engine off. It will still make nearly the same sound when the electromagnet fires. Try and find one rated for at least 100 amps continuous. Some solenoids also might be the latching type, and this type only needs a momentary switch of 12v to engage and disengage, rather than a 12v continuous on or off. make sure to test the type you get as the product descriptions are not always obvious. Apply 12v to the smaller contacts, does not matter which is which and there should be continuity between the larger contacts. Remove the 12v from the smaller contacts and there should be no continuity between the larger contacts.
Latching solenoids do have advantages in that they do not consume any power when engaged, but they must be wired with a momentary switch and a light to indicate what position they are in. If you apply a constant 12v to a latching solenoid, it will get seriously hot and fail, So test what you get, and wire it accordingly.
To prolong the life of the contacts, you do not want the solenoid allowing the house batteries to assist engine starting. In my Dodge the Blower motor circuit is deactivated during engine cranking, so I would choose this circuit to activate the solenoid. You can put an illuminated switch inline too. this can be great when the batteries are really low, and the belts damp. YOu wait for a few minutes for engine to warm and dry the belts before flipping the switch and letting alternator juice flow to depleted house bank. With hungry batteries and a damp belt, it can squeal as the belt slips. Bad for the belt, bad for the pulleys, bad for the eardrums.
Also, if delicate electronics are hooked to the house batteries, then voltage spikes will not travel back to house batteries when starter is turned off and possibly damage sensitive electronics.
I recommend a separate voltmeter for engine battery and house batteries one can easily see from driver's seat. If both voltmeters always read the same, then the solenoid's internal main contacts have fused. You can also see when the engine battery is getting weak as voltage will drop lower and lower during engine cranking.
These 3 wire voltmeters have a voltage sense wire. Run i this sense wire( white) right to the battery(+), and calibrate it with a trusted digital multimeter. Mine come on only with my ignition.
http://www.amazon.com/DROK-Digital-...qid=1443500797&sr=1-4&keywords=drok+voltmeter
Do not get the less expensive 2 wire voltmeters as they take voltage readings from wherever you take power from, and this can be significantly lower, or higher than the battery terminals and that is where you want the voltage readings to originate.
My only complaint with these voltmeters is they are too bright driving at night. I got the green ones before they offered the red ones. I put two layers of 35% window tint in front of them to tame them down.
having 2 voltmeters will also allow you to see how effective the alternator is in charging the distant house bank. Far too many people act like a 15 minute drive is all that is required. It takes about 4 hours of driving after the batteries reach 80% charged for 100% to occur, and that is if the voltage is held at ~14.5 for those 4 hours.
Few vehicles will allow voltage to be held this high for this long.
Get a Hydrometer:
http://www.amazon.com/OTC-4619-Prof...502419&sr=1-1-catcorr&keywords=otc+hydrometer
And when you believe the batteries are fully charged, this tool will usually show you they are not, and more time at absorption voltage, ~14.5v, is needed. This specific hydrometer is great as it tells you how much to add or subtract from the readings to compensate for electrolyte temperature. The Cells in the middle will be hotter when charging for a while.
It is best to establish a baseline Specific gravity reading when the batteries are new and fully charged. Measure and record each cell's reading and take note of level in the cells. They do not come perfectly even out of the factory. Some will always read a little lower than others SG wise, so it is important to note which ones, and then later on you can just test that one weakest cell if time is an issue.
Some cells also tend to use more water than other cells, so you can just check that cell when you are curious as to how much water they are consuming.
You do not have to take these extra steps if you do not really concern yourself with getting the most out of the batteries. If you do initially, then you can get a good feeling for how they respond to your charging sources and not concern yourself as much with thier care. but since they need watering regualrly, dipping the hydrometer when you think they are full can be quite enlightening.
I use my hydrometer to fill the cells with distilled h20 as well. The number of bulb squeezes to fill them up can be recorded too. Batteries use more water as they age, so the SG readings and battery levels can tell you when the end is approaching.
