I don't know if Ford has its voltage regulator in the alternator or in the engine computer.<br><br>Either way the batteries take what they can at the voltage allowed by the voltage regulator, and the fatter the cabling the more current they will take at the voltage allowed, recharging faster. <br><br>One side of the solenoid gets connected to either the engine battery, or directly to the alternator(fused).<br><br>Directly to the alternator is shorter and can keep the fully charged engine battery from limiting current to the house batteries. It also bypasses the OEM charging circuit which was not designed for 2 more deep cycle batteries tacked onto the circuit.<br><br>The engine battery after starting does not take much current for very long. Mine starts in the 30 amp range and tapers to below 10 amps within 30 seconds, but my engine starts very quickly and does not need much replenishment. Depleted house batteries are at their hungriest after engine starting and will ask for everything the alternator can make at every rpm until 14.5 is reached. (14.5 is my Dodge, not sure about ford). This can be several minutes of 60+ amps before the battery voltage climbs that high, and only then does the alternator get limited by the voltage regulator. And once my alternator heats up, it is not able to hold 14.5 at idle speeds.<br><br>At some point my voltage regulator decided that 13.7 is plenty,, well before the house batteries are nearing 80%, cutting back alternator charging significantly. Nothing to really be done about it. It was not designed to charge additional batteries perfectly, only top up the slightly depleted single engine starting battery without overcharging, and it is not very good at that either.<br><br>If one does hook the solenoid directly to the engine battery, then one should run another parallel (fused) cable from alternator to the same stud on the solenoid which attaches to the engine battery. No need to touch any original wiring. Adding this wire will significantly increase the charging current to house batteries, and even starting battery if it too is depleted.<br><br>These solenoids should be rated for at least 90 amps continuous, and a couple hundred amps intermittent. I installed an old one of unknown rating in a friend's van and the thing got too hot to the touch in 10 minutes of providing 25 amps to the house batteries, and the contacts inside welded themselves together. If one were not aware of this having occurred, then there would be no battery isolation with the engine off. So the Mac Daddy 200 amp $olenoids can pay for themselves.<br><br>Depending on how they are wired to be triggered, they can allow the house batteries to assist in engine starting, so the contacts within need to be large and durable to pass starter current.<br><br>I'd prefer the house batteries not automatically contribute to cranking the starter. In most vehicles the blower motor circuit is not live during engine cranking, and this circuit is a good trigger for the solenoid. <br><br>That PD product you link to is overkill. A simple converter like the PD9260c will fully charge the battery with grid power when available. Get yourself a regular 10 gang ATC fuse block to distribute power from house batteries to the house circuits. Consider moving the stereo power feed and stock interior lighting to the house battery fuse block too. <br><br>Monitoring State Of Charge of the house batteries can be done with voltage, but is only accurate on a rested battery, one not seeing discharge or charging currents for several hours, but you can notice trends and get a better idea of the SOC of the batteries.<br><br>Other products exist which count the amps into and out of the batteries, and keep track. These cost an additional 200$ or so and require all the vehicle grounds be run through a 500 amp shunt, which sounds complex, but is not that big of a deal. Basically no ground wire can go directly to the battery, but to one side of the shunt, the other of which then goes to the battery(s).<br><br>Even these are only 90% accurate.<br><a href="
http://www.bogartengineering.com/products/TriMetric" rel="nofollow" target="_blank">
http://www.bogartengineering.com/products/TriMetric<br><br>O</a>verkill for most, but an awesome tool to understand how much charging and discharging is going on. I installed one after 6 years of van dwelling, and it was surprising how wrong all my earlier assumptions as to battery state of charge were.<br><br>Like many I acted like the alternator was some magical instant battery recharger. That short drives were all that was needed to top them off, and the batteries just needed replacement every 6 months because they were junk, not that I was chronically undercharging them.<br><br>In many ways, Ignorance was bliss, and I'd have a lot more free time not typing novels trying to convince the masses how ineffective the alternator ultimately is in the task of fully charging a distant battery bank. <br><br><br>
