Matt71 said:
So a ford econoline alternator is going to be somewhere between 95 amps and 135 amps depending on what it has, so I'd need 4 gauge wire with a 125 amp fuse, or 2 gauge wire with a 200 amp fuse.
So what's the final veridic on running the line directly from the alternator to the house battery?
All lead acid batteries will regulate the amps they can accept at a given applied voltage.
The thicker the cable between charging source and the battery the higher the amp flow, into depleted battery, will be.
The shorter the cabling between alternator and battery, the higher the amp flow, into depleted battery, will be.
When a battery is in the 80% + charged range, it cannot accept much amperage even at high voltages of 14.7v, and as such, thicker cabling does very little or nothing in this range to speed charging, with an Asterix involved explained a few paragraphs below. The higher the amp flow, the more the voltage drop along the charging path will be, and since batteries above 80% charged cannot accept much amperage, the voltage drop along too thin of cabling is much less of a concern, and those powering winches are in a much different boat than a van dweller depleting the battery a certain % nightly, each and every night, and requiring the alternator to replace as much as possible in whatever drive times are to be expected.
But a battery at 50% charged can and will accept huge charging currents, if available, and can be recharged to ~80% quite quickly if a high amp charging source is available.
My single group 27 Northstar AGM easily accepts 110 amps initially, when depleted to 50%
My single group 31 USbattery Wet/Flooded battery easily accepts 75 amps when depleted to 50%
As battery voltage nears 14.7, then the amps begin to taper. Higher voltages would be required to keep the amp level higher, and voltages above 14.7v start getting in that uncomfortable zone for batteries, so voltage is limited by the charging source/voltage regulator and the battery determines how much amperage is required to hold the battery at that level.
If your house battery bank is not in the engine compartment, go from alternator to isolation device( Solenoid/ACR) to house bank over the shortest path possible. With batteries in the back of the Van, the alternator to house battery path is likely much much shorter than the path from starter battery to house battery, so not only is taking power from the alternator directly more effective for at least 2 reasons, it requires less outlay of $$ for copper.
If you can fit your house bank in the engine compartment, then do so. Here is makes sense to pull power from starter battery, but then the weak link is the original alternator charging circuit which is too thin to begin with, and much too thin when there is additional battery capacity tacked onto the end of the circuit. In this instance, One can either upgrade the original cabling from alternator to starter battery, or just add a parallel cable, properly fused, from alternator (+) to starter battery(+) or even to the starter battery side of the isolation device/solenoid/ACR.
Remember the (+) is only half the circuit. Ground cables from battery to engine/ alternator to engine, frame to engine( if one exists) also need to be upgraded. Upgrading the battery to firewall ground is also not a bad idea.
The thickness of the cabling used, will determine how much additional load is placed on the alternator and how much its lifespan will be degraded. Those with really expensive alternators that are very labor intensive to replace, will be in a different boat than the person with a 105$ alternator with a 5$ lifetime warranty, and can swap it out themselves in the AP store parking lot in 30 minutes or less.
I fall into the latter category, and will gleefully max out my alternator every time my batteries are depleted. I've got the tools and gauges to see Amp flow, have done the experiments, and an upgraded charging circuit greatly decreases the time it takes to get a battery from 50% to 80%, and my batteries gobble up these large currents and behave better for it the next discharge cycle, even if the solar does get them to full by sundown.
If my alternator was 350$ and took 3 hours of tool throwing frustrating labor, I'd be less inclined to max out the alternator with oversized cabling on depleted batteries, but my alternator is cheaper than my batteries, so I am going to work the SOB and work it hard via thick copper cabling.
* Even when a battery is above 80% charged, but has had a load recently placed upon it, it will briefly accept high amperages until battery voltage rises, so that 3 mile drive to ones overnight parking spot at 10PM can help replace more AH into the battery than a thinner cabling would, even if the battery is over 80% charged.
The surface charge voltage after such a short high amp drive also deludes people into thinking their batteries are more charged than they are. Those relying only on voltmeters will be deluded, those with amp hour counters will know better that surface charge is present and not representative of actual battery state of charge
It pretty much requires ~4 hours going from 80% to 100% state of charge, no matter the charging source, and anybody claiming they can get a true full charge on a depleted lead acid battery, in less time, are deluded, and their words and claims on this topic, and perhaps others, should be ignored as well.
Blinking green full charge indicators of various charging sources/ controllers are simply liars, only to be believed when the amps required to hold absorption voltages(14.4+) taper to very small numbers.
Alternator ratings are to be taken with a grain of salt. Their amp figure is the maximum that the alternator is capable of, and only briefly, when spun at a very high rpm, when cold, over cabling thick enough to carry the load asked of the alternator. Idle speed output will be some fraction of the alternator's rating.
Often, higher rated alternators perform worse at lower rpm's, and as such, recharging performance at idle and lower rpm's suffers.
The expected amp flow at idle speeds will vary greatly depending on vehicle platform. Most V belt Dodges do very Poor at idle speeds when hot but the serpentine belted dodges of '94 on, do better.
A clamp on Ammeter is very revealing. Voltage tells well less than half the story when charging the batteries, knowing how many amps the batteries are accepting at those charging voltages, however, is extremely enlightening.
A battery sucking 40 amps at 14.7v is nowhere near fully charged, a battery which only is accepting 5 amps at 14.7 is in the 90% charged range, a battery requiring only 1.2 amps to hold 14.7 might be 98% or higher charged. Depends on the battery, and its condition/ health.
While A person with a only a voltmeter sees 14.7 and thinks the battery is fully charged, a person with an Ammeter can see the amount of amps flowing at 14.7v and much more closely determine the state of charge of the battery.
http://www.amazon.com/Digital-Clamp-Meter-Uni-Trend-UT203/dp/B005HOPRRK
The above AC/DC clamp on Ammeter is reportedly very accurate. Do be aware many clamp on Ammeters are cheaper, but only measure AC current, not DC.
Such a meter is also a fully functional digital multimeter, which should be a standard tool in any van dwellers arsenal of tools, if the dweller knows how to use tools.
