Originally Posted by Bluepill
Also consider that you can charge fairly fast from your tow vehicle by using regular jumper cables attached between the tow vehicle and the rv battery. If you run the vehicle at idle with no accessories on (headlights, A/C, rear defroster, etc.) you can probably get 35-40 amps of charging current. That would give an 80 amp hour rv battery a full charge from 50% discharged in about 75 - 90 minutes.
Not going to happen. The voltage at the discharged battery determines the amount of current that will flow even if there is zero resistance in the cables.
If the RV battery is down around 50%, it can be charged at about 20% of its AH capacity. A Group 24 battery of nominal 80AH capacity will have a maximum charge rate of 20 amps, and probably will self-limit to 16 amps. To charge at 16 amps will require the alternator output be above 14.4 volts. Chances are very slim the alternator will go that high for more than about 5-10 minutes (to protect the TV battery from over-charging). So the charge rate even through battery cables will be on the order of 10-15 amps after the first 5 minutes.
It will take nearly 3 hours of TV engine on to charge the RV battery from 50% to 90%. It will take at least an additional 2 hours to get to 100%. If you don't believe me, put an ammeter on the RV battery charge line, and see how fast it tapers off.
All of which goes to show that solar is a very effective way to keep batteries charge because of a better fit with the real charging profile of deep cycle batteries. You typically have a 5-12 amp charge current going for 6 hours a day.
Running a generator to recharge a battery is slightly faster because you can get a converter (a WFCO is not your best bet) in your RV that is better tuned for recharging deep cycle batteries (will put out a sustained 14.4V in bulk mode). Even then, the best you can do is a little over 2 hours generator time for 50%-90%. It's not worth running the generator to get above 90%. But the generator also gives you capacity to run some 120V stuff while you are recharging batteries.
Things I learned from building remote communication sites in Alaska.
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan