by Hal W » Tue Jun 09, 2015 3:37 am
First post here, and I know this is an ancient thread, but I read through the 13 pages and want to add something that has been missed here. I have 30 years experience in auto tech so I have a little background with this subject, at least as far as auxiliary batteries on vehicles. Two very different batteries will have a slightly different charging requirement. That being said, if they are in parallel the alternator will see them as a single load, or in effect, a single battery. Think about it this way; if you connect a fully charged battery with jumper cables to a dead battery the 2 batteries will equalize, the dead battery will come up and the charged battery goes down. The gauge of wire that the jumper cables are made from prescribes the amount of amps that will flow between the batteries, (or, the speed that it takes to equalize them). This is why a set of cheap jumper cables get hot when you are trying to jump your neighbors stone dead battery with your fully charged car, the cable size is too small and they become a resistor as too many amps are trying to flow at once. I could go much deeper into this but if you're still with me...
The "Simple" RV system talked about here is fine, but there needs to be a couple of things in the system. There does need to be a heavy relay and a MANUAL switch. You do not want to have a dead battery on the TD and a full one on the TV that connect at the moment that you turn on the ignition to start the TV. Remember the cheap jumper cables. The cable size should be a minimum of 6 gauge and 4 would be better. (Think; full-sized battery cables are best). So now you are thinking "that is waaayy overkill". Yes, unless you have the above condition happen... Then it is properly sized to not smoke the 10 gauge typical charge wire running through your TV and TD. The way to avoid this problem is to install a resistor on the TV battery end to "slow down" the rate of charge to meet a smaller cable's capacity.
OK, so a "Simple RV system" set up like this will work great and be safe: TV Batt + to a Mega Fuse to a heavy relay run from a switched ignition source THROUGH a manual switch (to switch it off for starting the TV) to fire the relay, Then through a heavy resistor (50 amp 2 ohm? You need to wire it and check the voltage at the TD batt as you are wanting about 14.5 when charging). This is the only way I can see this type of system working properly and safely.
I think a Battery isolator system would be better in the long run if the cable size to the TD doesn't drop the voltage too low. I like the inverter to smart charger even better as the voltage step up eliminates the cable size problem and the charger is on-board the TD to plug into shore power anytime. There is no worries about efficiency here, a typical drive to and from your camp will take way more time then it will take to charge your TD battery fully. Think of that like "I have a 50 gallon bucket to fill up a 5 gallon bucket, who cares if I spill some?"