Originally Posted by Moshart
One question that we have, and maybe it was said and I just didn't understand. Does the 30 amp shore power system have anything to do with the the unit that we'll get? Would getting a 90 amp converter/charger be too much or would it be fine and allow us to add the maximum battery bank when the time comes? Do we even HAVE to have batteries connected right away, or would it be ok to leave it without for the time being?
We'll be parked and living/working in the same spot for some time to come, so we will be hooked up to shore power.
Well, first of all, it's important to always keep in mind exactly what you are referring to when you say "amps". You might be referring to the amps which the thing draws, or you might be referring to the amps which the things puts out. For example, an Iota 90a charger draws up to 22a of 120v AC on its input side, and can put out up to 90a of 12v DC on its output side.
So, having a 30a shore power plug, means that you can draw at most 30 amps of 120v AC from the shore power supply, and supply at most 30a of 120v AC to devices inside the RV.
What RV electrical rigging generally consists of is a 120v AC system and a 12v DC system.
Generally, the 120v system is fed either by shore power or generator.
The 12v system is fed by the batteries.
Then you have a converter/charger, which is fed by the 120v system, and which feeds the 12v system (converter). Since the batteries are part of the 12v system, they'll absorb power(charger) when the converter/charger is on, which is why it's called a "converter/charger". The more 12v loads that are running, the less of the converter/charger's 12v DC amp output is available for the batteries to absorb. I.e., if you have a 30a converter/charger, and have 20a of 12v loads running, then there will only be 10a available for the batteries to absorb.
You can then make it more complex, by adding an inverter, to feed the 120v system from the batteries. One way, is to use a small inverter which only feeds a couple of 120v plugs in the camper.
(INverter changes 12v DC to 120v AC - CONverter generally goes the other way.)
Another way, is to use a big inverter/charger (i.e., Xantrex ProSine), which feeds the entire 120v system in the camper, and can either take power from shore/gen - in which case it passes the incoming 120v through to the camper's 120v system, and fires up its internal charger to charge the batteries - or if there is no shore/gen, it turns off its charger and turns on its inverter and takes power from the batteries to feed the 120v system.
Getting a really big charger can be a problem in that it can draw too much power from the shore connection and possibly trip the shore power's breaker. Buildings with 30a 120v RV type power receptacles are rare. Campsites often have them, but it's not a sort of plug generally used in a building. You might (probably will) get stuck plugging into a normal 120v wall plug at some point - those receptacles are rated at 15a, though there may be half a dozen of them all being fed by a single 20a breaker.
Sophisticated units like the ProSine, can be configured to only draw X power. So if you were plugged into a 15a shore power receptacle, you could tell the ProSine (which has a 100a battery charger built-in) to limit the battery charger to say 50a. That way you could still charge the batteries, but not trip the 15a or 20a breaker on the shore power.
The Iotas aren't programmable like that. So if you install a 90a Iota, then when the batteries are low, it will go into full power mode. Fine if you are at a site with 30a shore power, but not so fine if you are plugged into a 15a receptacle (even if it is fed by a 20a breaker) because the 90a Iota can draw as much as 22a on the AC side, and there is no way to tell it not to:
No matter what, you have a finite supply of shore power. Even if you have a full 30a shore power connection - you can easily draw over 30a just by running a 90a battery charger and...well just about anything else. You won't run, for instance, a 90a charger and a/c at the same time on a 30a circuit, or a 90a charger and a microwave either. Or hair dryer. Or electric heater.
For factory RVs, which will have say a 30a shore power plug and a 13,500 btu or 15,000 btu roof a/c - about the biggest converter/chargers they usually install is 55a.
So it's a balancing act. If you plug into shore power and let your batteries get charged up overnight, then the next day when you fire up your a/c, the converter/charger obviously won't be in high power charging mode, so it won't be using much, if any, of the shore available power (unless you have a bunch of 12v loads turned on, then it will draw however much it needs in order to supply those loads).
Another option, would be to install say a 15a Iota, AND a 45a Iota. Then you could run the small charger when plugged into a 15a shore power plug, or turn them both on when you are plugged into a 30a shore power and have a full 60a charging capacity.
The general rule of thumb is that the bare minimum battery charger is 5% of the amp*hour capacity of the battery bank. So if you had say, two 6v 225ah golf cart batteries, rigged in series to make 12v, that would be a 225ah battery bank, and the bare minimum charger (by that rule of thumb) would be 11.25a - so say a 15a charger. 10%-15% of ah capacity would be better, so say a 30a charger would be about right as the minimum.
As for running only on shore power and not even having batteries. You can do that, but again, the power supply is finite. Even if you are plugged into 30a shore power, you might be able to run the microwave while the a/c is on, but you might trip the shore power breaker if the a/c compressor kicks in while the micro is running. You might be able to run the a/c, computer and coffee pot, but if you then turn on the hair dryer...pop goes the breaker.