Small PWM controllers fed from large grid-tie panels??

Re: Small PWM controllers fed from large grid-tie panels??

Wayne - Both my starting and house systems are 12V. Yes, I know that ordinarily a 2kW system should be 24V or 48V, but the complication of having the house system at a different voltage than the chassis system would be more trouble than it's worth, especially if needing to start the engine off the house batteries in an emergency. So, that's why it's 12V! I measured the constant draw as 8mA using my cheapo Goldstar DVM, but maybe I measured it wrong and it's actually greater than that? It probably is greater than 8mA, because either of the 8D batteries slowly depletes from just that load; other bus owners with the same DDEC II engine electronics also have their starting batteries deplete after a few weeks, so it's not just me.

Cariboocoot - I know that two MPPT controllers should not be on one set of panels because it causes them apoplexy: for that reason the panels are split into two banks of four, each bank feeding its own controller that in turn charges its own bank of house batteries. In effect I have two separate house systems running in parallel, and they aren't combined until the main DC breaker panels. I just wasn't sure if adding a PWM controller to a bank of panels that already feeds a MPPT controller was OK or not. I'll try to get a more accurate reading of this constant draw to know what size charger I need for it.

Thanks for your good advice. As you can see, I still have plenty to learn and understand. Converting a bus is not easy!
John

Re: Small PWM controllers fed from large grid-tie panels??

Another reason I have 12V for the house system is because 24V things such as RV water pumps are more expensive and much harder to find than their 12V versions. In a bus the amount of wiring really isn't much, so the difference in cost between heavier cables for 12V and lighter cables for 24V isn't consequential in the big scheme of things. My fridge will be a 120VAC converted freezer - I can't afford the $1,000-plus for a DC fridge/freezer like a SunFrost or Norcold! Apropos this, the reason I have two MPPT controllers is because each bank of PV panels could be tilted at a different angle to the other in winter, so each bank of panels would then be producing different amounts of power, more than a single controller can cope with. (The panels are hinged off a central walkway on the roof: in summer one side is down against the roof at 21 degrees below horizontal and the other side is up at the same angle, but in winter the raised side will be raised up to 45 degrees while the other side will still be down against the roof, obviously then producing different power. I can do this because my bus has a very curved roof compared to a typical S&S motorhome's almost-flat roof.)

When I was talking about needing to start the engine off the house batteries in an emergency if the starting batteries were down, the worst-case scenario I'm thinking about is the bus being parked in the path of a fast-moving wildfire such as are burning huge swathes of drought-ridden California and the western states now - if that happens, you have to move out RIGHT NOW, not after charging batteries for an hour or more: literally, every second counts if a fire is bearing down on you faster than you can run. I could well be boondocking in potential fire-risk areas, such as on National Forest or BLM land. I'm converting a bus instead of using a S&S because it will be better for my needs: what RV that I can afford can hold well over 200 gallons of water or is suitable for extended off-grid use?

John