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

Iceni John

My original plan for my bus's PV system was to use either a Morningstar SunSaver Dual or a pair of small SunGuard 4.5A PWM controllers to keep the two 8D starting batteries fully charged, in addition to the two TS-MPPT-60 controllers for the house batteries. (One of the two starting batteries must supply a constant 8mA draw for the engine's computer, and that's enough to slowly draw that battery down after a week or two.) I was hoping to run these small PWM controllers from the main bank of panels, but it seems I cannot do so - their specs mention limits of 80W panels and 30V maximum input. My eight panels are Sharp 255W with a Vmp of about 30V. I know that MPPT controllers self-limit their output to their rated limits if the panels have too much power for them, but PWM controllers don't seem to have this ability.

Is there any way I can run two small PWM controllers off my big Sharp panels just for the starting batteries, or will they need their own separate small "12V" panel? I have a 24" x 36" space that a small panel could fit in, but it seems silly that I would need another panel when I already have 2kW of panels on tiltable mounts on the roof!

The only other way I can think of keeping the starting batteries charged when off the grid for maybe weeks at a time is to use my present small 10A Schumacher charger off an inverter, but that's not an elegant way of doing things.

Any ideas, anyone?
Thanks, John

waynefromnscanada

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

Welcome to the forum "Iceni John",
Couple of questions, one observation and one suggestion.
What is the voltage of your starting battery bank, 12 volt, or 24 volt?
What is the voltage of your off grid system, 12, 24, or 48 volts?
An 8 ma draw, 24/7 for two weeks would total less than 3 AH, so if your starting batteries get discharged "in a week or two", either there is a far larger draw on those batteries than 8 ma, or they have a very high self-discharge rate, or both, or they're toast.
IF your main solar system has an inverter running 24/7, and IF the only draw on your starting batteries is indeed only 8 ma, that could very easily be supplied by a little "wall wart" type AC adapter or battery maintainer. Far cheaper and easier than installing an extra panel and or controller and would have practically no impact on your >2000 watt system.

Cariboocoot

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

What Wayne said.

You don't want the PWM controller on a GT style panel. For one thing all it will pass is the current. So a 250 Watt 30 Vmp panel with an Imp of 8 becomes in effect a "12 Volt" panel of approximately 145 Watts: 100 Watts of power potential wasted.

You also do not want multiple controllers connected to the same PV. It tends to confuse the MPPT function.

Just 'borrow' some power from the main system via a battery tender. Control it so it only comes on when the array is producing (instead of pulling off the batteries at night). And especially look out for whatever it is that is causing the starting battery to go dead so quickly, as 8 mA should not do this in a couple of weeks. Not from a battery that is probably >100 Amp hours as a starting battery would be in a large vehicle.

Iceni John

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

zoneblue

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

Iceni John wrote: »

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!

Oh contraire. A 24V house and 12V SLI, would precisely help you out there. To trickle charge a 12v pack from a 24v is super trivial. A 2 dollar voltage and current limited buck converter, set to 13.8/0.01C or something would be all you need. OTOH charging a 12v battery from a 12v battery... now thats much harder.

24v RV house banks are great. 24v pumps, 24v fridge. Amps all lower, wire smaller, less losses, less charge controllers. End of discussion!

Emergencys? Run a 12v ac charger off the inverter, in an hour or so your good to go.

Iceni 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

zoneblue

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

Iceni John wrote: »

...24V things such as RV water pumps are more expensive and much harder to find than their 12V versions.

The likes of shurflo, flojet etc all make there pumps in 24v and 12v, price is the about the same, and they are readily availble.

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,

It the unlikely event, its not going to hurt anyone to jump off one of the two 12v house bank sections either, no?

2kW pv is just not 12v terrritory. No matter how you dice it.