Upgrade Options for RV

xmineman · March 2015

I have slowly built, piece by piece over the past year, a pretty decent system for our RV. We live in it 7 months of the year while snow birding. It consists of 3 250 watt panels on the roof, classic 150 w/WBjr. 4 each 6v agm Trojan for 12 volt system,2kw Exeltech inverter, 40 amp industrial charger and 2 each Honda 2000i for bad weather charging and backup. I have a second LA battery bank of 180Ah which I charge by back feeding from large bank. During cold months I have been using up to 150 Ah per night including coffee in AM. Using the general 50% DoD rule I have 1 to 2 days of autonomy. Non furnace months are greater. I understand furnace fan consumes a lot of juice. In New Mexico I may have excess array capicity. In Michigan I do not. I have been looking for more loads of opportunity such as modifying water heater with 12v element to use dump power to heat water. I would like to increase my rainy day capacity to 4 or 5 days. Looking to add a 4th panel to increase charging to 80 amps.I am not sure if Classic 150 can handle 5. I would like 6 2v cells rated at 1100 Ah but may have to settle for around 800 AH. What would be the best all around option?

westbranch · March 2015

xmineman wrote: »

I am not sure if Classic 150 can handle 5

Use the Classic array sizing tool found here for the answer http://www.midnitesolar.com/sizingTool/displaySizing.php

Vic · March 2015

Hi xmineman, Welcome to the Forum,

The Classic should be able to handle four of the 250 watt PVs, but five would be a problem for your Classic 150. Most probably, with five PVs, the Voc would be too great, and on an RV, the lowest temperatures that the PVs might be exposed to may be uncertain.

One issue with an odd number of PVs, is that they would need to all be connected in series.

Here is the Link to the Classic String Sizer, if you have not seen it:
http://www.midnitesolar.com/sizingTool/displaySizing.php

A single string of 1100Ah batteries, like you are considering, is a smart idea, especially on a 12 V system. But, with only about 700 or so watts of PV output possibly available from four PVs, that might not work optimally.

Just a drive-by post. Have Fun, Vic

xmineman · March 2015

I currently have the 3 panels wired in parallel using 6 gauge wire to combiner approx 12ft away in electrical closet (back trunk). I did this because of possible partial shading issues. Is that harmful?I thought I might wire 4 panels in series/ parallel

BB. · March 2015

Shading is difficult for any solar electric panel. And how best to address shading (series/parallel) to some degree depends on solar panel layout and the type/location of shading (i.e., if you lay panels "landscape" orientation, then snow at the bottom of each panel only blocks 1/2 of the cells--So you will get 1/2 current until the sun melts the rest of the snow).

Otherwise, if you can reduce shading (shorten vent pipe, move panels way from chimney, move antenna so they do not shade panels from ~9am to 3pm--at least)--You will be much better off.

The MPPT solar charge controllers usually work most efficiently around 2x battery charging voltage (~14.8 volts charging means ~Vmp-array in the range of 30 volts or so--Or 2x "12 volt" panels in series for Vmp-array ~ 36 volts).

While the MPPT charge controller can run upwards of Vmp-array ~ 100 VDC (for "high end" MPPT charge controllers), the losses inside the MPPT controller can increase--Instead of being 96% efficient with Vmp~30 volts on a 12 volt battery, a 100 Vmp on a 12 volt battery may be down around ~93% efficient or so... The MPPT charge controller will run hotter (need good ventilation).

Typically we use "high" Vmp-array voltages when the solar array is 10's to 100's of feet from the charge controller/battery box... It allows the use of much smaller diameter copper wire from the array to the charge controller.

-Bill

Vic · March 2015

BB. wrote: »

Shading is difficult for any solar electric panel. And how best to address shading (series/parallel) to some degree depends on solar panel layout and the type/location of shading (i.e., if you lay panels "landscape" orientation, then snow at the bottom of each panel only blocks 1/2 of the cells--So you will get 1/2 current until the sun melts the rest of the snow).

Typically we use "high" Vmp-array voltages when the solar array is 10's to 100's of feet from the charge controller/battery box... It allows the use of much smaller diameter copper wire from the array to the charge controller.

-Bill

Hi BB Bill,

Great point about RVs often encountering numerous shading issues, and that series strings can greatly magnify the issue.

I should stick to stationary systems. Thanks - Vic

BB. · March 2015

Vic,

Your answers were fine... There is the difficulty of using a Prime number of panels (3, 5, etc.) that cannot be mixed/matched for series/parallel applications. If I had 5 panels and that was all I could fit, I probably would bite the bullet and just put them all in parallel. 12 foot run to a combiner box (plus breakers or fuses per panel) should work fine. At the expense of a comber box plus a spool of of heavier copper wire (if needed) to keep the voltage drop from getting too large (less than a 0.5 volt drop on a Vmp~18 volt set of panels).

-Bill

techntrek · March 2015

Suggestion, to move this to the RV sub-forum.

BB. · March 2015

OK... Moved thread to RV Forum.

-Bill

scrubjaysnest · March 2015

xmineman wrote: »

I have slowly built, piece by piece over the past year, a pretty decent system for our RV. We live in it 7 months of the year while snow birding. It consists of 3 250 watt panels on the roof, classic 150 w/WBjr. 4 each 6v agm Trojan for 12 volt system,2kw Exeltech inverter, 40 amp industrial charger and 2 each Honda 2000i for bad weather charging and backup. I have a second LA battery bank of 180Ah which I charge by back feeding from large bank. During cold months I have been using up to 150 Ah per night including coffee in AM. Using the general 50% DoD rule I have 1 to 2 days of autonomy. Non furnace months are greater. I understand furnace fan consumes a lot of juice. In New Mexico I may have excess array capicity. In Michigan I do not. I have been looking for more loads of opportunity such as modifying water heater with 12v element to use dump power to heat water. I would like to increase my rainy day capacity to 4 or 5 days. Looking to add a 4th panel to increase charging to 80 amps.I am not sure if Classic 150 can handle 5. I would like 6 2v cells rated at 1100 Ah but may have to settle for around 800 AH. What would be the best all around option?

Good answers above. Here are some additional thoughts. We boil the coffee on the gas stove so that would be a big power savings if it would work for you. Running the water heater on gas will also be a big savings over a 12 volt element. We actual had better solar this last summer in MI then this winter in AZ with out 2 80 watt panels. I don't know what you run the heater at but ours uses 2.25 amps, call it 30 watts. It is rarely used at night and we are quite comfortable at setting the thermostat at 55. You might want to look at your usage and see how/where you can conserve. LED lighting will give about an 80% savings over incendensent lights. Just some other thoughts that may save you a panel.

Upgrade Options for RV

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