Kid? and Wiring Suggestions for RV pv

[E350]

I have a 4x4 Sportsmobile-type mini-RV  (i.e., E350 Ford van converted to rv)

I have four BP 380J pv panels fixed on the roof of my Sportsmobile. (As soon as I can find another BP 380J, will install the 5th panel, bringing my combined wattage to 400 watts and my combined amperage to 22.5 amps.

The electrical characteristics of each BP 380J panel are:

Maximum power (Pmax) 80W
Voltage at Pmax (Vmp) 17.6V
Current at Pmax (Imp) 4.5A
Warranted minimum Pmax 76W
Short-circuit current (Isc) 4.8A
Open-circuit voltage (Voc) 22.1V
Solar Cells 36 cells (125mm x 125mm) in a 4x9 matrix connected in series

I have a single 250Ah 12v Universal AGM house battery.

My main current draws are:

1. an Engel 12v all electric refrig/freezer, figure 3.5 to 4 amps 100% duty cycle (because it is warm inside in the Summer and warm inside in the Winter with the Webasto furnace and I don't care what they say, if you want to keep things frozen you gotta rotate the dial to "5" which is the 100% duty cycle.)

2. a Webasto 12v diesel furnace for heat in the Winter which uses 4 amps when the furnace fan is cycling on.  Probably 25% duty cycle?

My uses are:

A. In the Kirkwood Mountain Resort ski area (7800' elevation, near Tahoe, CA) parking lot or nearby where my wife and I sleep on the weekends and ski 40+ days a year where temps can get down to minus 17F at night, and there can be some daytime shading, but strategic parking can avoid shading.

B. At Sherman Island (Sacramento County, CA ) in the Summer where we are beginner windsurfers/beginner kiters where temps can get to 100F.  There is no shade.

Questions:  1. What solar charge controller should I get?  (I was thinking about the Kid, but I am totally open to suggestions.)

2.  And should I wire parallel or in series or both?  E.g., if I don't get the fifth panel would I wire each pair in series and then combine the two pair in parallel and then connect to the controller?

3.  Anything else I am missing or should consider?

Your help would be greatly appreciated.

[mike90045]

With an "odd" number of 17V panels, you will either have to wire all in series & pray for no shade, or wire all in parallel.

17V panels see nearly no benefit with MPPT since the panel voltage is so close to the charge voltage (14V) and a volt or 3 of loss in the controller.  But if you were to wire them in series,  you are only at 130V and that's within range of many MPPT.

But I worry that one 250 ah battery can manage 18 hours of 5A (average) loads (90ah) and still charge while loaded, in short winter days.

[E350]

Mike:  Thanks for helping.  Since I have only four pv panels now, let's stick with an even number of panels for this discussion.  I think I am learning...

So:

1.  MPPT controllers work by converting the voltage from the panel to increase amperage.  And an MPPT controller will provide very little benefit at the the BP 380J's Voltage at Pmax (Vmp) 17.6V output per panel, because it there is very little difference between the battery's charge input of 14.2V and the panel's output of 17.6V for the MPPT controller to work with (i.e., there are only 3.4 "extra" volts over the 14.2V battery input  for the MPPT controller to convert to amperage).  (Because charging can only occur is the input voltage from the charging source is at least one or two volts higher than the battery's voltage at full charge, say 12.5-13.5v?)

2.  But wiring in series is additive to volts, so 17.6V x 4 panels = 70.4V which gives enough room between the battery's charge input of 14.2V and the combined 4-panels series-wired output of 70.4V

3.  But panels wired in series are detrimentally affected by shade.  Such that it kills the output?

4.  So is there any detriment to using an MPPT controller such as the Kid but wiring my four panels in parallel?  (Say the extra cost for the MPPT controller is not an issue.)

5.  Or is there a particular benefit to using a PWM charge controller and wiring in parallel over using an MPPT controller and wiring the panel's in parallel?

10-4 on the undersized battery.  I will top of the house battery by running the engine in the morning in the Winter or maybe convert to a bank of Trojans for more amp hours.  PV is intended to counteract a complete drain during the day.  (And furnace cycle's rarely during the day, but Engel fridge/freezer is still near 100% duty cycle if we want to keep stuff frozen.)

Edit:  FYI, I bought my 21"W x 47"L, 80w 17.6 Vmp 36-cell negative ground polycrystaline panels in 2005 when I designed and originally installed the system.  There are apparently newer identically sized 100w 18.5 Vmp 36-cell negative ground monocrystaline panels out there appropriate for RV use!  See Grape Solar:

http://www.grapesolar.com/specs-100w-mono-gs-s-100-fab36.html

I would assume that these newer panels would make even better use of the Kid MPPT controller because of the additional Vmp "headroom."

[vtmaps]

2.  But wiring in series is additive to volts, so 17.6V x 4 panels = 70.4V which gives enough room between the battery's charge input of 14.2V and the combined 4-panels series-wired output of 70.4V

With four panels there is a third choice (other than all in series and all in parallel): series-parallel with a Vmp=35.  That would be my recommended configuration when shading is not an issue.

3.  But panels wired in series are detrimentally affected by shade.  Such that it kills the output?

4.  So is there any detriment to using an MPPT controller such as the Kid but wiring my four panels in parallel?  (Say the extra cost for the MPPT controller is not an issue.)

5.  Or is there a particular benefit to using a PWM charge controller and wiring in parallel over using an MPPT controller and wiring the panel's in parallel?

Tough question.  And complicated to analyze.  First of all, a bit of shading on a series string of panels is not good, but it may not be so bad either.  It depends on exactly where the shading is, and the internal configuration of the cells and bypass diodes in the panel, the Vmp of the series string that contains the panel, and the number of parallel strings.

The purpose of bypass diodes is to bypass shaded cells.  If the shading only activated 1 bypass diode, the panel would still put out its full current, but at a reduced Vmp.  (reduced by the number of cells bypassed by a single diode).  Thus, if you had all your panels in series and had 2 (? just guessing) bypass diodes per panel, and had a spot of shade on one panel, you would lose half the voltage of one panel... your Vmp would drop by 8.5 volts and your power output would be 61.5 volts ÷ 70 volts = 88% of normal output.

Since you have true 12 volt panels (Vmp=17) and shading issues, I think I would stick to keeping your panels all in parallel and using a PWM charger (or a Midnite controller in legacy mode).

--vtMaps

[E350]

vtmaps:  Thank you for your reply.  In my late night Foster's Premium Ale inspired reading, your name came up many times helping people.

So, given my legacy panels (I am glad to see that there has been some progress in pv over the past 10 years), do you have:

1.  Any recommendations for a good PWM controller for now?

2.  And for the future what do you think of this Grape Solar panel for my mini-RV application:

http://www.grapesolar.com/specs-100w-mono-gs-s-100-fab36.html

3.  Or do you have any other panel suggestions for when I upgrade the entire system in the future?

4.  Do you think that a system based on the likes of the Grape Solar panel above would benefit from using a MPPT controller such as the Kid?

5.  And finally, assuming that I wire my existing four true 12v (17.6 Vmp) panels in parallel, your statement about using a Midnite Solar controller in "legacy mode" implies that the Kid (or likely other MPPT controller?) would not (at least without some kind of adjustment) perform as well as a true PWM controller with my existing panels wired in parallel?  If you have time and the interest, this is intriguing and I would like to know why.

Thank you for sharing your knowledge.

[vtmaps]

So, given my legacy panels (I am glad to see that there has been some progress in pv over the past 10 years),

They are not legacy panels... True 12 and 24 volt panels are still made.  They are for a niche market: battery charging (with either PWM or MPPT controllers).  The PV market has gone to larger panels (physical size, watts, volts) which are put in series to get high DC voltages needed for grid tie.  A very common size has been the 60 cell panel (30 volts Vmp, 200-250 watts).   Lately I've noticed that 72 cell panels are becoming more popular... these are true 24 volt panels with Vmp = 36.  So your comment about your panels being 'legacy' is not quite true... they're coming back into fashion.

2.  And for the future what do you think of this Grape Solar panel for my mini-RV application:
http://www.grapesolar.com/specs-100w-mono-gs-s-100-fab36.html

3.  Or do you have any other panel suggestions for when I upgrade the entire system in the future?

No brand suggestions... it's a crap shoot.  I usually like to maximize the watt to dollar ratio, but in an RV other considerations are more important.  You want to maximize the watts to size ratio (high efficiency panels).  You may also have other size and location constraints.  What controller to use depends, in part, on the panels and configuration you choose.

5.  And finally, assuming that I wire my existing four true 12v (17.6 Vmp) panels in parallel, your statement about using a Midnite Solar controller in "legacy mode" implies that the Kid (or likely other MPPT controller?) would not (at least without some kind of adjustment) perform as well as a true PWM controller with my existing panels wired in parallel?  If you have time and the interest, this is intriguing and I would like to know why.

I did not mean to imply (or for you to infer) that a Midnite controller in legacy mode is no better than a PWM controller.  The MPPT controller in legacy mode still sweeps, and still down converts PV volts to battery volts while increasing current.    MPPT controllers, in general, have higher tare losses than well engineered PWM controllers... I expect that Midnite's forthcoming 'Brat' PWM controller will be lower tare loss than the kid or classic.  In most systems the MPPT controller is able to harvest more than enough energy per day to make up for the losses. 

--vtMaps

[acorn]

I'll start backwards and address question 3: "Anything else I am missing or should consider?"

You don't mention if you have a battery monitoring system in place at the moment. If you have one then it should tell you more about the load you are actually placing on the battery. Perhaps try running the RV for 12 or 24 hours and see what your actual energy use is. This will help guide your upgrades or changes to the system.

If you don't have a battery monitoring system, perhaps that should be a requirement for your next upgrade. It can be built into the solar charge controller (such as the Kid with Whiz Bang Jr. and shunt, or a Blue Sky Energy controller with the IPN Pro Remote and shunt); or you can use a standalone monitor such as the Trimetric, Victron BMV, etc.

Since it looks like you will often be running an energy deficit when on solar only it will be important to know what your actual state of charge is. Then you will know when to plug in to shore power, run the engine, etc. With the numbers you specify for daily power needs, it seems like you could easily push the battery to 50% depth of discharge on a day with marginal sun. Doing this often will shorten the battery cycle life.

If you don't have a battery monitor, you might consider the cost of the charge controller and the battery monitor as a unit. I've found that doing this makes the Kid an attractive value proposition. The cost of a less expensive charge controller combined with a battery monitoring system will often be greater than the cost of the Kid + WBJR + shunt.

You could get by pretty cheaply installing one or two modest sized PWM controllers, but this wouldn't provide useful information about your battery state of charge. Your system doesn't sound like one where the charge controller will reach the "float" stage very often, so you won't have much information about how drained the battery is. Since you're using an AGM battery checking specific gravity isn't possible.

How do you charge the battery from the RV engine? Is there a DC-DC charger, separate alternator, isolation relay, or similar? When you do charge with the engine how do you know how much charge the battery is getting? Another motivation to have a battery monitoring system...

It sounds like your RV supports some fun activities.

-Scott

[zoneblue]

Lately I've noticed that 72 cell panels are becoming more popular...

It makes sense when you consider that 150mm is the biggest mC cell that they can make, and yet the cost overhead of the frame, connector, cable and junction box, then more cells is all that they can do to bring costs down. 

I guess 72 cells gives you a bit of advantage in that you could use PWM or no controller at all in the event of an emergency/ controller failure etc. One 300W panel also just happens to fit way better on our motorhome roof than 4x 80W panels ever did. A lot less wasted space.

[E350]

acorn:  I do have a Victron BMV 600 battery monitor and it showed that my refrigerator repeatedly drew down my battery over 50% overnight and system (the four panels mentioned above wired in series and Xantrex MPPT charge controller) was unable to replenish the battery during the day.

So, given the limitations of my existing panels, what controller type (PWM or MPPT) and what controller model should I go with, and how should I wire my panels. 

Without purchasing a controller to replace the missing Xantrex, I have no pv charging.