Controller advice for small RV system

Doug888 · August 2009

I would like to set up a small system for my RV starting with one 135watt Kyocera KD135GX-LP panel now and adding another one later. My question is on controllers, I am thinking about the Morning star Morningstar SunSaver MPPT Solar Charge Controller or the flex charge NC25A that one dealer recommended. What is your opinion on these two controllers.

Some background: I currently have two 12v group27 batteries, but will probaly be replacing them (two 6v) in the near future due to their 6 year age. I currently can camp for 2-3 nights before needing to recharge and would like to extend my camping time a few days. I have generator as backup, but in some cases I don't want to use it because of noise issuses. My power needs are to keep the propane refrigerator controls working, a 6w fluorescent light about 2hr/day, charge cell phone, camera battery, or light laptop use occasionally, 1 hr/day of small 12v led light (.5 watt?).

Thanks

mike95490 · August 2009

Re: Controller advice for small RV system

My vote:
SunSaver MPPT and get the Battery Temp sensor too.
When you get the 2nd panel, wire it in series.

BB. · August 2009

Re: Controller advice for small RV system

The two chargers you are typing about are about from the extreme ends of the spectrum...

The MorningStar 15 amp MPPT is a leading edge full function charge controller.

The NC25A (from a quick google search--don't know anything else about the product) is a simple PWM, minimal feature, charge controller.

You can read NAWS' (our host) FAQ about the two types of charge controllers here:

All About Charge Controllers
Read this page about power tracking controllers (MPPT type)

Either will work in your installation (if the panels are setup in parallel to run at Vmp~!7 volts). There are advantages to the MorningStar (MPPT, multi-stage charging, optional computer interface, etc.). But--it is much more expensive (and does not have any LCD display--the computer interface or remote LCD both option$).

If you cannot justify the price for a MPPT controller--You may want to look at some better PWM types in MorningStar Family.

For any charge controller you get (and especially for the MS MPPT), get the Remote Battery Temperature Sensor Option... Your battery will, most likely, be charged closer to 100% capacity and should help it last longer.

It sounds like you have been treating your batteries well--pretty long life.

One suggestion for you to spend some extra money on is for a Battery Monitor... Can be very helpful for you to better manage your batteries and loads.

Also--probably take a quick loot at the Battery FAQ... It is the once thing that requires good care for long life--and mistakes can take a bank out quickly (as I am sure you know).

Deep Cycle Battery FAQ

Regarding how much power you will generate... Depending on where you live and how you mount the panels, etc.... Roughly, you will get around 3 to near 6 hours of "full sun" (noon time equivalent sun; winter/summer)... The typical end to end efficiency (solar panel to charge controller to flooded cell battery to AC inverter) works out to around 52%...

Winter:
3 hours * 270 watts of panels * 0.52 = 420 Watt*Hours per day

Summer:
5.5 hours * 270 watts * 0.52 = 770 Watt*Hours per day

Power usage:

6 watts * 2 hours CFL light= 12 Watt*Hours
3 watts * 2 hours cell phone= 6 WH
4 watts * 2 hours camera batt= 8 WH
60 watts * 2 hours laptop= 120 WH
0.5 watts * 1 hours LED lamp = 0.5 WH
==============================
146 Watt*Hours per day (SWAG based on your post)

Now--most people probably use more power than the realize... If you are using an inverter, there are losses and idling power.

For AC loads, get a Kill-A-Watt meter... Also very handy for use around the home to identify energy hogs.

For DC loads, either use the Battery Monitor or get a cumaltive Amp*Hour or Watt*Hour meter (like this type).

The above is based on a lot of rules of thumb and estimates. Can get more accurate with more information (what your loads are, your location, can you mount the panels at an angle to face the sun, etc.).

But is more than close enough to figure out what will work or not.

-Bill

Windsun · August 2009

Re: Controller advice for small RV system

An MPPT controller, like the Morningstar is a much better choice due to the MPPT, which the FlexCharge does not have. However, the SS-15-MPPT would probably be overpowered much of the time if you add a 2nd panel, since that would total up to 270 watts, and the Morningstar is limited to 200 watts for a 12 volt system.

Currently there are no MPPT controllers between the 15 amp SunSaver and the 60 amp Outback Power and Xantrex units. Morningstar will be coming out with a 45 amp MPPT "soon" (guessing 3-8 weeks).

mike95490 · August 2009

Re: Controller advice for small RV system

Windsun wrote: »

the SS-15-MPPT would probably be overpowered much of the time if you add a 2nd panel, since that would total up to 270 watts, and the Morningstar is limited to 200 watts for a 12 volt system.

Well, the panels *could* put out 270W, but when heated, and off axis, I don't think they will be over 200W. Valid point, but I glossed over it, favoring the series PV for future use.

Maybe another tack would be get a respectable PWM controller, and upgrade to the new 45W Morningstar MPPT when you get your 2nd panel

icarus · August 2009

Re: Controller advice for small RV system

Not to argue with WindSun, but what is the matter with the BlueSky series of MPPT controllers? I know that they don't really compare with the Outback and Xantrex 50 amp versions, but for a small array <~350 watts they seem to work fairly well. I have little to compare mine to, except a PWM controller, but I know that I can meter ~10-15% greater harvest during bulk charging. Not an insignificant amount.

Tony

Doug888 · August 2009

Re: Controller advice for small RV system

I some what understand how both controllers work, but what I would like to get opinion on is which one would supply more total energy to the battery in real world use. The flexcharge controller is the somewhat controversial normally closed relay type that is connecting the panel directly to the battery until the battery reaches a certain voltage and then temporarily opens the relay to let the voltage drop back down a little then reconnects. My understanding is that this cycle continues in a programmed manor to keep the battery charged, but limiting the water loss.

Would the mppt type with its current boost in cooler weather put more energy into the battery then the direct connect method of the flexcharge controller.

thanks

dwh · August 2009

Re: Controller advice for small RV system

Doug888 wrote: »

I some what understand how both controllers work, but what I would like to get opinion on is which one would supply more total energy to the battery in real world use. The flexcharge controller is the somewhat controversial normally closed relay type that is connecting the panel directly to the battery until the battery reaches a certain voltage and then temporarily opens the relay to let the voltage drop back down a little then reconnects. My understanding is that this cycle continues in a programmed manor to keep the battery charged, but limiting the water loss.

Would the mppt type with its current boost in cooler weather put more energy into the battery then the direct connect method of the flexcharge controller.

thanks

The MPPT is not really a current booster - it's more of a voltage lowerer.

(Actually, it's both since lowering the voltage with the same wattage will increase the amperage - i.e., current.)

If the battery is at say...12.2 volts, then it is more efficient to feed the power to it at say 12.4 volts than it would be to try to feed it the power at 17 volts (what the panel puts out).

As the voltage of the battery rises, the MPPT *tracks* that, and adjusts the charging voltage to stay just ahead of battery voltage. The point just above the battery's voltage - where the charging is most efficient - is the "maximum power point".

The other type, just connects the 17v from the panel to the battery - that does work, but the battery won't accept as much power as fast with such a voltage difference.

Either way works, but the MPPT will get more power into the battery faster, thus using more of the energy available from the solar panel.

icarus · August 2009

Re: Controller advice for small RV system

An mppt controller will put more net energy into a battery in many cases. The advantage is gained if you have , consistently cooler Pv temps, lower battery states of charge. The advantage goes away as the Pv temp rises and the battery becomes closer o fully charged.

Some Mppt controllers do it better than others, and some (Like the BZ series of MPPT controllers) don't do anything well.

From personal observation, with out any scientific controlled testing, I get ~10-15% more current into the my battery bank in the mornings while the battery is somewhat lower AND the Pv temps are cooler. I know this because I have a ammeter that reads PV output before the controller, and another that reads after the controller. A simple PWM controller can't increase the current (amps) after the controller. Since I don't ever let my batteries get below ~80 SOC I am not getting much gain due to low battery voltage.

In the real world, with smallish systems, it probably doesn't make a lot of difference. If you really NEED the 10 or 15% that you get from a mppt, then you probably should reduce your loading, or increase your Pv. 10% of a 50 watt panel is ~5 watts, but 10% of a 2500 watt array is ~250 watts, certainly enough to justify the added cost.

Tony

Doug888 · August 2009

Re: Controller advice for small RV system

If you connected the panel directly to you battery in the morning and you measured the current, do you think it would be less or more then you currently measure on the battery side of your mppt controller?

BB. · August 2009

Re: Controller advice for small RV system

Their documentation is somewhat misleading...

Yes, the controller can be called 99.5% (or whatever number they pick) efficient.

And a MPPT controller, on its best day, is probably 93-95% efficient.

However, the MPPT controller is the DC equivalent of a "matching transformer"...

A PWM / or simple on-off switch controller takes a 17 volt 10 amp panel (just round numbers for a 170 watt panel) and connects it to a 13-14 volt battery giving:

13.5 volts * 10 amps * 0.995 = 134 watts PWM/On-Off controller
17 volts * 10 amps * 0.93 = 158 watts MPPT Controller

158W/134W= 1.18x (18%) more power from the "less efficient" MPPT controller. However, the MPPT advantage is only when the batteries are below 80-90% State of Charge--all controllers cut back on current flow as the batteries near 100% charge--so there is no efficiency advantage at those levels.

Add the fact that a more sophisticated/expensive controller has several charging modes which can charge the batteries faster, and use less water. Also, highly recommended it the remote battery temperature sensor to ensure that the battery is charged according to its temperature (hotter batteries require lower charging temperatures).

The On-Off charge controller can have one advantage--most PWM and MPPT controllers can interfere with AM (and other) radios.

An On-Off controller can be very quiet electrically. But--that is stretching its advantages.

The details can affect the numbers (few panels, huge controller, bad vendors, etc.) -- but from a 50,000 view, that is how it work.

-Bill

icarus · August 2009

Re: Controller advice for small RV system

It would be less. The Pv can only put out as much as it can put out. For example a 50 watt panel, rated at 17 vdc might under "perfect" conditions put out 2.94 amps (50/17=2.94. So if I connect that same panel under the same "perfect" conditons, and it puts out the same 2.94 amps, but only into 12.1 volts (low battery) the total power would only be 35.75 watts.

What an MPPT controller does is convert that extra "unusable" voltage to useable amperage. So then the situation might be convert the 17 vdc/ 2.94 amps , to say 12.1v to say 3.25 amps, using MORE of the available power, but never all of it as there are conversion losses. (55 watt/12.1volts=4.55 amps. That would be ~100%)

I hope this doesn't make this clear as mud.

T

PS Cavet emptor,,, Don't trust my math!

Doug888 · August 2009

Re: Controller advice for small RV system

Thanks for the info. I understand it now. The mppt controller works like a power supply with a higher current limit when the panel is at a higher voltage then the battery charge voltage. Now I just need to pick the MPPT controller to buy.

From my reading the morning star Mppt is efficient and well thought of. But does not have a meter built in and is limited to 200watts. (if I use a pair of 135watt rated panels I believe it would be rare that the controller would be limiting me under real world conditions but I may be wrong)

The Blue sky 2512ix would not limit me but also does not have a built in meter.

The Blue sky SB200E only has a two stage charger (no float) but does have a built in meter.

Solar Guppy · August 2009

Re: Controller advice for small RV system

Sunsaver Mppt will work well and I doubt it will ever limit with two of those panels. You can get an optional display or maybe get a system monitor instead ...

mike95490 · August 2009

Re: Controller advice for small RV system

Doug888 wrote: »

The flexcharge controller is the somewhat controversial normally closed relay type that is connecting the panel directly to the battery until the battery reaches a certain voltage and then temporarily opens the relay to let the voltage drop back down a little then reconnects.

Relays ? Avoid them. Real controllers use solid state switches of some sort, relay contacts, with 10A DC, buzzing all day long, won't last a month.

icarus · August 2009

Re: Controller advice for small RV system

You're right, the 2512 doesn't have a meter, (which is too bad) but it is always a good idea to have a battery monitor like the Bogart Tri-metric in addition to a controllers meter.

I also have a ammeter & shunt (~$25) on the inlet side of the controller.

T

PS I would favour the Sunsaver over the Bluesky, but for my set up, it is too small, and they didn't make the bigger one when I bought my 2512ix.

Controller advice for small RV system

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