Sunsaver Duo with 20V panel?

solowiej

Hi, my partner and I have been struggling with a configuration to keep our RV charging off-grid. The Morningstar Sunsaver Duo charge controller makes great sense, as it keeps the house battery bank and the chassis battery going. BUT we're hung up on whether we can use this with a higher-voltage panel like the Canadian Solar CS6P-250M 250 Watt 20V panel. Its Vmp is 30.4V & the max solar voltage for the Sunsaver Duo is 30V.

We'd like to use this panel because it's significantly cheaper than a 12V panel. Will this work?

Thanks for any insights!
Sean and Leah

BB.

Re: Sunsaver Duo with 20V panel?

Probably not...

First, the charge controller's maximum input voltage is based on Voc-cold (voltage open circuit, cold solar panels). Vmp~30 volts at STC can get Voc-cold in 48 volt range (i.e., controller turned of because battery bank is full).

Second, running a Vmp~30 volts on a PWM controller with a 12 volt battery bank will waste almost 1/2 of the solar panel's output wattage rating (power=Volts*Amps; if your battery bank is ~15 volts, and the panel is Vmp~30 volts, that is cutting the output voltage of the panel by 1/2, so power is 1/2 too... The panel is "safe" to run at low output voltage--But it is a loss of usable power).

Your only solution is to find Vmp~18 volt panels to use with the PWM controller--Or, get "cheaper"/higher voltage panels and go with more expensive MPPT type charge controllers (and get a DC to DC battery charge controller so it can keep the vehicle battery charged too).

There is another solution, instead of a MPPT charge controller, you can get PWM controllers with higher array input ratings (Xantrex C40 is one)--But it is still ~50% efficient.

-Bill

solowiej

Re: Sunsaver Duo with 20V panel?

Thanks so much Bill!

We're also considering a small MPPT (Sunsaver SS-MPPT-15L) but are concerned by its max input power rating of 200W (for a 12V system). The Pmax of the 20V panel is 250W. Overall, the PWM (w/ a 12V 135W panel) & MPPT (w/ 20V 250W panel) systems are close in price. It seems like the MPPT system still gives us more bang for the buck, even with the reduced efficiency. Any thoughts?

BB.

Re: Sunsaver Duo with 20V panel?

More or less, you "cost effectively" (and safely/reliably) put on a 15 amp MPPT charge controller:

• 15 amps * 14.5 volts battery charging voltage * 1/0.77 panel+controller derating = 282 watts "max cost effective" array

So, on cold clear days, the controller may spend some time at 15 amp current limit--But overall, you will "lose" very little in the way of energy over the year. (note, there is no "magic" to 282 watts--just a rough number to hang your hat on).

And, the controller can safely/reliably take more solar panel wattage than above just fine. In fact, the controller is used by some people to take energy from a 24/48 volt battery bank and down convert/charge a 12 volt battery bank--So having more wattage/current than needed does not hurt the unit.

Also, with this unit, highly recommend the remote battery temperature sensor option. This controller tends to "over estimate" the battery temperature (and charge slower/lower set point voltage) using its internal temperature sensor.

-Bill

PS: Yes--you will be very happy with the 250 watt panel and the MorningStar 15 amp MPPT charge contoller. It's only drawback is you cannot add any more panels to the controller later if you wish to expand. But, otherwise, is a very nice unit.

solowiej

Re: Sunsaver Duo with 20V panel?

Thanks a lot Bill!

BB.

Re: Sunsaver Duo with 20V panel?

You are very welcome.

By the way, here is an example of a battery to battery charger:

http://www.xantrex.com/power-products/power-accessories/auxiliary-battery-charger.aspx

-Bill