RV Solar Power Set-up Advice Needed

Re: RV Solar Power Set-up Advice Needed

Phil,

You’re still alive! Springdale and Skylab: I know it!

In general, I wouldn’t worry about any bad interactions between the chargers. Both are so-called “smart” chargers. They’ll individually track the battery bank’s voltage and adjust their operation according to the voltage measurement and the batteries temperature. For example, once the batteries reach their absorb voltage setting (~14.35 V at 77 F), the chargers will only supply enough current to maintain that voltage for the length of time you’ve selected. If the voltage starts to rise -- even just briefly -- one or both the chargers will reduce current to compensate.

My system stated on my small camping trailer. It started at ~180 W and grew to 300 W. With the latter configuration, we were able to generate >100 Ah per day when camping in the Rockies in the summer (6-7 hours of full Sun per day), and it worked very well. We really enjoyed the peace of being able to set up in a no-generator loop at Colter Bay campground in Grand Teton National Park.

We'll be using about 100 to 150 Ah/day. Do you think that 6 each KC130 modules should be enough solar power to get the batteries up to "float" each day in average California weather - say 4-5 hours of sun each day?

The answer to your question is complicated. Correctly aligned, your 780 W STC array should be able to generate ~48 A from full summer Sun. Assuming 98% coulombic efficiency, the system will easily replace 150 Ah in four “hours” of full Sun, although the actual amount of recharge time will be longer – perhaps six or seven hours, including ~three hours for the absorb stage.

There are several key parts to successful battery charging strategy. One is timing, another is load shifting, and a third is day-skipping. In general, I like getting the batteries to their absorb voltage by ~local solar noon. This will usually allow enough time to complete the absorb charge stage before the Sun sets too far, and it somewhat synchronizes the absorb stage’s falling current requirement with the gradually decreasing array current available from the setting afternoon Sun.

The absorb stage limits charge current to the battery bank. Accordingly, there’s often power available that not being used to charge the batteries. Load shifting takes advantage of this phenomenon by shifting loads from the morning bulk stage to the afternoon absorb stage, but you have to be careful to might divert current away from charging the batteries.

The day-skipping approach is to only worry about achieving a float state every other day. It’s a variation of load-shifting, and entails concentrating loads on day one (say, 150 Ah), lightening up loads on day two (say, 100 Ah), and allowing the system to complete a full absorb cycle on the second day. To me, this is better than leveling loads and never quite getting to float on any day.

As you and Bill have discussed, local weather patterns will greatly influence your system’s operation. I lived in Marina del Rey when I first moved to SoCal, but I moved to the SFV after three cycles of June gloom – I was literally and figuratively in a fog and just could not function. IOW, don’t expect miracles when there’s little- or no direct sunlight striking the PV array.

Another issue will be PV alignment. Flat mounted PV modules do well enough in the summer when the Sun is high and the cosine error is low. However, the combination of “short” days and low Sun will greatly reduce winter energy generation.

The insolation link Bill provided is a good one. The same data is available from here: http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/sum2/state.html My link isn’t better, but the downloads are shorter, and, being a creature of habit (both good and bad), I’m stuck with where I got started.

Finally, your 1,000 Ah battery bank will give you some flexibility. It should be able to meet your energy needs for ~three days without too deep a discharge, and, if the Sun is still hiding, you’ll need to crank up the generator for a while.

HTH,
Jim / crewzer

Re: RV Solar Power Set-up Advice Needed

Phil,

I like the Midnite Solar breaker boxes. The 15 A breakers for the PV subarrays sound good (assuming two modules in series per breaker), and the 63 A breaker sounds OK for your application of the MX60.

Magnum’s input spec for the 2012 inverter is 190 ADC (full output power from a “low” battery voltage), so, assuming a 100% continuous duty rating, the typical breaker spec would be 190 ADC x 125% = 238 A. The closest size is 250 A – a tad larger than the 175 A breaker you’ve spec’d. You may want to reconsider... However, if you're using "small" cable from the batteries to the inverter, a 175 A breaker may be appropriate, but this configuration will likely limit full power operation.

See: http://www.magnumenergy.com/Literature/Data%20Sheets/Inverters/MS_series_mobile_data_sheet_revc.pdf
and: http://www.magnumenergy.com/Literature/Manuals/Inverters/MS Manual (64-0007 12-04).pdf   (page 17)

Surge/lightning arrestors need a good ground in order to be effective. If you decide to add one, you’ll also need to address the grounding issue we discussed earlier.

Sounds like your making progress!

Regards,
Jim / crewzer