Battery Load modes

[tecnodave]

@E350

Over paneling depends on many factors, in my case I have enough to make it through several days of fog without running a generator to top batteries. A kid at 24 volts can safely handle 800 watts but I have seen 942 watts from one kid on my motorhome with 1020 watts of panel. It was maxing out at 30 amps.  That is a bit over paneled. To be more conservative of my equipment now I am using two kids to power motorhome.  1020 watts on two kids is very conservative but I can run on one alone if need be. I have installed extra breakers so system can be two separate controllers or bully mode or follow me mode.

The main panel racks for the motorhome covers the entire roof except the cab over area. The rack is built from awning arms mounted to the roof so that they can tilt to left or right. they are 7' X 8' each and mount three Sharp 170 watt poly panels in top row in portrait orientation and four Siemens 55 watt mono panels for the 12 volt system in landscape orientation on each half.

I have a total of 1460 watts on the roof and an additional set of Suntech 280 watt panels 560 watts total on a ground mount for a total of 2020 watts in.

This motorhome is not used on the highway........it lives in the country as a mobile house....maybe moved twice yearly. This is not a huge motorhome....it's a Tioga Class C 24 foot on a Chevy van C-30 chassis.  Main battery set is 500 pounds and standby is 250 pounds.

I just love the kid controller, in the past I had used a MPPT controller that could not be set to match the battery set. That caused me to have to top off the batteries with the generator set which is the most wasteful time to use the gen but the non adjustable controllers just cannot top off anything but the most common batteries. I almost ruined a good set of L-16's using non adjustable controllers and at this point if I cannot set every value to fit my system then I don't want to mess with it. The Brat comes from good lineage but it is limited in that it is not adjustable. I'm sure that it will have a better charge profile than the competition which is Chinese or the Chinese Morningstar.

I'm using an old standard PWM controller on my 12 volt system rather than newer MPPT controller because the old Trace C-40 can be set to the voltage I need and my much newer and more efficient Chinese Tracer MPPT controllers can not be set. They leave the battery at 1.220 s.g.... To me that is dead! I want to see 1.265 s.g.

I am a technologist and I need to control my entire system to the nth degree....I need a lot from it
I demand a lot from it.....I built it!

I think that if I had a retail Kid with 600 watts of panels on a 12 volt system that I would be sure that the Kid has plenty of ventilation and maybe set the current limit down to maybe 25 amps or so unless you really need all that power, in that case two kids teaming together would be way more conservative design and would be more reliable in the long run as you would not be running at max all the time.
If the panels are flat on the roof then 600 watts panels corrected for angle would be more like 400 watts output max so one kid can handle that without a problem. I don't like the idea of running at max power for extended lengths of time, it does not lead to longevity.

td

[E350]

Mtn Don and tecnodave:  My panels are mounted flat-nontiltable.  But I want to modify my design to add the hinges in VanGuy6678's ingenious preliminary tiltable design.  See it at:

http://sportsmobileforum.com/viewtopic.php?f=14&t=14746&hilit=how+do+you+mount+your+solar+panels%3F&start=15

tecnodave:  I hope you don't mind that I am reposting some of your posts (with attribution to you and links to here) over at the SportsMobileForum.com where there are some much smarter and technologist type guys are very interested in your input.  Your discussion re overpaneling was quite helpful.  Among other things, I learned that "over paneling" does not refer to hanging your panels over the edge of your rv's roof, but refers to a condition where the maximum array output wattage exceeds that of the charge controller's recommended maximum input wattage.  Good to know.

If I can ask:  First, for a 12v dc rv house battery set up, do you believe that 12v panels or 24v panels (or other) are best?

The reason I ask is that I have done some reading on this issue and came to the conclusion that maybe the "headroom" (i.e., difference between the 12v battery voltage and the 17.6 - 17.8 or so max panel output voltage (Vmp) in a parallel wired 12v panel array is not large enough for an MPPT controller such as the Kid to exploit, and, thus a PWM controller might be "better."  Or can the Kid be as efficient as the Brat PWM?  Can it know automatically when to operate in PWM (legacy?) mode or can it be manually set to operate that way?  Or is it irrelevant how it operates?

The second reason I ask is because the 12v panels seem to have the right length for an rv (i.e., 47" long) and I have had trouble sourcing 24v panels of the same dimension.

My third question is:  What is "s.g." ?   (E.g. 1.220 s.g. vs. 1.265 s.g.)  And how do you monitor it?

[TomW]

E350;

I can give you the answer to #3:

S.G. is Specific Gravity. In this case it refers to the battery electrolyte compared to pure water. 1.000 would be pure water and electrolyte is heavier than water so it has a higher SG. If that helps.

Tom

[tecnodave]

E350

I have good luck with my ancient Siemens 36 cell "12 volt" panels but I do run them 2 in series for 34 volts in for my kid or tracer on 12 volt systems. A little more headroom allows more power to be produced in marginal times.....shading , fog and such.

As I do have matching pairs of controllers I have been able to do direct side by side testing and I think that best efficiency and output will be with panels at 34 volts for 12 volt systems and at 68 volts for 24 volt systems. I have tried 105 volt strings on a 24 volt battery and have gotten more heating from the Chinese Tracers, Classic 150, and the Kid controllers. I conclude that 105 volt string is too high a voltage conversion ratio for most MPPT controllers. I have some 30 cell and 60 cell panels but I don't use them much as the voltage is a bit too low. If I use modern grid tie 60 cell panels then I need to run three in series for a volts in of 90 volts which is less efficient than two 72 cell panels at 68 volts in.

I still have over 50 of the old Siemens but the newer panels are more effecient so I have been switching to the 32" X 64" form factor "24 volt" 72 cell @ 125 mm. Size cells. These are much easier fo fit in tight spaces than the latest version panels which have 156 mm. Cells and are a handful to handle. There are some good " 24 volt" panels out there that are reasonable in size for R.V. /Sportsmobile applications. There is one S.F. Bay Area company that makes a good one for that application. Check out ML Solar of Cambell,CA they have MLS-A-100 100 watt "12 volt" panel 46"X26". Nice quality.  ML Solar is a buyer of overstock panels and cells and markets panels under there own label as well as others . I bought Toppoint 190 watt 32"X 64" panels for $130. From them with full warranty . Good service, good folks......They do sell some panels/cells with a very limited warranty.......do ask!

You do have my permission to repost my posts on Sportsmobile.com. I have been very busy lately as I have taken on another major project which is eating all my spare minutes. It is a real joy for me to be able to share my experiences with others as I enjoy tinkering electricity

td