Size of Solar System

pebbledropper · March 2016

Bill (or anyone else),

You've posted this formula before to advise people how to balance their number of batteries and panels:
battery bank Ah* 14.4 * 1/0.77 * 0.10 = W needed from array, with 10% being the system's target efficiency to achieve on a good day.

I want apply that to the system I'm about to build and see if I'm correct. Please ck me out.

I can fit 6-L16 6V's rated at 415Ah ea, or 1,245Ah (at full 100%) in my motor home. I can fit 9-160W, or 1,440W of panels on top (8.64 Imp ea). By these calculations (1245 * 14.4 * .77 / 1440), I can hope to achieve an efficiency of 10.5%. First, am I right? Second, what signicance does the percentage efficiency have, again. IOW, why am I targeting 10-13%?

An assumed question that I'll actually state is how do you feel about the balance of my1,245Ah bank and 1,440W array. FWIW, inverter is Magnum MSH 3012 w 120 charger; the batteries will be Fullriver AGM; the resulting 622Ah usable @ 50% is needed for residential refer (110Ah/day), medical equipment, other usual MH 120VAC stuff, and occasional use of front air conditioner (150Ah/day).

Jerry L

vtmaps · March 2016

1440 watts of panels X .77 = 1108.8 watts into the battery, assuming no loads. The ".77" accounts for hot panels and controller losses. You may not get as high a number as .77 on an RV because you probably don't have much airspace between the roof and the panels (which helps cool the panels). RVs often have shading issues from vents and other roof penetrations. Also, RVs usually don't have their panels with optimum orientation to the sun.

But assuming you really do have 1108.8 watts going into the battery at a charging voltage of 14.4 volts, then
1108.8 watts ÷ 14.4 volts = 77 amps of charging current. That is 77 amps ÷ 1245 ah = 6.2% charge rate. And that number is probably optimistic and assumes no daytime loads while the battery is charging.

Regarding the rest of your system, rather than six 6-volt L16 batteries, buy six 2-volt L16 batteries. This will allow you to have a single string of batteries, rather than 3 strings in parallel. Do you understand the very significant advantages of a single string? If not, we can have that discussion also.

--vtMaps

FFSolar · March 2016

We us for fixation the Panel Mount ( http://www.sunstore.co.uk/Solar-Panel-Aero-Edge-Mounts-Large-White-set-of-2.html),
her you have a idea, ther you have a natural ventilation and you can glue them with Sika.
We also suggest to use most efficiency Solar Panel, for Exempel Sunpower, you save space (20-30%) and have a higher V, less looses and us a MPPT Charger.
Sorry for my poor Eng.

pebbledropper · March 2016

vtmapps,

I looked at Concord's site. Both 6V and 2V result in Ah of 1,200 (at full charge). So, you're right, I don't know the advantage of a single string. Same amps, same cables, same size; I'm clueless.

scrubjaysnest · March 2016

pebbledropper said:

vtmapps,

I looked at Concord's site. Both 6V and 2V result in Ah of 1,200 (at full charge). So, you're right, I don't know the advantage of a single string. Same amps, same cables, same size; I'm clueless.

It is very difficult to get proper charge/discharge balance with parallel strings of batteries because the same current doesn't flow through all batteries. With a single string of 2 volt cells the same charge/discharge flows through all batteries since it is a single series string.

Take a look at this site for an explanation on wiring and its losses. http://http//www.smartgauge.co.uk/batt_con.html

vtmaps · March 2016

pebbledropper said:

I don't know the advantage of a single string. Same amps, same cables, same size; I'm clueless.

Here's a good starting point:
http://forum.solar-electric.com/discussion/13179/series-rule-of-thumb

There are many other reasons to have a single string. For one thing there are stochastic considerations. The capacity and lifespan of your battery bank is NOT the average capacity and lifespan of the individual cells... rather it is the weakest cell that sets the capacity and lifespan of the entire bank.

If you have a 2 volt system, you have a 1 in 2 chance of getting an above average cell. If you have a 4 volt system you have a 1 in 4 chance of getting 2 above average cells. For a 12 volt system it's 1 chance in 64 of getting 6 above average cells. Don't count on it!

A more useful way to look at it is to consider the below average cells... you will get some. But how far below average? The cell quality has a Gaussian distribution... The more cells you have, the greater the chance that you will have a far below average cell. If you have 3 parallel strings you have almost 3 times the chance (of a single string) of getting a really weak cell.

An optimal design for a system has as few cells as is practical. If you have a 2 volt system, you have a battery that will last longer than most 12 volt batteries.... of course, 2 volts is not practical for the power levels you will use in an RV.

And then there is points of failure. Every cable, crimp, bolted connection, circuit breaker, etc. is a potential point of failure. Three strings of batteries has more points of failure than a single string.

--vtMaps

littleharbor2 · March 2016

pebbledropper said:

vtmapps,

I looked at Concord's site. Both 6V and 2V result in Ah of 1,200 (at full charge). So, you're right, I don't know the advantage of a single string. Same amps, same cables, same size; I'm clueless.

Definitely NOT the same cables. For a 6 battery series string you only have 5 single, short battery interconnect cables, quite simple. 3 parallel strings of 2 require 3 interconnect + 4 parallel cables, many more points of potential unbalanced resistance leading to unequal charging characteristics.

pebbledropper · March 2016

You caused me to think of an interesting question: I don't have room to put 6 2V side by side. I will have 3 on one row and 3 on another, 2 x 3, IOW. So, if I make cables JUST long enough to get to,the next post, there will be two (the ones jumping over to the other row) that will be longer.

Therefore, should I make all the series cables the same length, regardless how close to the next battery?

A related question: when cabling just in series to make a 12V system here, should I make the series cables the same size as the cables going from the batter(ies) to the main buss, which for me is 4/0? Hooking 6-6V's together I would have used 2/0 for the series and parallel connections.

vtmaps · March 2016

In a series circuit the current (amps) is the same everywhere. Therefore all battery cables need to be able to handle the full current. Usually, in a 12 volt system you make the cables thicker than that... the reason being that with only 12 volts you can't afford to lose any voltage in the cables. The longer they are, the thicker they need to be.

One of the nice things about a single string of batteries is that none of the cables need to be the same length. That includes the battery interconnect cables as well as the cables from the battery to the inverter (through a circuit breaker, of course). It is the total length of all the cables that counts... the shorter and thicker, the better.

Try to arrange your batteries to have the shortest overall cable length. Here's a picture showing three ways to configure the batteries.... two of the examples are optimal.

--vtMaps

Size of Solar System

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