Solar wiring questions (newb)

Re: Solar wiring questions (newb)

Yes, you can go with 2-3 parallel strings of lead acid batteries. Just keep the wiring equal electrical lengths (i.e., resistance) for each of the parallel current paths.

And I still will push a DC Current Clamp Meter (here is a $60 one with DMM functions that is "good enough" for our use here).

Go out a few times a month and measure "heavy" charging/discharging current through each parallel path (make sure currents are roughly equal between strings). And measure the voltage across each cell (or each battery). You are looking for "variations" between the strings... Anything too high or too low needs to be investigated because the current is (probably) not sharing properly (bad/loose connections, corrosions, open/shorted battery cell, etc.).

Also, you should have a large amperage fuse or circuit breaker for each parallel string (in case there is a short circuit somewhere) to protect each string from over current.

High Amperage Inverter Fuses & Breakers

-Bill

Re: Solar wiring questions (newb)

frohickysmolder wrote: »

I will try to pay attention to the applications I use for the solar (less resistance = more applications). I still don't agree with the usage of CFL. But then again I have mercury in my mouth from evil corporate dentistry and I would only like to use objects that could be put in my garden (mad hatters anyone?) without sending me to an early grave. Sure you can bring it to home depot, but my guess is even a good 30% of them end up in the trash (and another 20% ghana). I gather batteries have similar destructive attributes to my garden, but batteries aren't replacing a benign technology such as the conventional light bulb. (light bulbs have also been the major subject of planned obsolescence so pardon me if i dont trust the technology establishment selling us higher priced poison filled bulbs, new and improved!) I also question the detriment of the light bulb to the environment. I believe there are far too many more destructive things such as cars, factory production, and the cattle industry for me to care about a light bulb. on top of that CFL's become an excuse for people to leave the light on, and they may well end up using just as much electricity.

I try to point to conservation as being a very important goal... But I understand your concerns about mercury, lead, etc. in the environment. Personally, I have concerns about LED fixtures that use a pound of aluminum for heat sinking vs the tiny amount of glass and metal used to make the old filament bulb (mining, smelting, transportation, disposal--shifting operations from relatively regulated US/Europe to poorly regulated Asia and Africa, etc.).

That carries through the batteries, acid, plastics, copper, solar panels and the heavy metals used in many thin film and as doping agents in silicon, etc...

BUT back to topic, I still need some help! would 3 strings of 2 batteries at 24 volts be ok? if I could add a battery that would leave me around 600 ah which is about what someone said my panels would handle, is that right? maybe i should just add a battery then rather than the other scheme of rotating in a spare. Would i change the voltage from 24 to 36 with 2 sets of 3 batteries? and any suggestions on an inverter for this system would be much appreciated.

However, I am also practical. Work with folks to help define/design a system that meets their needs (not mine, not the guy down the street, etc.). We do push conservation because off grid solar costs something like 10x what grid power costs in the US. So, any power you can save, can be a big help to keep system costs down. But, because power usage is highly personal--I (we) respect your choices in life. They are not "uninformed".

So, work from the Battery Bank as you have spec'ed. 24 volt @ 600 AH.

First, I am not a big fan of paralleled battery strings--But here is a recommendation on how to properly wire them to balance charging/discharging current through them (I like 1 strings, with 2-3 parallel strings maximum).

Next, typically we recommend 1-3 days of "no sun/no charging" with 50% maximum discharge. A "two day" battery bank and 50% maximum discharge is a "balanced" off grid system and will use ~25% of the energy per day.

So, such a bank would supply:

• 24 volts * 600 AH * 1/2 days no sun * 0.50 max discharge * 0.85 inverter eff = 3,060 Watt*hours = 3.06 kWH per day

Note that is enough power to run a 1,500 watt typical electric room heater for 2 hours per day from the battery bank.

That is a "nice value". I suggest around 3.3 kWH per day for a "nice sized" cabin/conservation minded home, cost effective system (~100kHW per month). Note--You can draw down to 20% state of charge (80% of capacity) in an emergency too--Your choice (batteries will not last as long--but will certainly last for months of emergency usage with very deep discharge).

Next, the typically maximum load for a flooded cell battery bank would be ~C/8 (~12.5%) of 20 Hour Battery Capacity:

• 24 volts * 600 AH * 1/8 rate of discharge * 0.85 inverter eff = 1,500 Watt "nominal" max continuous inverter output

And, a flooded cell bank can reliably support around a C/2.5 rate of discharge for surge loads:

• 24 volts * 600 AH * 1/2.5 * 0.85 inverter eff = 4,896 watt surge (starting a well pump, etc.).

So, the recommended inverter would be around 1,500 watt to 5kW maximum... "Good inverters" can supply upwards of 2x of rated power for a few seconds, so your rated inverter range would be around 1,500 to 2,500 watts recommended (or a bit higher). Larger inverters have higher losses--Sometimes folks buy two inverters--smaller one to run electronics, computers, lighting, cell phone chargers, etc... And a larger inverter to run tools in shop, washer, etc.

Now, to recharge the battery bank. Two ways to "look at the problem". First is the rate of charge into the battery bank. Second is hours of sun on the solar array to replace a day's worth of energy use.

So first, recommended rate of charge is 5% to 13% (of 20 Hour battery capacity rating). 10% is a "nice healthy" off grid value--But perhaps you will choose 5% for now because this is an emergency system and you will only plan on days or a few weeks of off grid power with backup genset and fuel supply... Your choice. If you wanted "full off grid", probably choose 10% or larger:

• 29 volts charging * 600 AH * 1/0.77 panel+charger deratings * 0.05 rate of charge = 1,130 Watt array minimum
• 29 volts charging * 600 AH * 1/0.77 panel+charger deratings * 0.10 rate of charge = 2,260 Watt array nominal
• 29 volts charging * 600 AH * 1/0.77 panel+charger deratings * 0.13 rate of charge = 2,938 Watt array "cost effective" maximum

Above are just rule of thumb suggestions... Nothing "magic" about the numbers.

Next, how much sun do you get in a day? Using PV Watts for Berlin Germany with a fixed array tilted to 53 degrees up from Horizontal, we get:

Month   Solar Radiation (kWh/m2/day)
1      0.84     
2      1.92     
3      2.34     
4      4.30     
5      4.67     
6      4.51     
7      4.77     
8      4.54     
9      3.25     
10      2.21     
11      1.47     
12      0.62     
Year      2.96      

Not great sun, If we assume 4 hours of "noon time equivalent" sun per day, such a system would need a genset for 7 months of the year to help the solar array (you pick your numbers/location/needs). Assuming 4.0+ hours of sun per day, a 3.06 kWH load would need:

• 3,060 WH * 1/0.52 panel+controller+battery+inverter derating * 1/4 hours of sun per day = 1,471 Watt solar array minimum

So the "optimal" solar array for your system would be in the range of 1,471 watts to 2,938 watts to support a 3.06 kWH load for 5 months of the year (4 hours minimum sun per day) with a 24 volt @ 600 AH battery bank.

Questions/changes about my assumptions/clarifications?

-Bill