battery bank

Yes. That is correct.

-Bill

Here is a link about connecting multiple strings of batteries for "even" discharge among the strings:

http://www.smartgauge.co.uk/batt_con.html

So, the above folks are correct about the basic electrical connections that add voltage (in series) and add Amps/Amp*Hours in parallel.

The details about choosing between parallel and series connections and larger Amp*Hour rated cells--There are lots of details.

The main reasons I do not "like" a bunch of 12 volt batteries for a 12 volt battery bank:

• First, wire like the Smartgauge link above, Rick's (Raj124) post is correct electrically, but physically you want to "balance" the wiring resistance.
• When measuring voltage of the battery bank, you cannot "see" the voltage (and health of each battery)--They are all paralleled together. With reasonably priced DC Current Clamp DMM (Sears), you can measure the current into/out of each battery--Makes things a bit easier.
• More cells to check water levels on. You can have 4x 6 cell (12 volt batteries) @ 100 AH, or 2x 3 cell (6 volt @ 400 AH) batteries in series.
• Fewer wiring connections.
  
• In theory, when you have ~3 or more parallel strings of batteries, you should have a breaker/fuse per string to lessen the chances of a short circuit on a large parallel battery bank causing the wiring to overheat and catch fire (somebody drops a wrench into the battery bank, shorted cell, etc.).
• Secondary issues--Each cell can fail/age/vary in capacity/voltage from the rest of the cells in the bank. Just more things to fail, if you have variations in temperature between cells, charging / ouptut voltages vary, etc.

Smaller cell (AH rating) batteries have less life (it seems). We have had people try to put a bunch of random 12 volt "car/marine" batteries in parallel, or a bunch of 35 AH @ 12 volt batteries into parallel/series banks.

When you have a lot of batteries in a bank, you will eventually have to find bad batteries and replace them. And after a couple of failed, then think about replacing the entire battery bank or chasing battery/cell failures for months/years into the future.

Also, when picking batteries--You have to figure out how big a battery you can handle at your place. A 12 volt @ 100 AH is about the same size/weight as a 6 volt @ 200 AH (3x cell) or 2 volt @ 600 AH (1x cell). They each store the same amount of energy (E=V*AH rating). You just have to series them into the bank voltage you want (batteries are just cells put in series--12 volts--6x cells in series without the external wiring).

You can get heavier batteries (or even cells) that weigh >300 lbs. Or fork lift batteries that weigh >1,000 lbs. Then you need to figure out how to get it off the truck and into the battery shed (steps, concrete floor/driveway, crane, pallet jack, etc.).

In general, even for a small system, 2x 6 volt @ ~200 AH golf cart batteries make a really nice starter set. They are relatively cheap (~$80-$100). Will last 3-5 years, and if something happens (many people "murder" their first battery bank), you are not out much money.

2x series GC to 2 series x 2 parallel GC batteries make a really nice starter bank. 2s GG batteries are 12 volt @ 200 AH or 2sx2p is 12 volts @ 400 AH.

A 4 battery bank of GC batteries will give you (using our off grid system rules of thumbs):

• 12 volts * 400 AH * 0.85 AC inverter eff * 1/20 hour discharge = 204 Watt (120 VAC) average load
• 204 Watts * 5 hours a night * 2 nights = 2,040 WH for two nights (50% maximum battery discharge for longer life)
• 12 volts * 400 AH * 0.85 invt eff * 1/8 hour discharge rate = ~510 Watt maximum continuous AC load (~4 hours to 50% State of charge)
• 12 volts * 400 AH * 0.85 invt eff * 1/5 hour discharge rate = 816 Watt max AC load (minutes to 1 hour or so)
  
• 12 volts * 400 AH * 0.85 invt eff * 1/2.5 hour discharge rate = 1,632 Watt max AC surge (seconds to start well pump, etc.)
  

More or less, that makes a 300 watt (typical) to 800 Watt max AC inverter "cost effective" for this configuration. A 300 Watt Morningstar 12 volt TSW AC inverter is really nice for this bank (plus the Morningstar has remote on/off, and low power "search" mode).

To charge such a battery bank--Two sets of calculations. The first is sizing the solar array to the battery bank (larger battery bank needs more solar power for proper charging). 5% rate of charge is OK for weekend/seasonal usage. 10%-13% is better for full time off grid usage:

• 400 AH * 14.5 volts charging * 1/0.77 panel+charger derating * 0.05 rate of charge = 377 Watt array minimum
  
• 400 AH * 14.5 volts charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 753 Watt array nominal
  
• 400 AH * 14.5 volts charging * 1/0.77 panel+charger derating * 0.13 rate of charge = 979 Watt array "cost effective" maximum

And then there is sizing the array based on your daily loads and average hours of sun per day.

• 400 AH * 12 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 1,020 WH per day

Say installed fixed array near Palm Beach Florida:
http://www.solarelectricityhandbook.com/solar-irradiance.html

West Palm Beach
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 63° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.46	5.14	5.53	5.82	5.63	4.87
Jul	Aug	Sep	Oct	Nov	Dec
4.92	4.91	4.71	4.98	4.52	4.30

If you want to run the system during bad weather--The minimum average sun is December--Lets say 4.0 hours of sun (average) per day (still a lot of sun for most people):

• 1,020 WH per day * 1/0.52 AC off grid system eff * 1/4.0 hours of sun per day = 490 Watt "break even" solar array

So, I would suggest a 490 Watt to 753 Watt array for a nice/capable system to learn on. The system is a bit small for running a standard full size Energy Star rated refrigerator (need ~1,200 Watt inverter minimum, and 6-8 GC batteries in series/parallel). Or a larger 24 volt system with 4sx3p GC batteries (24 volt at ~600 AH) would give you ~3,300 WH per day (fridge, lighting, radio, small water pump, laptop computer) and a ~3.3x larger solar array.

Anyway--Suggesting to not focus on the battery bank until you have a good handle on the loads and what you want/need the system to do.

-Bill

48v/400ah has 2x the energy capacity of a 24v/400ah battery bank.

Finding 48 volt rated breakers & inverters is a little bit more difficult. But sizing the battery bank voltage+ah is based on your needs. 48 volt systems tend to be bigger (more powerful).

Solar systems are not cheap. You want to design+build one that is the right size for your needs. Too small or oversized, each has their downsides.

-Bill

Robert, where (roughly, nearest major city) will the system be installed? (figuring out how much sun you get)

Also, running at battery bank at ~35C, the batteries will last about 1/2 as long as batteries running at 25C... The only batteries (I think) that would last that long would be fork lift batteries or some other Lithium chemistry battery (and that chemistry is still a bit "cutting edge" also known as "bleeding edge" for a first time off grid).

• 2,900 Watts * 5 hours per day * 1/0.85 inverter eff * 1/48 volt battery bank * 2 days storage * 1/0.50 max discharge = 1,422 AH @ 48 volt battery bank

Typical 10% rate of charge for an off grid power system:

• 1,422 AH * 59 volts charging * 1/0.77 solar panel + charger deratings * 0.10 rate of charge = 10,896 Watt solar array "nominal"

Minimum average hours of sun needed to keep system "happy":

• 2,900 Watts * 5 hours * 1/0.52 off grid system eff * 1/10,896 Watt array = 2.6 Hours of sun minimum per day

Conceivably, if you run the AC system only during the middle of the day and not at night/during bad weather, you could possibly get away with a 1/2 size battery bank (711 AH @ 48 volts) and a ~5,500 Watt solar array (depending on where the system will be installed).

That is a lot of solar power (large load). And a fair amount of risks--You need to have "free cash" available--There may be some mistakes along the way. Do not spend all of your savings on such a system.

One thing you could do--Run a genset + your AC system for a few days with a Watt*Hour meter and measure how much energy you really will be using.

-Bill