Pros and cons of 1 big battery vs several small ones?

I personally recommend that people have one series string of batteries as "ideal", with 2 to 3 parallel strings of batteries as "OK". Some people run more than 3 parallel strings (cannot get larger batteries, cannot afford them, etc.) and can do OK with that.

However, Lead acid batteries tend not to share current very well. And fewer parallel strings usually is better, and less work for you (fewer electrical connections, fewer cells to check water levels--if flooded cell, etc.).

For paralleling batteries, check this website:

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

Also, I would suggest that two x 6 volts @ 200 AH "golf cart" type batteries in series are better than two x 12 volt @ 100 AH batteries in parallel. You have 1/2 the number of cells to check (6 cells vs 12 cells for 2x parallel batteries--Plus you can take your volt meter and do a quick check on each 6 volt battery's voltage (charging, discharging, resting). It is much easier to find problems this way. Measuring across 2x 12 volt batteries in parallel--Even if one battery has a "weak cell", you could not find that with just a volt meter.

If you are going to be paralleling battery strings, and just very handy for general debugging--See if you can justify a DC Current Clamp DMM (digital multi-meter). This one from Sears is ~$60 USD and is "good enough" for our needs. A DC Current Clamp meter is very easy to use (and very safe)--You just clamp the cable for each battery string and measure the charging (or discharging) current. You are looking for "differences" (one string taking too much charging current or supplying very little load current--A couple of examples).

Now--The next major question is the use of GEL batteries in a solar power system. In North America, most GEL batteries are not very good for solar power. The major issue is that when charging, you can only charge them at the 20 Hour Rate (C₂₀) or 5% rate of charge (i.e., a 100 AH battery charging at 5 amps). For off grid / solar power systems, it is usually much better to charge the batteries are around a 10% to 13% or so rate of charge--Simply, because the sun is not up in the sky long enough to fully recharge a well discharge battery bank at 5% rate of charge....

For example, a 50% charged battery would take ~10-14 hours to fully recharge at a 5% rate (2-3 days of solar charging). At 10%, it would only take ~5-9 hours--A little more than 1 to 2 typical days.

If you are up north, then in the winter you may not even get more than a couple of hours per day charging.

And similar with a genset--5% maximum rate of charge vs 13% (or sometimes higher) with flooded cell (or AGM).

GEL batteries, if they are charged too fast, develop permanent gas pockets in the GEL and ruin the battery's capacity.

However, there may be some European (primarily) GEL batteries that can take higher charge rates--As always, check the battery manuals for exact charging instructions.

Lastly, if this is your first set of batteries--Getting a good quality hydrometer and a flooded cell battery bank will give you a lot more accurate data on your state of charge and how well you manage the battery systems. If you have sealed batteries (GEL, AGM, etc.)--Getting a Battery Monitor may be a good idea (although battery monitors can have their own issues).

-Bill