Originally Posted by PNW_Steve
There are two basic types of controllers: PWM & MPPT.
A PWM controller will limit you to the lower voltage panels like the 100watt you mentioned.
An MPPT controller will allow you to use higher voltage panels like Grey Coyote mentioned. The MPPT controller will be more efficient. It will also cost somewhat more.
With 4 x 6v - 215ah batteries you will wind up with a series/paralell string giving you 12v - 430ah. As you never want to discharge beyond 50% that will leave you with 215ah of usable capacity.
Edit: To be correct: there will be some system losses so the 600watt system will NOT put 40amps into your battery bank.
Your max charge rate should be between 21.5 - 43 amps. Check with the battery manufacturer for specifics on your batteries.
43 amps @ 12v = 602 watts.
A 600watt set of panels and a 45 amp MPPT charge controller should be reasonable to keep your batteries charged and likely support minimal 12v loads at the same time.
If you are going to be supporting significant 12v loads at the same time you are charging the batteries then you will need to upsize your panels and controller to accommodate.
One question that I have not answered for myself is: If I install a 60 amp controller because I will have significant 12v load that I want to support while charging as well, how do I prevent having an excessive charge rate on the batteries? I expect that I will find something in the charge controller setup that will address this but I have not found a definitive answer yet.
As long as the charge controller has both terminals for the batteries AND terminals for the load, as well as a setting for the type of battery that is connected (flooded and AGM have different charge profiles even though they are both lead-acid), then the charge controller will properly sense what the batteries can take and shunt the rest of the energy to the load. (In a battery's first stage of charging - "constant current" - if it is fed too much current too quickly, its voltage rises above certain setpoints set by the type - flooded or AGM. The controller should sense that and slow down the current feed. In a battery's second stage of charging - "constant voltage" - the controller holds the battery at a specific voltage and lets the battery's internal resistance "taper down" the current as it finishes charging. In a battery's third stage - "finishing charge" - the charger "pings" the battery with a high-frequency signal for about 1/5 of a second every second or so to de-sulfate the battery's plates. This third phase is optional and a lot of lower-end chargers just leave it out. Its good for long-life but is only triggered when the battery is fully charged.)
However, if the charge controller is an "el-cheapo" that only has battery terminals, then you will need to worry a bit about that. I absolutely do NOT recommend these controllers (these controllers are usually low-wattage types anyway, hobby-type stuff).
In fact, some high-end charge controllers even have a terminal for "energy dump" to shunt power to if the batteries are fully charged and there's no-to-light load on the load terminals. Usually these charge controllers are in fixed locations (i.e. home or business), so the dump is connected to an electric water heater or water pump (for example to pump water from a well into a tank or cistern - something to do when there is excess energy and nothing else to do with it).
I should draw your attention to your idea of putting batteries in parallel. I would recommend against doing that. No two batteries are exactly equal, so the weaker battery (or weaker string of batteries) will discharge the stronger until they are "equal" - which never exactly happens (due to age, temperature, etc.). Furthermore, the weaker battery or string will never allow the stronger to charge to its full potential, causing the stronger to "age" faster. And for flooded batteries, the weaker will electrolyze more water than the stronger, causing you to have to fill it more often - and maybe cause you to carry around a bomb in your battery bay until a recombiner can take the hydrogen/oxygen and recombine it back into water (you are using a recombiner for your flooded batteries or at least a plastic vented battery box, aren't you?).
Now, if you break the parallel connections every so often (say, once a week or so) and apply the finishing charge to the banks individually, then you can "get away with" the parallel structure. However, if you aren't willing to do that, then save yourself some headaches and don't use the parallel connections.
You can also use battery isolators to keep the weak battery or string from discharging the strong one (an isolator is basically a big diode - a one-way valve for electricity). But juggling the isolators, charger, and load could be interesting.