Generally if you have enough solar, you will not need to isolate one bank to fully charge it. The final 5% of recharging requires time, and not very high amperages, so the size of the loads present on the batteries during the 80% to 100% charged range are important, so one can maintain absorption voltage of 14.8v at 77f for trojan t-105 GC batteries.
Many people do choose this approach of dual battery banks, but instead of two controllers, they use a Marine style 1/2/both/Off switch or two to move all the loads or charging sources to the other battery bank. No secondary solar controller necessary with this.
There are advantages to one large battery bank, regarding the peukert effect, but if your loads are small this is not much of a factor. Peukerts law basically says the larger the load on the battery, the less capacity the battery has to give, so a larger battery bank is less affected by large loads than a smaller one on the same loads.
I use 3 1/2/both Off switches. One for the original Van ignition system, one for all house loads, one for the Solar and other charging sources. This was most convenient with my previous petulant group31 house battery which needed a 16volt EQ charge every 14 cycles or so. I'd Switch ignition and loads to the other battery and crank up the solar voltage to 16v for as long as it would take to max out the specific gravity and 'reset' the battery capacity to the maximum of its remaining capacity..
That battery lived out its cycle life and I now only have one single 90AH AGM battery for both house and engine starting duties, So all three switches are set to "2"
I use and recommend these Switches over other brands:
http://www.amazon.com/Blue-Sea-Systems-m-Series-Selector/dp/B015ZPJL7M
I'd recommend not going for two separate battery banks and solar systems, but instead just getting more Solar, if you can actually fit more on the roof, unshaded.
EQ charging, trojan recommends 15.5v, does present an issue, and EQ charging can significantly extend battery life when performed when needed, and regularly. over 15 volts is danger territory for SOME 12 volt electronics, so it is nice to be able to not expose them to these voltages, at the turn of a switch.
More than one way to skin the cat. Getting more battery life can be accomplished with just more solar wattage which can get to absorption voltage earlier in the day, and be able to maintain it under higher loads, and having a programmable solar controller so one can hold absorption voltage until specific gravity maxes out.
The actual gains of a dual battery bank/solar system might never be realized. If maximizing battery longevity is the goal, then the correct absorption voltage for the battery temperature, held long enough, is the key, and this is most easily accomplished by having a surplus of solar, and a solar controller which allows one to fine tune voltage and duration to approach maximum baseline specific gravity of the cells each recharge cycle.
In general getting from 80% charged to 100% takes 4 hours, no matter how powerful the charging source might be. The Hydrometer, excess solar wattage, a programmable solar controller, and the will and knowledge to use all 3 to their potential, will have the happiest longest lived batteries.
The more often one gets to do a 100% recharge, the less often a 16v EQ charge is required, and it will take less time at 15.5 to 16v to max out specific gravity and be less abusive to the battery.
Only a Hydrometer can definitively say when a flooded battery is fully charged. Many people assume that their solar controller dropping to float voltage signifies a fully charged battery. This is not correct, especially on those systems whose batteries are discharged regularly to 50%. Premature sulfation usually does not ring its ugly head until it is too late to do anything about it. For most people they declare a ' just fine' and 'performing just like new', until that one day when their remaining battery capacity has declined to the point it can no longer power their overnight needs.
A battery slowly loses capacity. It loses it slower when properly fully recharged as promptly as possible.
A solar controller dropping to float voltage does not signify the battery is fully charged, only that absorption voltage was held as long as programed to do so. The correct absorption voltage, and duration at which it is to be held for a specific battery temperature, is a moving target depending on depth of discharge and health of the battery, so a 'one size fits all' charge algorithm common on less expensive controllers is more of a one size fits none.
So, an adjustable solar controller programmed correctly, and more than enough solar wattage to get to absorption voltage earlier in the day, is the easiest way to prolong battery longevity.
I'd aim for more well than one solar watt for 1H of battery capacity, and the deeper the % of discharge, the higher the ratio required to keep batteries happy, but other charging sources can alleviate this ratio's requirements if applied regularly.
A thickly wired alternator can relatively quickly get batteries upto 80% charged as charging below 80% is pretty efficient. It is once batteries reach absorption voltage that they start restriciting the amperage they require to not exceed absorption voltage.
When using a plug in charging source, trojan recommends 10 to 13% charge rate, or ~23 to 30 amps for a single pair of t-105s in series for 12 volts.
Keep in mind that a pair of t-105 batteries drained to 50% can easily accept 80amps.
A 1 to 1 ratio is really quite light, in terms of charge rate percentage Rolls surrette, the premier flooded battery maker recommends sizing an off grid solar system so that it approaches the 20% rate which is closer to 3 watts to 1AH of capacity.
So, go for more solar wattage if the goal is maximizing battery longevity, and performance during that lifespan. It is far too easy to have too much battery capacity for too little solar, but nearly impossible to have too much solar for the battery capacity.
