Regarding battery capacity vs solar wattage, it is entirely possible to have too much battery capacity for the amount of solar.
In a van dwelling scenario, where one discharges nightly, it is important to get the batteries to as high a state of charge as possible before the next discharge cycle begins late afternoon.
Batteries in the 80%+ charged range begin to limit what they can accept, making charging from 80% to 100% much less efficient than below 80% charged, which can be 99%+ efficient.
So if one has enough battery capacity that they remain above 80% charged almost always, it can be argued that they have too much battery capacity.
But lots of battery capacity = lots of warm and fuzzies, and bragging rights, though this can be a little misguided.
Battery manufacturers have charging rates recommended anywhere from 10% to 40% of the AH capacity, 10 amps to 40 amps per 100AH of capacity. These assume one is plugging in a grid powered charger and has 8 to 12 hours or more to recharge fully. 100 solar watts = about 5 amps, at noon, on a good sunny day.
Solar amperage has a slow ramp up and peaks at solar noon and then tapers, so the textbook 10-13% rate trojan recommends is a bit out of Whack with reality concerning solar.
Generally the 1 watt to 1AH capacity is the absolute minimum one should employ, but AGM batteries would be much happier with a 2 to 1 ratio minimum. If one utilizes the alternator to recharge, and does so in the morning, when batteries are most depleted, and can efficiently accept everything extra the alternator can make, then one can get by with less solar.
The Key to good to excellent battery longevity is achieving a true 100% recharge, ASAP, as often as possible. Anything less, is a compromise to some degree.
The key to achieving a true 100% recharge is getting the battery upto the manufacturer recommended absorption voltage, compensated for battery temperature, and hold it for the proper amount of time.
The precise time at which absorption voltage needs to be held, varies greatly with the battery, its temperature, and its health, and how many cycles it has accumulated since its last true 100% full recharge. A true 100% recharge on a flooded battery can be determined with a hydrometer. On an AGM battery it can be determined by the amperage the battery accepts at absorption voltage. When a 100AH agm battery can only accept 0.5amps at 14.4v, then it can be considered fully charged. Flooded batteries are usually 2 to 3% of battery capacity at absorption voltage.
I've seen my Northstar AGM take only 2 hours at absorption voltage for amps to taper to 0.05% of capacity, and I've seen it take 10 hours after a week in which it only got recharged back upto 90 to 95% each day. 10 hours at absorption voltage is not possible in my usage. This requires being plugged in, or one night of NO battery discharge and 2 full days of solar.
So if all the above is confusing to you, the short version is, More solar is better. Huge amounts of battery capacity and not enough solar to feed it might have the battery never quite reach a true 100% and its capacity might decline faster than less capacity cycled deeper, If the time at absorption is adequate. But shallower cycles, assuming a full recharge often, = longer battery lifespan, so it is a catch-22.
Really it comes down the the amount of cycles achieved before battery capacity declines to the point where the user needs new batteries. One can go to tremendous lengths and efforts to achieve the maximum cycle per$$ ratio, but they might spend more money on equipment to do so, defeating the goal, as replacing the batteries sooner is not really that big a deal, most of the time. It is only when the batteries declining capacity comes as a huge surprise to the user that they freak out and usually blame the battery for what really is their own fault, as they trusted that soothing blinking green light and the marketers of their controller/charging source..
Carrying huge amounts of battery capacity is a lot of weight. I once had 345AH total capacity, with only 130 watts of solar, and improper settings regarding absorption voltage and duration, and those batteries failed before 2 years,( first one shorted a cell at 13 months, being cycled shallowly.
I now have 198 watts of solar and only 90AH capacity total, and cycle much deeper, but do achieve absorption voltage and hold it as long as needed every cycle, and my 29 month old battery is still performing admirably, and I do work it hard.
But I utilize my alternator often for bulk charging. And I have a very capable grid powered charger too, and make use of that often too.
'Bulk' is when the battery can take everything the charging source can give it. When the battery voltage eventually rises to absorption voltage, the amount of amperage needed to hold absorption voltage declines, and continues to do so as this constant voltage phase is held. When amperage at absorption voltage declines to a certain level, then the battery can be considered fully charged, and then, and only then, should the charging source lower voltage to 'float' levels.
Beware of premature float voltages. Almost every automatic charging source/solar controller will revert to float voltage prematurely.
Most people assume that when their solar controller drops out of the absorption/acceptance stage of charging, this means the batteries are full. This is entirely false!! Dropping to float voltage means only that the controller held absorption voltage as long as it was programmed to do so. In might have been long enough, but 90% of the time it is not, especially on a hard working deeply cycled battery.
Trust if you feel you must, but confirm, with a hydrometer, or an Ammeter.
A programmable solar controller will pay for itself with extended battery longevity, assuming one actually uses the ammeter or hydrometer to confirm that their allotted time at absorption voltage is adequate. Hint, 2 hours is rarely enough on a deeply cycled battery!
It is much easier to achieve and then hold a lesser amount of battery capacity at absorption voltage, than it is a larger amount of capacity.
Many variables exist here, and much ignorance too.
Until the battery capacity declines to the point where it is noticeable to the casual user, everything appears 'just fine', and might be shouted from the hilltops in between chest thumps. Reality can be much different. The Lead acid battery (flooded/AGm or Gel) which gets 100% recharged nearly every cycle, can easily last twice as many total cycles as the battery recharged to only 95 to 98% each cycle.
It is the time held at absorption voltage that determines this, and getting from 95% to 100%, can take many hours as charging in this range is very inefficient. The less healthy the battery, the more cycles it has accumulated since the last ACTUAL 100% recharge, and the more time at absorption voltage is required. So if the controller is set for 2 hours, and 5 hours was actually needed, the total battery capacity will decline much much more rapidly, yet it can still be many months before the user notices the underwhelming performance( voltage held under loading), and many will only notice when stuff simply no longer works. At this point a hail mary true full recharge is usually attempted, but it is too little too late, and any possible regained capacity from an extended 'equalization charge' which is a forced overcharge' is short lived.
Another topic is the suitability of any particular battery for deep cycling. Most every flooded/wet 12v battery that proudly states' deep cycle' on the side of it, is mocking anybody who believes it. Beware of marketing. With AGM there is more of a blurry line. Gels are too finicky in their charge parameters for a Newb to consider.
A pair of 6v golf cart batteries, wired in series for 12 volts, is simply the best bang for the buck. Their ~230AH of capacity
is much easier to fully recharge, and more tolerant of less than perfect recharging, compared to 2 group 27 or 29 or 31 marine batteries in parallel for the same total capacity. The only area where 12v flooded batteries will outperform 6v golf cart batteries, is under large inverter loads, such as when using a microwave, as the 12v batteries will hold a higher voltage for longer and not sound the low voltage alarm on the inverter, as they are more closely related to a starting battery than to a deep cycle battery.
Regarding interior lighting, 12v LED all the way. I can't imagine buying disposable alkaline batteries or swapping out nimh or nicad rechargables every so often, unless one has no house battery and minimal charging abilities to begin with.
My best interior lights have been Automotive reverse lights in the t10 and t20 wedge type bases. I get the brightest ones I can find and use a PWM dimmer to tone them down. Some of these have been true junk though, too blue and too dim, but they have gotten much better as of late.
I want to try these next:
http://www.amazon.com/Philips-White...&qid=1458842669&sr=1-7&keywords=t+10+phillips
I have these already:
http://www.amazon.com/Philips-retro..._UL160_SR160,160_&refRID=0R6BGCANXHQ7675BA1EH
But in the 4000k warm white color, not 6000K which is more of a cool white. These are a few years old and not as good as newer offerings, but they produce the most lumens per watt of any LED I've utilized. Much cheaper LEDs exist, I've not tried them all, but many I have tried have been underwhelming and some are short lived.
I do make use of a nitecore hc50 headlamp which uses 18650 lithium cells, but rarely inside the Van.
