6 volt vs 12 volt battery systems?

fishrus
fishrus Registered Users Posts: 5
I have a houseboat that I am upgrading the battery system on. I don't have the reserve capacity in the current system that I would like to have. I have heard some people say that a 6 volt battery system is far superior to a 12 volt system when it comes to power storage. Can someone explain the difference to me or point me to a place to find the answer?

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    no, they aren't superior or inferior to each other. simply, a 12v battery winds up being extremely heavy for the same amount of capacity as the 6v counterparts are and so it is easier to lift 2 100lb 6v batteries than 1 200lb 12v battery. one could take this down to the comparison of 6v batteries and 2v cells as well. it's just easier to carry is all.
  • bryanl
    bryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    If you look at specs and measure, you'll find the 'true deep cycle six volt golf cart' hype to be bogus.

    For this class of batteries, available usable (cost effective) capacity is about 10 to 15 watt hours per pound of battery and that is not voltage dependent - it is a characteristic of modern, wet cell, lead acid batteries of somewhat portable size.

    The life you will get from your batteries depends mostly upon how you use and maintain them, not on their voltage.

    Keep in mind that batteries just don't store much energy and that factors such as temperature, cycle to cycle variation, age, and use profile can each impact available energy by 10% or more. What that means is that minor changes to your bank are not likely to have much impact on the end result.

    The general rule of thumb is to have enough battery to last a weekend - adjust either bank size or usage as needed. Then provide at least one watt of solar panel for each pound of battery to help keep things charged. Then provide for a backup energy source to help provide optimum charging and to help out when the sun doesn't.

    A lot of folks have very very strong misconceptions about batteries and many supposedly 'authoritative' sources need a bit of skepticism as well. To avoid getting scammed, try to stick to objective measure that the manufacturer or retailer will stand behind, things such as spec sheet data, cost, and warranty. Interpret measures carefully keeping mind the pertinent variables.

    Watch out for folks who use poorly defined terms (e.g. "deep cycle battery") and make claims about brand or whatever based on very small sample sizes.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,398 admin
    Re: 6 volt vs 12 volt battery systems?

    I think there is a disconnect here...

    There is battery bank voltage (typically 12, 24 or 48 volt--older cars had 6 volt, and some boats at 32 or 36 volt battery banks).

    From a simple energy storage point of view--Remember Energy=Volts*Amps*Hours (time).

    If you double the voltage and 1/2 the current--you still have the same amount of energy storage...

    For example, 2x 6 volt 225 AH "golf cart" batteries... You can connect them for a 6 volt bank (two batteries in parallel) or the two batteries in series (12 volt bank). Parallel connections add Amp*Hour capacity... Series connections add Voltage:
    • 6 volts * 2x225 AH = 2,700 Watt*Hours of storage at 6 volts
    • 2x6 volts * 225 AH = 2,700 Watt*Hours of storage at 12 volts
    So--If you have 400 lbs of batteries, and want to reconfigure them from a 6 volt bank to a 12 or 24 volt bank, there is no difference in the amount of raw energy stored/available.

    However, there are reasons why larger systems are higher battery bank voltage... And that is simply the amount of current flow to support a load. And if you (for example) double the bank voltage, you have 1/2 the current (smaller switches, wires, fuses, etc.).

    Say you have enough room (and money ;)) for 4 standard sized storage batteries. And you can get either 6 volt or 12 volt batteries in that size.

    You have a choice of connecting 4x 12 volt batteries in parallel. Or 4x 6 volt batteries in series parallel (2 in series, then connected into two parallel for a 12 volt battery bank).

    Several issues... One is that batteries must be connected in parallel "correctly" with balanced paths (equal resistance). If you don't do this correctly, one battery may carry all of the charging/discharging and the others will do much less work (and one battery will probably wear out before the others). Here is a web page that describes how to make the connections:

    Smart Gauge Battery Bank Wiring

    Another issue is that paralleling a bunch of batteries makes "current sharing" and balancing more difficult. And a weak/failed battery/cell can drain the rest of the bank (and is very difficult for you to notice the problem). Personally, I don't like to recommend more than two or three parallel connections.

    Plus, every parallel battery string should have its own over current fuse/breaker... 200+ amp breaker/fuses/wring is not cheap or easy to do.

    Lastly, if you have flooded cell batteries--4x 12 volt batteries in parallel is 24 cells to check water level in.

    2x 6 volt cells (next size larger battery, just two 6 volt in series) and you have just 6 cells to check water levels.

    And--that does not get into the main bank voltage discussion either... 12 volt is very common (12 volt DC car equipment/cigarette lighter adapters/etc. are everywhere). And there is some 24 volt items too (truck/boating).

    But, for an off-grid system, you also need to look at the total power needed... For example, a 1,200 watt inverter, worst case, would need to have branch circuit designed to handle this amount of current:
    • 1,200 watts * 1/10.5 battery cutoff * 1/0.80 efficiency * 1.25 NEC wiring and fuse derating = 179 amp circuit
    So, you are looking at fuses/switches/breakers/wiring to support ~200 amps continuous current flow and do it with only ~1.0 volt drop maximum.

    Above 1,200-2,000 watt loads, you should be looking at a 24 volt minimum battery bank. And above 2,400-4,000 watts, you should be looking at a 48 volt battery bank.

    Since this is a house boat--if the Boat's electronics/starting motor is 12 volts--you may wish to keep the living space battery bank at 12 volts too--so you can use the generator on the engine to recharge both sets of batteries (use a battery isolator or A+B type switch). Gives you redundant battery banks with the flip of a switch (i.e., starting bank fails, just switch in the house battery bank to get going again).

    Sorry--lots of stuff here. Questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    Well I've never heard of a 6 Volt power inverter so I'd presume the OP was wondering about the advantages/disadvantages of using two 6V batteries vs. one 12V. One characteristic that I think was missed is that it may be two 6V vs. two 12V for the same Amp hour capacity, in which case the advantage of the serial connection for the 6V in keeping current flow even can be significant.

    For example, if the choice is between two 100 Amp hour 12 Volt batteries in parallel or two 200 Amp hour 6 Volt batteries in series, the 6 Volt batteries would be the better choice because you're not trying to balance current flow through parallel wiring, which is often tricky.

    The bigger the system, the greater the advantage to serial connections - and higher system Voltages if possible.
  • fishrus
    fishrus Registered Users Posts: 5
    Re: 6 volt vs 12 volt battery systems?

    I'm learning alot here from you folks, Thank You so much! I see that I didn't completely describe the situation. The battery bank is for the house only. I have two ~175 watt solar panels hooked to the charging controller. There is a generator charging backup. Currently there are two 12 volt batteries wired parallel (I'm not sure of the AH rating). I'm sure that the batteries are part of the problem because they are older. I guess my question is, if I am going to replace all the batteries and start over, how can I get the most reserve capacity out of two batteries and have a setup that is worth spending the money on but not breaking the bank?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    Ah, tricky. The thing about RV applications (as opposed to a fix location install) is you never know quite how good the insolation is going to be. Under most circumstances you could do some basic calculations like this:

    two 175 Watt panels = 350 Watts @ 77% efficiency = 269 Watts
    269 Watts / 14.2 Volts charging = 18.9 Amps which would be 5% (min) or 378 Amp hours or 13% (max) of 145 Amp hours.
    269 Watts over "4 hours equivalent good sun" would yield 1076 Watt hours per day.

    Given that, a couple of the 6 Volt T105's @ 225 Amp hours would fit, the 18.9 Amps being 8% charge rate and the 1076 Watt hours being roughly 89 Amp hours or 40% of the capacity maximum.

    But there are of course other options.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,398 admin
    Re: 6 volt vs 12 volt battery systems?

    I would suggest we start with knowing your loads... How many Amp*Hours (at what voltage) or Watt*Hours or kWatt*Hours do you use per day. And what is the peak current / watt load (i.e., 100 watt TV plus a few lights, 1 hp sump pump and 1,500 watt microwave running 2 hours per day or what)?

    Usually, knowing your loads (and starting with lots of conservation) is usually an easier starting point.

    Once we know that, we can size out your system (solar panels, charge controller, battery bank, inverter, wiring, etc.) and see what can be salvaged from your current system.

    If you are currently using 120 VAC AC--A kill-a-watt meter is a great place to start.

    Otherwise, we can size the equipment to your existing solar array--but that still leaves us short in knowing your peak loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    Bill,

    If I'm reading this right the OP is replacing batteries in an existing RV application. I'd guess his panels are fixed at the two 175's and there's already an inverter which can handle whatever loads he has, plus there is a generator back-up. There are probably space limitations for the batteries as well; tucked into some compartment. Under those circumstances it comes down to "what's the best way to fit the most Amp hours into the space available?" - with all its associated problems.

    Which brings up an issue I completely and stupidly missed: are these batteries inside the living quarters or outside? If inside, then choosing from available AGM's becomes important because you don't want batteries off-gassing in the house, so to speak.

    I was just trying to show the most Amp hours the available panels were likely to be able to support. When you've got built-in gen back-up load considerations become less critical.
  • fishrus
    fishrus Registered Users Posts: 5
    Re: 6 volt vs 12 volt battery systems?

    You guys are good. The batteries are outside. The boat is on Lk Powell in Southern Utah where high temps are in the 100s F during the summer and there is no lack of sun. The load on the battery bank is actually pretty minimal, I feel. No TV, microwave and all appliances are 110 v on the generator only, no inverter. All lights are 12 volt flourescent or whatever they put in RVs that have round bulbs and maybe 5 run at the same time in the evenings. The marine radio and car stereo system with no amp are the biggest draw and run most of the time until sleeping. Water pump is 12 v on demand for sink, shower and toilets. The boat was a rental that we purchased and are trying to fix and upgrade. We installed the solar panels to help charge the batteries as the only other charging mechanism was the generator or the outboard rectifiers that were disconnected because they are expensive to replace when you fry one from over use. From what I learned from this website about batteries, I'm sure we have a battery issue, old and new mixed and a mixture of wiring to hook them up in parallel. I want to rebuild the system to solve the low voltage situation in the late evening everyday. Space is an issue as the battery box can just hold 3 pickup truck size batteries. I can modify it if I need to, but that is where I need help. What size do I expand to or is this space enough and I just have to buy the right batteries? Some of the boats on the lake don't have any power issues and also power a small house chest freezer on an inverter with less solar panels than I have with no generator. These guys were the ones telling me about the 6 volt battery system they are running that handles this and that is all the info I have as I never looked at what they had on the boat. I want to duplicate this in the most cost effective and efficient manner.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    If you haven't read these I suggest that they are very informative.

    http://www.batteryfaq.org/

    http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

    I would also try to get a good realistic handle on your loads. Most people over estimate the amount of power they can actually harvest, and at the same time underestimate their loads. For example, you lights might be incandescents and they might draw way more than you think. For example, a simple 12 volt reading light like they put in RVs have a brake light bulb, and can draw better than 15 watts. 5 of those burning for 4 hours would 300 watt hours. The marine radio probably draws about 15 watts and the car stereo probably burns another 15 or 20, so running those two for 16 hours/day might be another 480 wh. Add in the pump and you could be over 1kwh per day.

    Just for comparison, we live with 400 watts of fairly ideally oriented panels feeding into 450 ah of t-105 batteries. We routinely draw ~6-800 wh/day. On an ideal day, we can generate 1-1.5 kwh of power, but on an ongoing day in day out basis we generate just about the 6-800 wh/day.

    Tony
  • bryanl
    bryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    re: "there are reasons why larger systems are higher battery bank voltage" -- the context was fairly clear in the OP, I think, and that was not for "larger systems" but rather for 'portable' systems which are usually well under 10 KwH energy storage.

    As for the current balancing, that is indeed an important issue but it also can get rather over-inflated in this context. I'd posit that the usual use profile - in this context - has a rather low average current (below the 20 hour rating for the batteries) with occasional, rather large (10x 20 hour rate), peaks. With lower currents (both charging and discharging), the current imbalance problem is a minor concern. With large current peaks, the Peukert effect becomes an issue favoring parallel arrangements.

    As for AGM's and indoor - the code does not discriminate and requires proper battery boxes with proper ventilation without regard to wet cell or AGM.

    As for the number of cells and individual cell maintenance - I don't think that is an issue in this context. Many reasons from bank size to Hazmat needs to utility to requirements for proper charging and maintenance equipment to cycling patterns.
  • hillbilly
    hillbilly Solar Expert Posts: 334 ✭✭
    Re: 6 volt vs 12 volt battery systems?
    bryanl wrote: »

    As for the current balancing, that is indeed an important issue but it also can get rather over-inflated in this context. I'd posit that the usual use profile - in this context - has a rather low average current (below the 20 hour rating for the batteries) with occasional, rather large (10x 20 hour rate), peaks. With lower currents (both charging and discharging), the current imbalance problem is a minor concern. With large current peaks, the Peukert effect becomes an issue favoring parallel arrangements.

    Curious what you mean by all this. I've always thought that a parallel arrangement would have greater potential for imbalanced charging/discharging currents. At larger currents how would the Peukert effect favor parallel vs series battery interconnections?
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    i think what he's saying is that the small currents drawn will not play much of a part in creating large imbalances and the high current draws are brief enough not to worry about in a parallel arrangement. series batteries are not going to have a balancing problem at all as any excess resistance will affect the entire bank fairly equally in its performance.

    as to agm batteries needing a vented box, i do believe the code goes too far if that's the case. this is like requiring a fire truck to follow your car because it could possibly catch on fire. let me guess, wiles?:roll::cry:
  • hillbilly
    hillbilly Solar Expert Posts: 334 ✭✭
    Re: 6 volt vs 12 volt battery systems?

    I think the idea that smaller currents are much less of an issue for batteries to balance out, but the last part of the sentence seems to be saying something more entirely:

    "With large current peaks, the Peukert effect becomes an issue favoring parallel arrangements."

    ???
  • BB.
    BB. Super Moderators, Administrators Posts: 33,398 admin
    Re: 6 volt vs 12 volt battery systems?

    Peukert's Law:

    t=H(C/(I*H)^k
    • H is the rated discharge time.
    • C is the rated capacity at that discharge rate.
    • I is the actual discharge current.
    • k is the Peukert constant, dimensionless.
    • t is the actual time to discharge the battery.
    Some known values for k:
    • Trojan T-105 = 1.25;
    • Optima 750S = 1.109;
    • US Battery 2200 = 1.20.
    Since k is an exponent and its values are >1, then higher values of current will have greater than a simple 1:1 effect on available amp*hours

    Now does the higher current depress battery output voltage, causing a negative feedback that reduces the spread in shared currents--Very possible. The miss-match of R (wiring resistance) is linear (1:1) and the Peurkert factor is non-linear--But with low values of k--the effect may not be useful at "normal battery loads":

    Making numbers up: R may be off by 10-20% in a well designed system... The ratio difference with k=1.2 with a 2:1 unacceptable mismatch in current:
    • (10 amps / 20 amps) ^1.2 = 0.435
    • vs 0.5 linear ratio:
    • 0.435/0.5 = 0.87
    Do the same thing with 10 amps vs 12 amps miss-match in current:
    • (10/12)^1.2 = 0.803
    • 10/12 = 0.833
    • 0.803/0.833 = 0.96
    Or another 13% net effect vs a 10-20% resistive effect when you have a 2:1 current mismatch... So the effect does not look to be large with respect to what I would expect to R value differences in a typical wiring system...

    And for an "acceptable" 20% (1.2:1) current mismatch--the difference is only 4%--I would expect that to be swamped by R mismatches in wiring (and battery construction).

    So the negative feedback effect of the Peukert numbers for typical maximum acceptable difference in current flow (using 20% difference as acceptable and 2:1 difference as unacceptable) and assuming that the Peukert effect can be modeled as a resistive element (i.e., battery voltage is depressed as if there was a "Peukert" variable resistor in the circuit) then I would say the Peurkert effect could be ignored for lead acid storage batteries.

    If the Peukert effect only affect battery capacity and cannot be modeled as a non-linear resistive element (i.e., is a chemical conversion loss of some sort that does not appear as a depressed output voltage)--then the Peurket effect would have zero effect on instantaneous current sharing.

    One could take this further in that battery capacity is roughly equivalent to resting voltage--One would assume that operational voltage under load would be affected by actual state of acid specific gravity. Roughly 12.7 volts full charged vs 11.6 volts 20% charged.

    If one battery string is being discharged faster than its neighbor, then its faster falling specific gravity would cause voltage depression... If we assume that the difference between a full and empty battery is ~1 volt (for a 12 volt battery).... That is roughly the maximum supportable voltage drop of 12 volt wiring.

    And since the difference due to resistance in the voltage drop would be on the order of 20% due to resistive mismatches (again, just my experience in electronics)--I would guess that you could see a 20% difference in battery state of charge (under heavy loads) that could provide a 20% negative feedback to a resistive mismatch...

    So--battery state of charge differences of 20% between strings would match a 20% resistive mismatch under heavy loads.

    At lighter loads, the specific gravity effect remains the same, but the overall voltage drop differences due to reduced current flow (say 10% average current of 20% mismatch would be 0.02 volt mismatch due to resistive losses) would be much less than the effect--While the 20% or 0.2 volt mismatch of unequal specific gravity would be a dominate factor...

    So, at lower current flows, I would hypothesize that specific gravity differences will force parallel strings back into sharing and overwhelm a typical worst case parallel wiring mismatch.

    The above is not very rigorous--But seems to give some useful results...

    Peukert factor is not a dominate factor in guiding current flow through a battery bank... It is either equal to, or much less than typical resistive mismatches. So for heavy loading, make sure your cables match between strings and the connections are clean and tight.

    For low current flows, specific gravity has much greater effect on forcing proper current sharing than either Peukert or Harness Resistance--So matching battery temperatures across the bank and having equal specific gravity fills (100% charged resting s.g.) is important.

    Of course, I could be all wet with the above analysis. :blush:

    -Bill "just ignore me" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    Something else to consider:

    Two 6 Volts: one negative cable, one positive cable, one cable between batteries.
    Two 12 Volts: one negative cable, one positive cable, two cables between batteries.

    Thus the 6 Volt batteries again are the better choice. The current flow issue is real and only ignored by the foolish, even on such a small install. As for the Peukert effect, this is moot because the 6 Volt and 12 Volt batteries are unlikely to have the same factor.

    No one has to agree with me or follow my advice. Do as you like. It's not my money you are spending. :p

    But I've a pretty amazing track record of being right about oh-so-many things for 50 years. :-)
  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    Marc
    It's always easy to say you're always right...if, like me, you ignore the attempts that failed before the ''right'' attempt worked! :p:-)

    Be sure to take the above with the humour it was intended to provide!

    Ralph
  • bryanl
    bryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: 6 volt vs 12 volt battery systems?

    On the one hand, sorry for being a bit vague or unclear, but on the other, it certainly generated some interesting ideas to think about. (and a need for some caution about over-engineering or going overboard on precision that exceeds any possible accuracy)

    As for the final arbiter with the only way to wire things - my banks aren't wired that way ...

    In regards to the serial vs parallel thing, consider why most diesel pickups have starting batteries in parallel. Or maybe think about calculating circuit impedance in parallel vs serial circuits.

    As for current balancing, keep in mind Kirschoff's laws, battery voltage (not SG), charging patterns, and use profile. The analogous issue is that of dissimilar batteries in a bank. Give the batteries half a chance and they'll even things out themselves.

    The Smartgauge.com discussion is very good but you do need to keep in mind what he is comparing and the conditions assumed - and it isn't serial vs parallel with highly variable loads. Like always, critical reading is needed when it comes to technical issues.

    And I'd always suggest strong skepticism when it comes to strong advocacy for technical issues that have many variables involved. Anyone saying there is only one best way in such circumstances is often missing something.
  • fishrus
    fishrus Registered Users Posts: 5
    Re: 6 volt vs 12 volt battery systems?

    I want to thank everyone for helping me work through this problem. There is alot of good info here. You folks have definately pointed out the flaws in the current system that are most likely the problem.

    So, if I understand this right, if I can fit a maximum of 3 batteries into this system, I can get the most AH storage out of 3 - 12 Volt batteries hooked up in parallel and the 2 - 175 watt panels should support the system, correct?

    I see that I need to pay close attention to the wiring to help balance out the batteries during charge and discharge.

    What do you folks recommend for battery type in this situation, taking into account the usage and the high temperatures?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 6 volt vs 12 volt battery systems?
    fishrus wrote: »
    I want to thank everyone for helping me work through this problem. There is alot of good info here. You folks have definately pointed out the flaws in the current system that are most likely the problem.

    And I'd like to point out that what appears to be disagreement amongst posters is really difference of opinion; several solutions are possible to any given problem, and each of us has a favourite based on personal experience. :D
    So, if I understand this right, if I can fit a maximum of 3 batteries into this system, I can get the most AH storage out of 3 - 12 Volt batteries hooked up in parallel and the 2 - 175 watt panels should support the system, correct?

    Yes, and then again no. In terms of filling up the battery box, with "room for three" then three 12 V is the best. But then again, what capacity those batteries are enters into the formula. If the total exceeds the panels' ability to charge then you'd probably be better off reducing to two. Whether that be two 6V or two 12V is debatable; you could make the choice on availability and/or actual capacity of the batteries. Id est: two 6's might have slightly more Amp hours than two 12's and yet not be over the panels' limit, or it could be the other way 'round.
    I see that I need to pay close attention to the wiring to help balance out the batteries during charge and discharge.

    Yes. Especially with three 12V in parallel.
    What do you folks recommend for battery type in this situation, taking into account the usage and the high temperatures?

    There are many good brands. It comes down to what will fit your electrical needs, your space, your budget, and is available to you. Batteries being heavy, shipping them from far away gets expensive fast. So look locally first.
  • fishrus
    fishrus Registered Users Posts: 5
    Re: 6 volt vs 12 volt battery systems?

    Maybe my question needs to be how big and what type of a battery bank will 2 - 175 watt solar panels efficiently operate under standard operating conditions?

    Somewhere, I thought I saw reference to rule of thumb "1 watt for 1 lb of battery" for roughly sizing solar panels. I do understand now all of the variables that influence this , but is this a good estimate for normal cycling of a battery bank?

    Let's say I put a 40% daily draw on 3 - 12volt batteries, will the panels keep up with charging this bank under standard operating conditions or am I better off going with 2 - 12 v or 2 - 6 volt.

    I guess by type I am looking for wet-cell, gel cell, AGM?

    Any insight?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 6 volt vs 12 volt battery systems?
    fishrus wrote: »
    Maybe my question needs to be how big and what type of a battery bank will 2 - 175 watt solar panels efficiently operate under standard operating conditions?

    Thought I already answered that. Maybe not. 350 Watts of panel will,under most circumstances, be good for up to 380 (5% minimum charge rate) Amp hours. I personally recommend "shooting for" the 10% rate, which would be 189 Amp hours. As mentioned before, the two 6 Volt units would give you 225 Amp hours and a peak charge rate of 8.4%
    Somewhere, I thought I saw reference to rule of thumb "1 watt for 1 lb of battery" for roughly sizing solar panels. I do understand now all of the variables that influence this , but is this a good estimate for normal cycling of a battery bank?

    Yes you probably did read that somewhere. I'd ignore if it I were you. It doesn't give the same results as the tried-and-proven 5%-13% of the "20 hour" Amp hour rating that is the industry standard recommended by every battery and inverter manufacturer as well as many knowledgeable people who have extensive experience with systems. :roll:
    Let's say I put a 40% daily draw on 3 - 12volt batteries, will the panels keep up with charging this bank under standard operating conditions or am I better off going with 2 - 12 v or 2 - 6 volt.

    The panels will keep up a bank sized as mentioned above, providing it is not drawn down too much. In terms of Watt hour harvest, you might expect to reap about 1 kW hour per day. That would be around 83 Amp hours @ 12 Volts, or 37% DOD on the 225 Amp hour batteries I mentioned. You could run a slightly larger bank and keep the DOD % even smaller. For instance L16's @ 320 Amp hours: a charge rate of nearly 6% (kind of minimal) and a DOD of 26%. But those bigger batteries really want a higher charge rate. This is where considering three 12 Volt units at around 100 Amp hours each would be good.
    I guess by type I am looking for wet-cell, gel cell, AGM?

    Any insight?
    I'd skip on Gel cells. Use AGM's if they are going to be in "living quarters" (no gassing). Otherwise the Flooded Lead Acid is usually the cheapest in terms of Amp hours per $.

    Others will have their own opinions. You get to pick from among them. :p

    You mentioned earlier that some others have recommended the 6 Volt systems. Take a look at what they've got if you can. There's no substitute for practical success! If their set-ups work good, go for it.