6- vs 12-volt

Welcome to the forum JS,

If the choice is to make a very large battery bank from "smaller" batteries, I would prefer two 6 volt @ ~200 AH batteries in series to make a 12 volt battery, then paralleling those--Vs just getting 12 volt @ 100-200+ AH in parallel.

The simple reasons--A) 6 volt @ 200 AH batteries are very common "Golf Cart" batteries and tend to be cheaper and a bit more rugged (larger and possibly thicker plates) that an equivalent 12 volt @ 100 AH battery. 3x higher amperage cells in series for 6 volt, vis 6 smaller cells in series for 12 volts--Both battery store the same amount of energy (6v*200AH=12v*100AH=1,200 WH of storage).

B) It is very easy to use a voltmeter and measure the voltage across each 6 volt battery to see the basic health of the battery bank. You will find batteries with high or low voltage, and can look deeper to see what is "wrong". With a bunch of 12 volt cells in parallel--They all have the same voltage and you have to do other things to find weak/failing batteries (measure specific gravity, use a DC current clamp meter to see how charging/discharging current is being shared, disconnecting a lead to each battery and measure the disconnected/resting voltage).

I am not a fan of making a battery bank out of 5-6 parallel strings of batteries--And I would be looking at something else (larger AH batteries or cells) and trying to get the number of battery strings down to 1-3 strings in parallel as a maximum (save on wiring connections and things that can go wrong).

I would also be suggesting looking at your loads and energy needs. Battery banks over ~800 AH, it is usually a good idea to step up to the next higher battery bus voltage... I.e., a 12 volt @ 1,200 battery bank becomes a 24 volt @ 600 AH battery bank (now 4x 6 volt @ 200 AH in series for 24 volts, and 3x parallel strings for 600 AH at 24 volt battery bank). And you will be down to 3 parallel strings.

Inverter wiring is now 1/2 the size, and voltage drop is less of an issue with 24 volts vs 12 volts. If you are using solar chargers (not sure--cabin in the woods may mean too much shading for solar), solar chargers are rated at current output... i.e., 60 amps at 12 volt = ~720 Watt array and 60 amps at 24 volts = ~1,440 Watt array support (same controller, just higher bus voltage).

You talk about time between charges and health of batteries? You run a genset to recharge your bank?

If so, this gets into you want to recharge your battery bank back over 90% state of charge, and come back within a month to run the genset again. FLA batteries typically can go only ~1 month between charging or they begin to sulfate and "die".

If you have a float solar array (1% to 2% rate of charge) and some decent sun (and cool weather), that may be able to float your battery bank between visits (recharge with genset before you leave, then float on solar).

By the way, how long did this bank last, and did it meet your needs? Would a propane refrigerator work, and allow you to reduce your electrical needs? For a weekend/seasonal cabin, sometimes propane makes more economic sense than a battery/solar/genset system.

I will stop here--I am not sure how far you want to go and review your present system and design.

-Bill

Yep, 6 volt is better value:

• 2x 6 volt @ 200 AH in series gives 12 volt @ 200 AH for $120 for the pair
• 2x 12 volt @ 100 AH in parallel gives 12 volt @ 200 AH for $260 for the pair

Just an "extra jumper" to series connect the two 6 volt batteries in 12 volt string -- Also, now 1x series string (6 volt batts) vs 2x parallel (12v batts). I am a big fan of reducing parallel strings. With 6 volt @ 200 AH batteries, you will have 1/2 the number of parallel connected strings.

Just to give you an idea of what a "balanced" system would look like... A 12 volt @ 1,200 AH battery bank would suggest a 3kWatt maximum inverter--But for 12 volt battery banks, it is difficult reliably get more than ~1,800 Watts because of 12 volt bus/wiring voltage drops.

We recommend 5% rate of charge minimum, and 10% to 13%+ for solar charging. 5% for weekend/summer cabin. 10%+ for full time off grid usage (Note that 1,200 AH @ 12 volt battery bank is the same storage capacity as 600 AH @ 24 volt battery bank--So the Array calculations are the same (p=v*I, 2xV and 1/2xI -- equals same power):

• 1,200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,230 Watt minimum array
• 1,200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,260 Watt nominal array
• 1,200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.15 rate of charge = 2,938 Watt array "cost effective" maximum

In general, more solar panels, less generator usage. Deep Cycle flooded cell lead acid battery mfg. typically recommend 10% minimum rate of charge for best battery life.

In general, weekend cabin batteries are generally going to age out before they cycle out of life. Avoiding a bunch of deep cycle to 20% state of charge, storing batteries >75% state of charge, and keeping batteries cool (for every 10C/18F over 25C/75F, batteries will age 2x faster--1/2 life).

Cheap golf cart batteries typically last 3-5 years. And good quality may last 7-8 years (assuming something else does not go "wrong").

Not sure where the cabin is located, but assuming not used in deep winter, you will probably get 4-5 hours of sun minimum per day (assuming you are up north). A 1,230 Watt system would output (on average):

• 1,230 Watt array * 0.52 off grid AC system eff * 5 hours of sun per day (sunny summers) = 3,198 Watt*Hours per day

That is enough power (and you have a large enough battery bank) that it should run your refrigerator+other loads quite nicely for non-winter occupation... Of course, you have to have the sun and no sunny area.

If I found the correct inverter, this is an MSW (modified square/sine wave) inverter.

http://powerbright.com/manuals/PW3500-12_MANUAL_english_french_02.pdf

If it goes bad, or you think about going 24 volt volt battery bus--I would be suggesting a 1,500-2,000 Watt PSW (pure sine wave) inverter--MSW inverters are hard on induction motors (refrigeration compressors) and some electrics (computer AC power adapters, and even some chargers for tool batteries) can cause early life failure too.

A 3,500 Watt 12 volt inverter (and remember it supports a 7,000 Watt surge) would draw:

• 3,500 Watts * 1/0.85 AC inverter eff * 1/10.5 volts battery cutoff = 392 Amps at rated load (and 2x that at 7 kWatt)

That inverter has some pretty heavy/short cabling and heavy currents for this size battery bank (AH and voltage).

-Bill