6 volt vs 12 volt battery systems?

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

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

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.

-Bill "just ignore me" B.