another st charger option

[Lloyd]

Lloyd,

I've missed the forum for the past day, so there was a lot to read.

Please remember, in a battery bank you can't go by amp-hours unless you're comparing the same voltage battery banks.   Like Bob says, you're comparing apples to oranges.   Amp-hours works great when you're at Walmart buying a battery for your truck:  they're all 12 V batteries.

You have to use kwh when comparing battery banks of different voltages.   Using the values in your last post as an example:

12 v bank:   6 batteries = 8.64 kwh     = $61.80 per kwh
24 v bank:   4 batteries = 5.76 kwh     = $61.80 per kwh
48 v bank:   8 batteries = 11.52 kwh    = $61.80 per kwh

See, cost per kwh is the same, unless you are getting a discount by buying batteries in volume.    The total potential stored energy in the banks is proportional to the number of batteries in the bank, not the voltage configuration of the bank.

Hi Marcus,

I think I was pretty clear about the kwh relation to a bat bank.

Bruce , don't get frustrated my noise will result in a more profitable signal.



Now to get any redundancy in the the 24 or 48 volt bank you have to dbl the cost of each choice. From here it doesn't matter what voltage bank we choose, all we need do is break down to actual wattage used(charge-discharge) to calculate, the true costs of the bat bank on kwh.

Just because the higher voltage has more available, doesn't mean we  are getting a value from that investment...much like the situation where the stand by ac gen is sized to the to the highest load it has to run as opposed to being able to sacle up and down to the actual load.

My whole point is to be true, we have to calculate over all efficiency...and that is in no uncertain terms, the total cost of the system as it relates to our actual energy usage.

Marcus,

maybe you can tell me how much a 48 v bat bank costs?

Lloyd

[mbryner]

I can only tell you about my 48 volt system (soon to be doubled in size):

8 x L-16 Deka batteries in series @ $300 each = $2400
370 Ah x 48 V = 17,760 wh or 17.8 kwh 
$134.83 / kwh

Of course these could be arranged in series/parallel at 12 V:

2 x 6 V in series = 12 V
Then 4 of those parallel.   Still 8 batteries.  Still same cost.
370 ah at 12 V x 4 parallel groups = 4440 wh x 4 = 17,760 watt-hr
Hmm, it's exactly the same total energy as in the 48 volt bank.

No arguing Ohms law with me.   I know what it says.   Done here.

[Lloyd]

8 x L-16 Deka batteries in series @ $300 each = $2400
370 Ah x 48 V = 17,760 wh or 17.8 kwh
$134.83 / kwh

Marcus,

How many kwh's are you burning per 24 hr period?

What is your generator run time for regeneration?

Why are you adding the duplicate bank?

I know you have a big PV planned, but lets leave that out for now.

Lloyd

[mobile_bob]

Lloyd:

correct me if i am wrong, i have been wrong before, just ask my ex!



amp/hrs vs kwatt/hr are two different terms all together

the former has no factor of voltage while the latter considers the voltage of a bank, that is what makes it an apples to apples comparison
irregardless of ohms law.


any time you try to calculate the true cost of a battery bank, you will end up in doing so in kwatt/hrs delivered to the load over the lifespan
of the battery, amp/hrs are not part of that equation at that level.

as an example lets work with 12volts first

i could buy a single 100amp/hr battery that is 12volts,  for say 100dollars  it can deliver 1.2kwatt/hrs over 20 hours
it is good for maybe half that if we only ask it to do 50% depth of discharge, so each cycle it can deliver us .6kwatt hours
at that rate it can do 1000cycles so
the battery over its life will deliver .6kwatt hours x 1000 cycles = 600 kwatt/hrs for the 100 dollar investment, or 16.7 cents per kwatt/hr
delivered to the load over its useful life.

now lets look at 48volts,  one of two ways, the first using the same battery as in the previous 12v example

we would need 4 such batteries, 4  twelve volt 100amp/hr batteries= a 48volt bank at 100amp/hrs, but that equals 4.8kwatt/hrs
48volts x 100amp/hrs = 4.8kwatt hours, now if we use this bank to 50% it too will last 1000 cycles and deliver 2.4kwatt/hrs each cycle
so 2.4 kwatt/hr x 1000 cycles = 2400kwatt/hour over its lifespan for our 400 dollar investment
2400 kwatt/hrs / 400 dollars = 16.7 cents per kwatt/hr... which is the same as  the 12volt example

so where is the rub? its a wash, except for two things

charging efficiency and do we need the added kwatt/hrs delivered each cycle

lets make things really apples to apples

if we engineer a system that only needs .6kwatt/hrs per day, then the 48volt battery bank has the capability of delivering 4x the daily requirement
so rather than increase the loading we stay with .6kwatt/hrs, and instead of using 50% of the battery capacity as we would in the 12volt example
we now use 12.5% of its capacity!

remember the non linearity of capacity as it relates to lifespan?

at a draw of 12.5% per day the battery will now last 3000 cycles, which is so

if the 12volt bank cost us 16.7 cents per kwatt/hr based on a 1000 cycle lifespan
and the 48 volt bank also cost us 16.7 per kwatt/hr based on a 1000 cycle lifespan, then
the increase lifespan in cycles reduces our cost per kwatt hour delivered to maybe one third
that of the 12volt system

now lets compare charging efficiencies, using an engine driven generator

in order to recharge the 12volt 50% depth of discharge bank will require about 25amps for our example
in charging capacity 25 amps x 14.4 volts = 360 watts / typical 12volt alternator efficiency of 54%= 667watts mechanical power

to recharge the 48volt bank at 12.5% depth of discharge will require about 6.25amps at 57.6volts or
6.25x 57.6 = 360 watts (same as above, but) 360 watts / typical 48volt alternator efficiency of 80% = 450 watts mechanical power

so the 12 volt system requires 667 watts mechanical and the 48 volt system requires 450 watts

667/450 = 1.48 or  in other words the 12volt systems fuel requirements are 48% higher than that required by teh 48volt bank.

to recap
the nonlinear relationship between depth of discharge and lifespan in cycles plus the decrease in fuel consumption to recharge
makes the use of 48 volts very attractive. the savings in fuel alone might well pay for the battery bank all by itself, and in any
event surely would pay the premium or added cost of a 48volt bank over a 12volt bank.

note: my numbers above are fictional, but non skewed, they are meant for illustrative purposes only. while some of the numbers
might well be higher or lower, the relationships will remain the same and the bottom line advantage will remain in favor of the 48volt
bank.

where am i wrong in this assessment?

bob g

[Lloyd]

Hi Bob,

Here it is:

   
Ohm's Law
   Ohm's Law defines the relationships between (P) power, (E) voltage, (I) current, and (R) resistance. One ohm is the resistance value through which one volt will maintain a current of one ampere.

( I ) Current is what flows on a wire or conductor like water flowing down a river. Current flows from negative to positive on the surface of a conductor. Current is measured in (A) amperes or amps.

( E ) Voltage is the difference in electrical potential between two points in a circuit. It's the push or pressure behind current flow through a circuit, and is measured in (V) volts.

( R ) Resistance determines how much current will flow through a component. Resistors are used to control voltage and current levels. A very high resistance allows a small amount of current to flow. A very low resistance allows a large amount of current to flow. Resistance is measured in ohms.

( P ) Power is the amount of current times the voltage level at a given point measured in wattage or watts.[/b]
Now with this equation, I can determine the watts if I know the volt and amps, or I can determine the amps if I know the watts and voltage, I can even determine the voltage if I know the watts and amps. Now that I can get an answer for watts I can get a kwh.


I fear this is degrading, and going nowhere fast.

My point all along is something you know very well.......make all you decisions based on test results.

I am just using economics as my test bed.

Bruce, and you agree that 12v makes a good small system voltage, I also agree.

We all agree that 48 volts is the best voltage in large systems. Maybe even higher voltages...I think that won't happen until we get better, and more affordable bat tech.

What I was trying to drill down to, is at what size kwh system, does it makes sense to change from 12 volt to 24 volt to 48 volt? Not based on the factor of higher voltage being more electrically efficient...bc in all cases 48 volt would be the choice.

So the only way to test over all system efficiency, is to determine the cost per kwh, against the cost to build the system, and arrive at the true costs per kwh. It's hardly arguable that as you increase system voltage, that it also increase system costs.

The question is at what point does the higher efficiency of 48 volts systems, amortize the increased system costs.

Lloyd

[mobile_bob]

Lloyd:

i understand ohms law about as good as the next guy 

my point with my dissertation is simply this

if you are going to have to recharge with an engine driven genset, 48volts will win out over 12 in "all" cases, even small systems.

while it may well cost more for the 48volt bank it really need not be dramatically more, because

i left out one more option in my analysis

that being the use of smaller lower capacity 12volt batteries to make up the 48volt string

while the ratio is not quite linear, a 25amp/hr battery will be dramatically less expensive than a 100amp/hr battery

so in the end you would be faced with using 50% depth of charge just as with the 12volt example, the addional cost would
not be 4x such as in the previous 48volt example, so you would not get the advantage of shallower cycling and the battery
life would be similar at 1000 cycles, but

you would still be more efficient in charging, enough so to pay for the bank of batteries in fuel savings within the lifespan of the batteries.


now having made the case for 48volts, a similar case can be made for 24volts as well, and

there is still a place for 12volts for certain applications, but there will have to be much research and development to come up
with a means of engine driven alternator to get the efficiency of either a 24 or 48volt system,, this much i know probably as much about
or more than most anyone you want to talk to.

i have put a lot of years in research and then more years in hands on research and development of alternators for high efficiency charging
and 12volt units albeit a long and reliable history, are not very efficient compared to what can be done at higher voltages.

it maybe that from your viewpoint and experience with 12volt systems, you see things from a completely different angle than i am because i am viewing the issue from the engine driven alternator viewpoint

you are looking at first cost of installation, and amortization of that bank 12v vs 48

i am looking at how many gallons of fuel at X dollars per gallon will an engine driven genset burn over the life of the batteries 12v vs 48v
and factoring that against first cost and amortization of the banks 12v vs 48v

to be fair i have to admit that it is possible to get the efficiency of an engine driven alternator up to around 75 plus % for 12volts, i just have
not seen anyone working on getting it done, and for now i have little interest in exploring that avenue myself.

although i would be happy to help in that area, via some idea's and discussion

i am working now on the threephase transformer pack and remote rectifier for 12volt battery charging, and am going to be setup to do relative
efficiency testing this weekend,  while i have no real need for such a system, i do have an interest to see if it can be done and to find out
what can be done to increase efficiency.

believe me, a 12volt system would be nice, and i would prefer on if i could get the overall efficiency up to anywhere close to what i can do with 24 or 48volts.

bob g

[Lloyd]

Bob,

Just for the record I agree with you.

Redundancy is an issue, so to have that in a bank we need at least two parallel groups of system voltage. This is good  for life cycle of the bats as well, it also in most cases covers the Peukert factor, that most don't think about.

As far as bat choices...I don't know of any bats, in the 100 amphr range that I would want to depend on. To me a conventional lead acid wins out over AGM, bc of costs, even with the more efficient charge characteristic.

All this no matter what voltage we choose.

The cheapest entrance into a well tested bat is the trojan t240 no matter the voltage choice of the system.

And again, in other threads you have hit the nail on the head. Usage on an off grid system has to improve dramatically. If we look at the economics of a true off grid solar/bat/gen. We would never choose the off grid bc we could never recover the investment, bc there is flat out no way we can make electricity cheaper then we can buy it from the grid. At the same convenience factor.

Now if we can scale back system costs bc we change our usage pattern, by conservation, more efficient equipment, we lower our over all investment, which lowers our cost per kwh produced.

Lloyd

Bob, I do thank you again for all you have done in testing my proposition on the 12v system.... Now if you would just send me your shipping address, and the specs, and schematic on that diesel cat....I have a present for you.


And here is an excellent online ohms calculator http://www.the12volt.com/ohm/page2.asp#9

[mobile_bob]

here is a link for the cat

magnaflow 54014D

http://www.car-sound.com/02product/displayuniversal.asp?universal=54104D

bob g

[Cornelius]

Now, i haven't read the last replies scrupulosly, but do have cable sizing and stretch in mind when choosing DC voltage.

[mobile_bob]

painful???

painful??

whatcha talkin about willis?



sometimes it is out of what some would call a painful discussion/debate/arguement that something totally unrelated comes to light and is helpful
to others.

just taking folks assertions on faith, is what got us into AWG, carbon trades, cap and trade, and now the epa regulating CO2 as a dangerous gas.

you guys think this has been painful, try following the global warming is a hoax over on the LEF.

just kidding, i like to argue so it isn't fair 

i often forget most folks don't like confrontation and heated debates, and find them painful and tedious.

guess i better keep that in mind!

(what am i saying???)

NAH,,, not me!



ok, back on topic

this whole debate of voltages has led to some interesting research that i find interesting anyway.
and it centers around the common need for massive cables for 12volt power transfer, the goal
of all the testing is to see how much power can be transmitted over a significant distance using
10/3 cable instead of dual 4/0 positives and dual 4/0 negative cables

it may prove to be helpful to others down the road?

bob g

bob g