Battery Bank sizing

larrybc1 · November 2012

Hi,

I have a solar installation in the Phils and need some advice on the battery choices. I am off the grid and I have 25 - 210 watt evergreen panels and 2 wind turbines. I anticipate using about 8000-9000 watts per day--max may be less, and want to have a couple of days of autonomy if I can. The system is 48 volts and I have everything but the batteries. here are my choices:

Trojan 6 volt 225 AH battery
Trojan 6 volt 435 AH
Trojan 2 volt 1110 AH --most expensive one!

What is the best way to go? thks Larry

Cariboocoot · November 2012

Re: Battery Bank sizing

Let's do a little mathematical analysis.
Assuming you mean 9000 Watt hours per day on a 48 Volt system that's 188 Amp hours, so the absolute minimum battery bank size would be 2X that or 376 Amp hours. If you want to keep the DOD to 25% (instead of 50%) you would need 2X that or 752 Amp hours.

For 376 Amp hours @ 48 Volts:
with 6 Volt 225 Amp hour batteries it would require sixteen; two parallel strings of eight in series for 450 Amp hours
with 6 Volt 435 Amp hour batteries it would require eight; one string of eight in series for 435 Amp hours
with 2 Volt 1110 Amp hour batteries it would require 24 all in one string.

For 752 Amp hours @ 48 Volts:
with 6 Volt 225 Amp hour batteries it would require thirty-two; four parallel strings of eight in series for 900 Amp hours (this is the worst choice in my opinion)
with 6 Volt 435 Amp hour batteries it would require sixteen; two parallel strings of eight in series for 870 Amp hours
with 2 Volt 1110 Amp hour batteries it would require 24 all in one string.

Depending on the cost and DOD required I would go with the 435 Amp hour 6 Volts.

westbranch · November 2012

Re: Battery Bank sizing

Weight per cell/tub can be a consideration too... might want to factor that in, mine are 150# + per and were a grunt to place... YMMV

stephendv · November 2012

Re: Battery Bank sizing

Hi Larry,

8kWh/day for 2 days = 16kWh. For a long life, 50% depth of discharge is common, so you'd need a 32kWh battery. At 48V = 666Ah. Since you'll draw that over 2 days it's closer to the C100 capacity but to be conservative, take it as the C20 rate. So you need about a 650Ah C20 battery. To compare batts and prices, I like to reduce them to a single value: $/Wh/cycle. $ is easy enough, Wh has to be measured at the same rate when comparing batts (using Wh instead of Ah means we can compare batts of different voltage), and the cycle is provided in the batt specs for a given depth of discharge. It's common to provide a number of cycles at 80% DoD value for a batt to indicate how long they're expected to last.

So the first option: Trojan 6 volt 225 AH (assume these are the T105-RE)
$166 for 1350Wh and provides 700 cycles at 80% depth of discharge. So that's 0.000175 $/Wh/cycle.

Trojan 2 volt (L16RE-2V)
$355 for 2220Wh and provides 700 cycles at 80% DoD = 0.000228 $/Wh/cycle.

Crown forklift batteries (http://www.solar-electric.com/crinba24vo75.html):
$3617 for 18000Wh and provides 1500 cycles at 80% DoD = 0.000133 $/Wh/cycle.

So the cheapest long term option are the Crown forklifts in my opinion; next best T105s.

Cariboocoot · November 2012

Re: Battery Bank sizing

BTW those Trojan 2V cells are the same as the 6V just internally connected in parallel instead of series.

I don't know if you can get Crown batteries there but they have a wider variety of 2V cells http://www.solar-electric.com/2voltbatteries.html
And some different 6V choices http://www.solar-electric.com/6voltbatteries.html

larrybc1 · November 2012

Re: Battery Bank sizing

Cariboocoot wrote: »

Let's do a little mathematical analysis.
Assuming you mean 9000 Watt hours per day on a 48 Volt system that's 188 Amp hours, so the absolute minimum battery bank size would be 2X that or 376 Amp hours. If you want to keep the DOD to 25% (instead of 50%) you would need 2X that or 752 Amp hours.

For 376 Amp hours @ 48 Volts:
with 6 Volt 225 Amp hour batteries it would require eight; two parallel strings of four in series for 450 Amp hours
with 6 Volt 435 Amp hour batteries it would require four; one string of four in series for 435 Amp hours
with 2 Volt 1110 Amp hour batteries it would require 24 all in one string.

For 752 Amp hours @ 48 Volts:
with 6 Volt 225 Amp hour batteries it would require sixteen; four parallel strings of four in series for 900 Amp hours (this is the worst choice in my opinion)
with 6 Volt 435 Amp hour batteries it would require eight; two parallel strings of four in series for 870 Amp hours
with 2 Volt 1110 Amp hour batteries it would require 24 all in one string.

Depending on the cost and DOD required I would go with the 435 Amp hour 6 Volts.

I am a little confused here..with 6 volt batteries and a 48 volt system wouldnt I need (8 ) 6 volt batteries per string ( 8x6=48 )? maybe I am missing something here..thks larry

Cariboocoot · November 2012

Re: Battery Bank sizing

larrybc1 wrote: »

I am a little confused here..with 6 volt batteries and a 48 volt system wouldnt I need (8 ) 6 volt batteries per string ( 8x6=48 )? maybe I am missing something here..thks larry

You're not missing anything, I am; a few functioning brain cells.

Fixed it.

larrybc1 · November 2012

Re: Battery Bank sizing

Cariboocoot wrote: »

Let's do a little mathematical analysis.
Assuming you mean 9000 Watt hours per day on a 48 Volt system that's 188 Amp hours, so the absolute minimum battery bank size would be 2X that or 376 Amp hours. If you want to keep the DOD to 25% (instead of 50%) you would need 2X that or 752 Amp hours.

For 376 Amp hours @ 48 Volts:
with 6 Volt 225 Amp hour batteries it would require eight; two parallel strings of four in series for 450 Amp hours
with 6 Volt 435 Amp hour batteries it would require eight; one string of eight in series for 435 Amp hours
with 2 Volt 1110 Amp hour batteries it would require 24 all in one string.

For 752 Amp hours @ 48 Volts:
with 6 Volt 225 Amp hour batteries it would require sixteen; four parallel strings of four in series for 900 Amp hours (this is the worst choice in my opinion)
with 6 Volt 435 Amp hour batteries it would require eight; two parallel strings of four in series for 870 Amp hours
with 2 Volt 1110 Amp hour batteries it would require 24 all in one string.

Depending on the cost and DOD required I would go with the 435 Amp hour 6 Volts.

I missed your last post with the new info based on 48 volt system...? thks

Cariboocoot · November 2012

Re: Battery Bank sizing

Okay, fixed it again. Sheesh! Have I got it right yet?
You need eight 6 Volts in series to get 48 Volts. So two parallel strings is sixteen and four parallel strings is thirty-two.

Cariboocoot wrote: »

Let's do a little mathematical analysis.
Assuming you mean 9000 Watt hours per day on a 48 Volt system that's 188 Amp hours, so the absolute minimum battery bank size would be 2X that or 376 Amp hours. If you want to keep the DOD to 25% (instead of 50%) you would need 2X that or 752 Amp hours.

For 376 Amp hours @ 48 Volts:
with 6 Volt 225 Amp hour batteries it would require sixteen; two parallel strings of eight in series for 450 Amp hours
with 6 Volt 435 Amp hour batteries it would require eight; one string of eight in series for 435 Amp hours
with 2 Volt 1110 Amp hour batteries it would require 24 all in one string.

For 752 Amp hours @ 48 Volts:
with 6 Volt 225 Amp hour batteries it would require thirty-two; four parallel strings of eight in series for 900 Amp hours (this is the worst choice in my opinion)
with 6 Volt 435 Amp hour batteries it would require sixteen; two parallel strings of eight in series for 870 Amp hours
with 2 Volt 1110 Amp hour batteries it would require 24 all in one string.

Depending on the cost and DOD required I would go with the 435 Amp hour 6 Volts.

I'd better go check and see if I forgot a pill today.

larrybc1 · November 2012

Re: Battery Bank sizing

Cariboocoot wrote: »

Okay, fixed it again. Sheesh! Have I got it right yet?
You need eight 6 Volts in series to get 48 Volts. So two parallel strings is sixteen and four parallel strings is thirty-two.

I'd better go check and see if I forgot a pill today.

ok got it!! thks
Is there any problem having 2 strings like that...I know 3 or 4 strings is a real problem..like to have just 1 string but it is expensive..so having the 2 strings of 435 AH is ok??

thks Larry

Cariboocoot · November 2012

Re: Battery Bank sizing

larrybc1 wrote: »

ok got it!! thks
Is there any problem having 2 strings like that...I know 3 or 4 strings is a real problem..like to have just 1 string but it is expensive..so having the 2 strings of 435 AH is ok??

thks Larry

Two parallel strings usually isn't a problem. The more parallel circuits you have, the greater the chance (inevitability) of different resistance between them causing some to handle more current than others. This causes an imbalance between the batteries which just gets worse with time.

Battery Bank sizing

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