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# Thread: How to calculate the capacity of storage battery?

1. ## How to calculate the capacity of storage battery?

In stand alone solar system, how do you calculate the capacity of storage battery? Is that any formula?

Please make an example. Thanks so much for any answers.

Helen

2. The battery manufacturer makes that calculation when they design the battery, and then they put a label on the battery with the specs.

example 12V 110Ah @ 20 hour rate

In reality, you can only use half the power stored, otherwise, you severely shorten the life of the battery. There is a good battery FAQ here:
http://www.windsun.com/Batteries/Battery_FAQ.htm That will answer a lot of questions I hope.

Mike

3. hi, Mick90250,

Thanks for your answers, but I think you did not catch what I said. You know if I want to make an off-grid system, I need to figure out which capacity of battery I should use.

Ok, let me make an example,
Load :2* 11W lamp; 10 hours working time per day.
system voltage:12V
Rainy days: 3 days

Then how will you calculate the capacity of the battery?

4. Originally Posted by solarf.helen
hi, Mick90250,

I want to make an off-grid system, I need to figure out which capacity of battery I should use.

Ok, let me make an example,
Load :2* 11W lamp; 10 hours working time per day.
system voltage:12V
Rainy days: 3 days

Then how will you calculate the capacity of the battery?
Much better question. I knew you were not going to manufacture a battery.

Batteries have a shorter life, the deeper you discharge them. example:
1000 cycles 10% discharge
800 cycles 20% discharge
300 cycles 50% discharge
80 cycles 90% discharge

Generally, discharge to no less than 50%, and size 1 day to about 10%

You are asking for 3 days. I 'll assume 12V CFL, so no inverter losses

Your load is 22W for 10 hrs = 220W hour x 3 days = 660w hour consumed in 3 days

So a battery bank of 1320wh would be the minimum size.

Working all in watts, allows conversion back to any voltage. Volt x Amp = watts

12V = 110A
24V = 55A

So a standard group 24 size battery would be just under the spec, but a group 27 would
http://www.mysears.com/DieHard-Marin...ze-27M-reviews
just make it. at 105A.

Then you have to recharge it with something.

5. ## Cycles vs Life

Originally Posted by Mike90250
Batteries have a shorter life, the deeper you discharge them. example:
1000 cycles 10% discharge
800 cycles 20% discharge
300 cycles 50% discharge
80 cycles 90% discharge
Generally, discharge to no less than 50%, and size 1 day to about 10%
.....
Then you have to recharge it with something.

6. Originally Posted by MarineLiner
so it differs with "cycle" in the AC (Alternative Current, Hertz/cycle).
and Depth of Discharge (DOD) is what i just know.
Right, battery cycles, are:
a) Discharge to some value
b) Recharge back to full
= one cycle

Not the same thing as AC powerline frequency (cycles)

7. Originally Posted by Mike90250
Right, battery cycles, are:
a) Discharge to some value
b) Recharge back to full
= one cycle

Not the same thing as AC powerline frequency (cycles)
So with that said that means that the average battery coming in with a 50% discharge would last less than a year?

8. It's a giodeline to start. Same batteries, same site, but more batteries, sized to only 20% dishcarge, should last over 5 years. (Deep cycle batteries, not Kmart deep/starter batteries)

9. ## need help.. confusing :(

which is ri8.. case1 or case2?
CASE I:
our fan needs 220 v and 0.2 A for an hour = 50 watts.
a 12volt 7AH battery @20 hour rate -> 7/20 = 0.35 A.
The battery can sustain a 0.35A load for 20 hours.
But we can use only the 50% capacity of battery so tat it doesnt get damaged.
So 0.35A load for 10 hours.
If we consider the losses in invertor, the time will be around 3 hours.
CASE II:
power consumed by the fan is 50watts.
lets consider that we want the fan to run for about 3 hours in battery.
power required = 50 watts * 3 hours = 150watt hours
So, the battery capacity should be 150 watt hours.
what we need : Two 12volt 7AH in parallel. (in parallel v is constant and I= 7+7, so P=12*14=168 watt hours)
Loses in invertor will occur. We can not drain the full power from the battery.

10. welcome. Lets' un-confuse this a bit.

power consumed by the fan is 50watts.
power required = 50 watts * 3 hours = 150watt hours
Good so far. If you use only 50% of battery, then you need a 300 watt hour battery.
At 12V, that would be a 25amp hour battery.

Now to recharge that battery, to account for all the system losses, you need to have a PV panel with a wattage nameplate that, when multiplied by your SUN hours
use PV watts to find out your hours http://www.nrel.gov/rredc/pvwatts/about.html
Ths solar SUN hours are quite different than when the sun is up , most areas get 6 or fewer hours.

Anyway, you need to harvest 300 watt hours, divide by ____ sun hours = ______ watts of PV

If you use an inverter , add 30% to the needed 300W harvest, you will need to harvest about 400 wh

Hope this helps

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