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Associate

Join Date: Dec 2008
Posts: 26

# How to calculate the required AH for ups

01/31/2009 4:04 AM

could any one help me how to calculate the required AH for ups.

my load is two motors have same spec.

p=2.2kw

I Rated=4.98amp

I starting=28 amp

i want this ups to supply for 30 min.

so could you please explain for me how to calculate it the required KVA and AH for batterys .

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Power-User

Join Date: Nov 2008
Location: Johannesburg
Posts: 298
#1

### Re: How to calculate the required AH for ups

02/02/2009 2:06 AM

Hi there

Simply put:

C = x * T where

C = capacity in amp hours

x = current drawn

T = Time in hours

C = (4.98 * 2) * 0.5

C = 4.98 AH

But you do not want your battery to be fully depleted so then add some spare capacity by:

Cnew = C/0.8

Cnew = 4.98/0.8

Cnew = 6.225 AH

Now you need to take rate of discharge into consideration. You will need to look at the following charts to get your value http://www.powerstream.com/BB.htm

I have used 0.5

Cfinal = Cnew / 0.5

Cfinal = 6.225 / 0.5

Cfinal = 12.45AH

In conclusion you will need a 12.45 AH battery to run your load for thirty minutes at 9.96A average draw.

To get the kVA rating you need to multiply the voltage by the amps. As you did not provide the voltage - you can calculate this yourself.

Regards,

Craig

Anonymous Poster
#2

### Re: How to calculate the required AH for ups

02/02/2009 4:36 AM

why u put T=2,

C = (4.98 * 2) * 0.5

although i want it for 30 minutes only

so could we make it like that

C = (4.98 *0.5)

Associate

Join Date: Dec 2008
Posts: 26
#3

### Re: How to calculate the required AH for ups

02/02/2009 4:38 AM

why u put T=2,

C = (4.98 * 2) * 0.5

although i want it for 30 minutes only

so could we make it like that

C = (4.98 *0.5)

Power-User

Join Date: Nov 2008
Location: Johannesburg
Posts: 298
#4

### Re: How to calculate the required AH for ups

02/02/2009 5:07 AM

Hi there,

You said that you have two motors of the same spec. So I took your 4.98A as the current of one motor, so - (4.98 * 2).

If the 4.98A is for both then you do not need to multiply by two. The divide by 0.5 is to get 30 min. Remember T is time in hours.

Regards,

Craig

Associate

Join Date: Dec 2008
Posts: 26
#5

### Re: How to calculate the required AH for ups

02/02/2009 8:42 AM

by the way,the supplier get for me 60 batteries a every one is 12 volt and 9.5AH

to supply this motor it`s voltage is 400volt 3 phase ac

Power-User

Join Date: Nov 2008
Location: Johannesburg
Posts: 298
#6

### Re: How to calculate the required AH for ups

02/03/2009 1:08 AM

Hi there,

That sounds reasonable - but it depends on the battery type. Refer to the link I posted to get your constants.

Regards,

Craig

Anonymous Poster
#7

### Re: How to calculate the required AH for ups

02/04/2009 11:33 AM

Craig,

But most UPS output is single phase, 240V 50Hz and not 3 phase 400V 50Hz! how will thus guy get his motors to run?

Rgds,

Percy

Associate

Join Date: Dec 2008
Posts: 26
#8

### Re: How to calculate the required AH for ups

02/04/2009 3:13 PM

there is 3 phase o/p ups.

this link for the ups we have it.

Power-User

Join Date: Nov 2008
Location: Johannesburg
Posts: 298
#9

### Re: How to calculate the required AH for ups

02/05/2009 1:18 AM

Hi there,

Industrial UPS 3 phase and 525/400V. Masterguard (Ex Siemens) makes a very nice range. It is a bit pricey - but well worth it. You can even have a Profibus link to it - nice for reading status' and diagnostics.

Cheers

Craig

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