×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Stepped DC load calcs

Stepped DC load calcs

Stepped DC load calcs

(OP)
We have a DC system that requires a battery to supply the following stepped load:
1.    129kW for 15 minutes, then,
2.    106kW for 15 minutes, then,
3.    100kW for 60 minutes, then,
4.    18.5kW for 90 minutes
I have contacted battery manufacturers and they disagree on the way the stepped load is simplified for battery autonomy.
Some say that the 129kW should be taken and then ratios used on the time which gives 129kW for 87 minutes.
Others say that the time is taken and then ratios used on the power which gives 62.17kW for 180 minutes.
A battery discharge curve is none linear and has different characteristics at the 87 minute rate compared to the 180 minute rate, e.g. Powersafe 2V275 1 hour wpc to 1.7V at 20 degrees is 329.4 and at the 3 hour rate is 136.8.

Which is the correct method?

UPS engineer http://www.calibrepower.co.uk

RE: Stepped DC load calcs

It all comes down to the amps.  That's all batteries care about. (in the small picture)

The amps drawn dictate the efficiency at each of those loadings.

You need to come up with the actual amps demanded at those various power loadings then go to the battery's charts and find the amp-hr rating of the battery at each of those rates.

Stir in the time verses those rates to calculate the percentage of discharge during each of those rate-periods.

That will give you the most accuracy in a calculated estimate.  To refine it you need to start considering battery temperatures during this exercise. Harder may be trying to get the currents being demanded at those various powers from the loads.

 

Keith Cress
kcress - http://www.flaminsystems.com

RE: Stepped DC load calcs

See
485-2010 - IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications
or
1013-2007 - IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic (PV) Systems
or
1115-2000 - IEEE Recommended Practice for Sizing Nickel-Cadmium Batteries for Stationary Applications
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources