Stepped DC load calcs 1

[raithrovers1]

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

 

[itsmoked]

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

 

[jghrist]

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