Battery Sizing (load profile)

[salem]

For battery sizing, can we convert a load (A) for 150ms to equivalent load for 1 sec, as a percentage value
e.g. 180 A for 150ms is equivalent to (150ms / 1000 ms) X 180 = 27 A for 1 sec (1000 ms)
This is required as the minimum time duration which we can use in battery sizing software is 1 sec.

 

[ScottyUK]

Not really. The battery doesn't behave as a perfect energy store, so you can't use a linear relationship between current and time. The physical size of the battery plates limits the maximum current you can draw. You need to look at the internal resistance of the cells when drawing high current, otherwise you'll find the terminal voltage will collapse.

 

[Parchie]

Let me try.

First, you need to know your battery nameplate ratings. There is this thing called CCA--> cold cranking amps. It simply means the available current that can be drawn from a battery at 0 degrees F at a cell voltage of 1.2 volts (7.2 volts from a 12-volt battery) for 30 seconds. With the info on CCA, you can estimate if your battery can deliver the needed amps (even at freesing temps).
If you don't know your battery's CCA, you can estimate using CCA ~ 5 x RC,

RC being the reserve capacity (RC is the ammount of time in minutes with the battery delivering 25 amps until the terminal voltage drops to 10.5 volts on a 12-volt battery).​

Also, if you only have the AH rating of your battery, approximate RC = (2 x Amp-hours rating)-32.
Note: I found those information here.

 

[jghrist]

I normally treat any load as lasting at least one second.

 

[HamburgerHelper]

I think in IEEE 485 it does state the shortest load as being one second.

I can't remember which supplier it is but there is a company that sells batteries and chargers that has a calculator that lets you put in a load profile, battery type, voltage constraints,number of cells,margin, and load characteristics (constant current or constant power) and then gives you what you need. At least what I remember for IEEE 485, it is a little more complicated than just adding up the amp-hours. Really hard loads at the start of the load profile cause the battery's amp-hour requirement to need to be larger than if the hard draws were nearer to the end of the profile. I think it was Hindlepower or someone that works close with them. Maybe, SBS?