battery specifications limit!

[ayounggeek]

how to we fix the battery specifications for a load ,consider a 400w 24v pmdc motor.
Is it that the voltage must be above 24v or does it have any limit.AH of battery indicates the stored energy so what about fixing voltage?

 

[tpetar]

Calc /10 discharge time, if 400W @ 24V this is 17A. 17A x 10 its 170Ah 24V battery capacity or bank of battery.

 

[enjunear]

Batteries will change voltage depending on their state of charge. When fully charged, a lead acid battery will sit around 13V (~2.20 V/cell) when unloaded and rested for 24 hours. When fully discharged, it will register around 11.7V (~1.95V/cell). Choose the number of batteries based on their nominal voltage. A "12V" battery will run from 13V to 11.7V, most devices are just fine with a little overvoltage... but check the spec sheet to be safe.

Like tpetar said, you want to have a battery pack with a charge capacity (C, in Amp-hours) of ~10x the discharge current. Using a C/10 discharge rate (or less) will keep you in the linear region of discharge for a Pb-acid battery. You can expect that you will get C amp-hours out of a battery that is discharged at C/10 or less.

However, if you draw more current than a rate of C/10, you'll start to see the non-linear effects of batteries, which are modeled with Peukert's Law. Read more on that here. For lead-acid's, k = 1.1-1.3, for reference. For discharge rates higher than C/10, you won't get the rated capacity of the battery, it will appear to be less.

 

[ayounggeek]

what will be the case of lithium ion battery?(lifepo4).
I believe c/1o law was for lead acid batteries,i didnt find high AH batteries for pmdc motors used in electric vehicles!:-|

 

[enjunear]

what will be the case of lithium ion battery?(lifepo4).
I believe c/1o law was for lead acid batteries,i didnt find high AH batteries for pmdc motors used in electric vehicles!:-|

Peukert's Law is well defined for batteries at a constant temperature and constant load current. Outside of those assumptions, the estimation begins to be error-prone. It was originally developed for Pb-acid, and is loosely correlating for other battery compositions.

Li-ion batteries are a different animal than Pb-acid, but they will still exhibit a decreased-capacity effect at high discharge current rates. However, the effect of decreasing capacity is correlated to the battery temperature in Li-ion. All in all, the k constant is smaller in Li-ion batteries than Pb-acid, for an identical test scenario, so the rate of discharge has a less pronounced impact... until the battery heats up, which is directly related to the rate of discharge. The problem still exists, but the conditions that lead to the degradation are slightly different for the two chemistries under consideration.

Check out this paper for a complete analysis.
A critical review of using the Peukert equation for determining the remaining capacity of lead acid and lithium ion batteries