pic: 2791's Chassis

[Matt H.]

Originally Posted by Chris is me

That being said this is a valid concern - does anyone know how I could predict battery discharge rates using math? I have no idea how to do it.

Looking at the spec. sheet for the batteries:
http://www.alliedelec.com/Images/Pro...A/610-0002.PDF

The last graph gives discharge characteristics. The C-rate or CA on the graph can be roughly calculated as Current Load/Rated Amp Hours so at your quoted 60 amps
CA~3.4
Visually extending the curve; you're right on the edge.

If you current usage rises to 80 amps with a greedy manipulator you'll almost certainly run of battery life.


Disclaimer: I'm not an expert on batteries and am only ~70% sure my interpretation of the C-Rate is correct.

Another method you could try is using Peukert's law.

Where the time to full discharge is approximated by:

t=H(C/(IH))^k
Here t is time to full discharge, C is the rated capacity, I is the current, H is the rated discharge time and k ranges between 1.1-1.5 for lead acid batteries (it is generally empirically determined, but we can use the discharge curves to estimate it)

Using the spec sheet again:
C=17.4 H=20 hours I=60 amps k~1.34

t~4.1 minutes

For 80 amps

t~2.8 minutes

So it all comes down to how much you want to trust the math. To me it looks like a very border line case especially if you have a high current (always on rollers, heavy lifting etc.) manipulator.

For more information:
http://en.wikipedia.org/wiki/Peukert's_law

[Chris is me]

That's pretty compelling, really. Though I doubt the drive will spend more than 30 seconds a match drawing maximum current, it is still something we should watch out for.

Our current plan as of today's meeting is to oder a set of pulleys to gear us for the more conservative 9.5 feet per second and to do extensive battery life testing. We've got a simple, probably low load manipulator planned - but we will still be careful. If during practice or competition we figure out we're going through batteries too fast - we'll swap those right out. Should take under 10 minutes.