Tetrx Minibot Battery Charge Cycle during Regional Play

[JesseK]

It's somewhat easy to account for the robot's actual power draw if you know how to calculate it's theoretical motor load + a safety factor. Multiply it out to get the mAh used, and figure out how long it takes to recharge that amount using the trickle charger. The average time between matches is 30 minutes in Quals at large Regionals (20 mins at smaller ones), so depending on your robot's characteristics and which Regional you attend, you can figure out if your charging abilities meet your minibot battery requirements.

However, in Elims (particularly Finals), that time goes down to ~7-8 minutes between matches. If you make some friends along the way, I'm sure they'll lend you batteries.

edit -- Another thing to keep in mind is that unless the minibot weighs a TON, or is grossly geared improperly, it will reach its goal in 4-6 seconds. Just thought I'd throw that out there. So good design and a shut-off switch when the bot reaches the top should help with cycle times.

[johnr]

Does putting these battries on short recharges condition the baterries never to completely recharge? Should you at some point completely discharge the battery? Or am i just showing my age and not keeping up with these new fangled batteries?

[Al Skierkiewicz]

Originally Posted by johnr

Or am i just showing my age and not keeping up with these new fangled batteries?

Not to be the pot calling the kettle...
That should not be a problem. You are referring to a NiCd principal that was the norm at one time. Manufacturers figured out the "memory effect" and have ways to prevent it. The FTC battery is a NiMH battery.

[IKE]

Originally Posted by johnr

Or am i just showing my age and not keeping up with these new fangled batteries?

Sometimes it is good to go back to school:
http://batteryuniversity.com/

There is a good section on Nickel Batteries that discuss the differences between NiCd and NiMH batteries (Tetrix battery).

[Hugh Meyer]

Chris,

Here is a discharge curve I did on one of ours. It shows good response down to about 1.6 AmpHrs, then below that it starts down hill. It looks like any calculations for life should us 1.6 AmpHrs as the useful life. This is with a discharge rate of 2 amps.

It looks like it will be important to carefully manage battery usage. We will likely top them off between matches and alternate batteries.

-Hugh

[JesseK]

Oh no, lol. /slighthijack

The differential equations we use to predict robot acceleration assume a constant voltage as amps get used. It doesn't matter much, as we have to assume every robot uses the same exact batteries in order to differentiate between different mechanical designs. Yet I wonder how much the drive train drag race sims (and even more of an extension, my Quadrotor thrust sims) would be affected by adding a V(t)/Vspec change.

Anyone who's looking for a ballpark figure of consumption, each iteration I've calculated has been under 15mAh for robots that weigh <=5lbs and/or are <=5 seconds.

[Al Skierkiewicz]

Originally Posted by Hugh Meyer

Chris,

Here is a discharge curve I did on one of ours. It shows good response down to about 1.6 AmpHrs, then below that it starts down hill. It looks like any calculations for life should us 1.6 AmpHrs as the useful life. This is with a discharge rate of 2 amps.

It looks like it will be important to carefully manage battery usage. We will likely top them off between matches and alternate batteries.

-Hugh

Hugh,
Could you run that test again at 900ma and plot against this one? It will be a while before I can run that test on ours.

[DonRotolo]

OK, 0.9 A x 3.7 hours x 0.9 (charge efficiency) = 3 Ah. So, the max it should take for a fully discharged battery is 3.7 hours.

10 seconds of use - let's call it 2 minutes just to be sure - relieves the battery of 0.033 hours * 8 A (2 Tetrix motors at full steam) = 0.266 Ah. You can put in that amount of energy in perhaps 20 minutes.

I don't think it'll be much of a problem, particularly if you have 2 batteries.

[Hugh Meyer]

Originally Posted by Al Skierkiewicz

Hugh,
Could you run that test again at 900ma and plot against this one? It will be a while before I can run that test on ours.

Yes, I will run another test at 900 ma. We were closed today from the snow. It will be Sunday or Tuesday before I get to run another test.

What are you looking for in the comparison?

-Hugh

[Al Skierkiewicz]

Originally Posted by Hugh Meyer

Yes, I will run another test at 900 ma. We were closed today from the snow. It will be Sunday or Tuesday before I get to run another test.

What are you looking for in the comparison?

-Hugh

Hugh,
I just think it would be a good test to show the difference in load vs. life for those that don't have the testing you have. I know I won't be able to run the test for a while either.

[Hugh Meyer]

Al,

Attached is the discharge curve comparison you requested. One curve shows a discharge rate of 2 amps and the other shows a rate of .9 amps.

It seems there is not a big difference.

-Hugh

[Al Skierkiewicz]

Hugh,
Very interesting. The amp hour rating stayed pretty much the same while the length of test was almost half. Was the battery warm at the end of the 2 amp test?
Thanks for the data.

[Chris_Elston]

Originally Posted by Hugh Meyer

Chris,

Here is a discharge curve I did on one of ours. It shows good response down to about 1.6 AmpHrs, then below that it starts down hill. It looks like any calculations for life should us 1.6 AmpHrs as the useful life. This is with a discharge rate of 2 amps.

It looks like it will be important to carefully manage battery usage. We will likely top them off between matches and alternate batteries.

-Hugh

That's pretty impressive that this battery hangs on to the voltage for that long in the upper 12 volts. That should keep the minibots pretty consistent in the race to the top even on a so-so charge. Nice work Hugh!

PS: We've been wanting to get one of those CBA II analyzers ourselves. This year might be the year...

http://www.batteryjunction.com/western-cba2.html

[chemipanda]

How long should it take to charge the battery for the first time?

[Bill_B]

Originally Posted by chemipanda

How long should it take to charge the battery for the first time?

Ours took 4 hours to charge when new from the box. that was when the charger light turned green. The battery pack was warm at 3h45m and warmer at 4hours.

BTW, I taped a piece of clear plastic with a small hole for the slide switch over the indicator and switch area of the charger. This will keep the small slide switch from reaching the 1.8amp charge rate position by accident.