Tetrx Minibot Battery Charge Cycle during Regional Play

[Chris_Elston]

I was wondering if anyone has put any for thought into the charging cycle of the minibot batteries. At $50 a piece, it's something to THINK about.

According to the spec's of the battery, the recommended charge rate is 0.9 A. This rate is a very slow trickle charge and take a few hours to charge the 12 V battery back to a green light status. Using a faster charge rate than 0.9 A will damage the battery packs according to some of my hobby experts I've talked with, the risk of killing the battery if one was to purchase a fast rate charger was not recommended.

So that leads the question...the minibot only needs to run 10 seconds at the most, there is quite a long period to recoup the discharge of 10 seconds of run time verses the charge time of 2-3 hours verses our next match is in 7 minutes....anyway...food for thought...

Suggestions? What energy studies has your team done? I suspect with a race, battery being fully charged or not in your next match could mean milliseconds of time which could catch up with you as the day wears on.

[perlgerl]

The battery capacity is 3000 mAh with a 20 A integral fuse. At 20A, the battery should be good for 540 sec (= 3000 mAh/1000 / 20A x 60 x 60). That's 54 matches, which should be sufficient for one competition. But I'd be a lot more confident with a second fully charged battery pack.

[perlgerl]

I just found these tests which estimates the stall current at 7.3 A. At that current, the battery should be good for 1479 sec or 148 matches.

[Luke Pike]

The problem is that while the battery technically lasts that long, the performance may not. Peak performance could be the factor (in addition to others) that wins the race.

[Al Skierkiewicz]

Michelle,
The amp/hr rating is usually specified at about 1/10 C or in this case, using the charge current Chris specified, 900ma. So under continuous discharge, 3000ma/hr and 900ma load, about 3 hours. I am guessing our use will be in the order of 5-6 amps for two motors, so with a little derating for increased current, I am guessing 30 minutes. Since we are only going up and should get there in 5-7 seconds, that's about 257 matches. If teams were drawing 10 amps for two motors, that would reduce to under 20 minutes or about 171 matches. Putting your batteries on charge between matches should keep them topped off. A caution, don't charge batteries when they are warm it confuses smart chargers.
First is planning to get at least 10 matches in qualifying at each event, so if one goes all the way to finals, you might have to play as many as 19 matches in a weekend.

[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

[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