Overly Optimistic Battery Estimations?
 

I remember hearing somewhere in the kick-off that a battery should last for six (6) rounds... It doesnt seem right. While the low bat light might not come on for 6 matches, you are surely not getting anything near what you would a fresh battery. What do you guys think is a good estimate for battery consumption? Our team (469) has plenty of batteries, so it matters not as much to us, but to other teams with 1 or 2 batteries, it may be important in deciding whether it is worth it for the team to invest in another battery.

Re: Overly Optimistic Battery Estimations?
 

We would never use the same battery for more than one match. We always switch them out for every match. Say you have a battery that is charged up to 13.6v, at the end of the match, it is usually around 11v.

Well since this year there is only going to be 2 rounds for every final rotation, you can possibly get by with only 2 batteries. But It would be worth while to get another battery and charger. Batteries usually take 2 hours to get a good 13.5v+ charge.

from my experience practicing and competing, after about five minutes of continous use you start to get control problems. Our robot always used to lose its response after that.

We tested (last year) and found that we could run for three matches, if we absolutely *had* to, without the battery dying. We usually never run more than one match without a new battery. The exception is during elim rounds, we may run two matches with the same abttery, so we can put a fresh one on in the finals! ;)

- Katie

Yes, I think a battery should be OK for six rounds. Last year we practiced with one battery for half an hour then stopped to let the "chip" motors cool. The controls should still work down to six volts (my designs do...) although I don't know how low the specs for the RF module go. Most teams just swap in a new battery as part of the prep. for a match.

As for an extra battery to swap-in for the finals, I would bet my own money that you could borrow at least a hundred batteries from the other teams...

the RC will stop working, period, below 8 volts. i believe below 9 volts it starts missing packets, and if it misses enough packets, it shuts it self off till it's power cycled. anything above 9 volts you can work with, but i've noticed that below 11 or 10 volts, controls start getting sluggish.

I have no doubt that you could "run" it for six matches as we've done some lengthy demos, but I use the term "run" loosley. To get peak performance out of the motors (especially with last years game) we always switched out every match.

I will go out on a limb here and say this, "if you can't run at least two matches on a battery you need to look at your mechanical design." There are a lot of factors contributing to battery drain but an efficient design should not drain the battery during one two minute match. If it does, your robot has either come dangerously close to reset or has reset during the match.

I agree. Make sure that the side load on the motors is minimal and try hard to have as little resisitance as possible in all of your components. Programing techniques to avoid backdriving the motors or blowing the breaker also aid in keeping the battery drain to a minimum.

If I remember correctly, we have faster chargers this year, so having a topped off battery will probably be easy until you get into the elimination rounds where you have to do matches very rapidly. Then you can always ask teams not competing in the elimination rounds for batteries to borrow :)

last year we lasted 3 minutes on a battery before we started noticing slowing down and the low batt light, and this year we are able to draw 120A, so i highly advise changing after every match

Umm... that seems really, really, really, really low. Assuming you are drawing 100amps continuously, that means your battery was effectively 5Ah. This is less than 1/3 of the 20 hour rating! Plus, I would highly doubt you could pull 100amps for 3 minutes without tripping a breaker.

The manual this year says "The 12vdc SLA batteries can deliver current in excess of 200 Amps for a sustained period of time (minutes)." I'd guess sustained would be about 3 minutes.

Don't be fooled by the statistics. The 120 Amp circuit breaker needs to be derated when periodic currents over 120 amps are drawn. Putting a demand on the battery of periodic bursts above about 200 amps will cause the breaker to trip in about 15 seconds.
Although our batteries are rated at 18 Amp Hour that really means they are designed to deliver 0.6 Amps continuously for 20 hours before the terminal voltage falls to to a specified voltage.
And yes, the battery is able to supply currents for short periods of time, well in excess of 200 amps. That is why it is so important to store, charge and mount batteries with caution. 200 amps flowing through a dropped tool or wire is capable of causing high temperatures resulting in burns or other injuries. All electrical teams should stress safety when working with the battery and other high current components.

Keep in mind that the main breaker used, at ~200 amp draw, could go for close to [edit] 30 seconds (remembered this number wrong, oops)[/edit] before tripping, so having a robot that draws 200+ amps for large parts of the 2 minute period without tripping a breaker isn't impossible, just unplausable and incredibly lucky.

Tyson