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

now that the circuit breaker specs are out, you will see that you can draw between 300 and 400 % of rated current for 5 seconds before the breaker will trip.

Battery acquisition
 

We are a Rookie Team and as such, we are sans extra batteries.
We noticed that in the spec sheet the designation is ES-18-12 -rather than the EX-18-12 designation which is actually on the battery. We have found ES -18-12 batteries (at about $86 each) but cannot find EX-18-12. (FIRST has stated that the EX-18-12 which we received was specially made for the FIRST competition)

When this thread was pursued in another instance we saw that FIRST has said that the EX-18-12 is the correct designation and is the only battery "official" for competition.

We would like to have a couple of extra batteries for use during competition.

This just seems so confusing to us and no one has actually stepped up to the plate to say where we might find an EX-18-12 battery OR whether we could use an ES-18-12 during competition.

By the way, the EXIDE TECHNOLOGIES website is no help either.

Any ideas or advice??

thanks

You are right the Exide site is not easily navigated and there is no place to search for part number. However the spec sheets are now available on the FIRST docs and they published data on the ES battery so I would bet that is the one you could buy on the open market. My guess here is that the batteries provided in the kits are new product for Exide and we are the test market. If that is the case, they are going to want some feedback from users so keep a log of pros and cons with the new batteries.

The Powerfit line of batteries is currently only available in europe. The ES18-12 is probably this S312/18 battery relabeled or a variation of it to be released for the US market.

Re: Battery acquisition
 

From The FIRST Forums :
So, it seems that there will be no extra batteries during competition matches, only the two in the kit. Of course, you can use spare batteries for practice and use in the pit, just not the matches.

Stephen

Thanks for the info
 

We did purchase a pair of ES-18-12 that are labeled as PowerFIT
I must assume, therefore that these are acceptable as competition batteries?? Does anyone think that we could be disqualified for using this battery in competition?

i hope not
Attached is a pic of the battery we bought

thanks for the help !!!!

Bob

Re: Thanks for the info
 

Please see my previous post, right above yours. I just edited it to emphasize the important part. I'm not sure if you would be disqualified, or what sort of penalty would result, but it seems FIRST only wants teams to use one of the batteries from the kit during official matches.

Stephen

Robot rules:

Copyright © 2003 FIRST THE ROBOT 7 3.1.2 Battery Recommendations / Cautions
· During any match, only one of the 12 Volt Direct Current Sealed Lead/Acid (SLA) batteries supplied by FIRST may be used to power the robot. You may charge the batteries through the normal operation of the battery charger that FIRST provides.

The "one of two batteries" rule is meant to insure only one battery at a time may be used on the robot.

That's what I thought as well, so I asked the question on the FIRST Forums. Someone else asked it too, and here is his question:
The answer is what I posted above, the last part of which I will repeat here:
Admittedly it only says you "should" use one of the two batteries provided from FIRST, but I think that can be read as a team is required to use only one of the two provided from FIRST.

Stephen

PS. I really do hope I'm wrong though! :)

Perhaps my team is just odd, but I compared last years batterys with this years, and I can't find a single diffrence. The part numbers are exactly the same. Does this mean they are the same? It makes sense in my head, but there is much talk of this years batterys being diffrent.

-Andy A.