We will be bringing all of our previous years chargers (~6) to both of the Michigan reigonals and we will be more than happy to help out teams that need them (as long as we get 1 or 2) this is a great day, I hate to run the robot on the 9V edge of losing communication it just gets too sluggish ya know.

Broken Chargers
 

The main reason FIRST allowed last years chargers at VCU was because about half a dozen teams had chargers in various stages of disrepair. Unfortunately Spare Parts (the table I worked as a volunteer) did not have any spare chargers as of Thursday, so rather than leaving teams with no options, they allowed other chargers. We did receive back-up chargers around noon on Friday from FIRST, but they did not rescind the ruling that other chargers could be used. All that being said, I have no idea what this means for future regionals, but I figured I'd give some more information for speculation.

I keep hearing some people say we can't use last year's batteries. I went by yahoo post #851 and used last year's batteries at KSC. We had them sitting out on our table and noone said anything. This year's game is very power intensive and I think last year's batteries should be allowed.

As for the chargers - I can understand their concern for safety, but they should allow up to 4.3 amp AUTOMATIC battery chargers. The batteries are rated up to 4.3 amps, and the charges should be automatic for safety.

Well we wound up (unintentionally) using old batteries now that I think of it, but didnt get called on it. As to the comment about the inspectors at KSC: We had to explain many of the points on the checklist to our inspector....

I'm confused
 

One of you EE types please help me out here (not the one with the 9V battery on the 12V charger, tho', thanks). Joe's battery photo shows the "capacity" of the battery as 17.2 Amp-hours. Now I know that a lot of hemming and hawing goes in to this rating, as far as discharge rate, final "discharged" voltage, and so on. But still and all, doesn't this rating basically mean that the battery will deliver about 1 amp at 12 volts for 17.2 hours before it's "flat"? Or about 17 amps for 1 hour? Or 34 amps for 30 minutes? Or 170 amps for 6 minutes? (Yes, I know there is a breaker that limits below that.)

Actually where I'm going with this is to look at the 1 amp charger. Doesn't it take a 1 amp charger 17.2 hours to recharge a flat battery with a 17.2 amp-hour capacity?

8 two minute matches at a sustained 60 amp draw is about 16 amp hours of energy draw, so one "fully charged" battery should in theory last pretty much through qualifying day. OK, so the 17.2 amp-hour rating says in the fine print that that is for a puny demand rate, maybe an amp or two, not for smokin' motors at 60 amps. I still suspect that the batteries are not getting nearly well enough charged on the 1 amp chargers in the time available.

Dodd

Regarding these questions about batteries and chargers-- is the concern only for the qualification and elimination rounds? Can I take any comparable battery (different make) and use it during practice? I have a supply of batteries that I use for emergency ham radio communications and I intend for our team to use them during practice and robot work. We'll use only the supplied batteries and charger during qual. (and hopefully elimination rounds!). Thanks!
Rookie Team 847 from the rainy state of Oregon.

What battery you use only matters during "official" activities. I'd have a legal battery on board during inspections (especially at "weigh in") as well as whenever you're on the field.

Other than that I don't think anyone really cares, I know I don't. In previous years we've used unofficial batteries routinely in the pits and in the unofficial practice areas that spring up around the venue.

Speaking of which, if you want to make ALOT of friends, build yourself a 50 foot tether and let other teams use it for testing. Last year I don't know just how many teams used ours. After a while we got so tired of having teams come to borrow it we just left it in the practice area. Fortunately everyone was graciously professional enough to leave it there so we could pick it up at the end of the day!

The Ruling from the Buckeye Regional
 

As far as I am concerned, the battery issue is dead. FIRST has run up the white flag and surrendered!

I was very concerned about batteries and in particular only using "this year's batteries and last year's Exide/Yuasa batteries" (approximate quote). I wanted to know if the batteries I had qualified as last year's batteries (I was not sure which exact batteries in our stash were the ones from last year) and I wanted to get a ruling on using Exide's model "ES18-12" which is the replacement model number from Exide for the battery we have in the kit, which is model "EX18-12.

ANYWAY...

Bottom line: After much discussion and consultation, the official answer we recieved was "any 12V sealed lead acid battery in that size and shape is legal" (paraphrased, but true to intent).

So... ...that ends that as far as I am concerned.

Joe J.

Dodd,
Here hopefully are some answers for you. The battery rating system is generally this...
Amp-hour capacity is what the battery can be expected to deliver when discharged at about 1/10 the rating. For our battery type, that is 1.72 amps for 10 hours before the terminal voltage falls to average of 10.5 volts. Please note that drawing more current reduces the overall amp-hour rating. Most robots that have optimized gear ratios and average drive efficiencies will not draw constant 60 amp currents for all two minutes of a match. Even the current spikes encountered when hitting stall conditions during the match will not trip the 60 amp breaker even though they are considerably higher than 100 amps. (The stall current for a single FP motor)
Now let's talk about our real world limitations. 8 volts minimum at robot controller, 45 amps sustained at any 30 amp breaker, 400 amps sustained at main breaker 450 amps at battery output max for very short periods of time.

The one to really worry about first is the 8 volt minimum at robot controller. Hitting this is a minimum 2 second dead time while it reboots, symptom is no rotating light for 2 seconds. (keep operator interface in voltage monitor and watch it!)
45 amps sustained at 30 amp breaker will trip this breaker even though it resets right away, symptom driving unstable, erratic behavior, noise from breakers tripping. (warning!!!don't touch a 30 amp breaker after a match where it has tripped repeatedly, it will be very hot!)
The main breaker should never trip under normal competition unless something is really wrong with robot design, wiring has failed or the breaker has taken a severe physical hit. (usually from the side)
Anyone who draws 450 amps from the main battery will have all of the above symptoms and melted wires, smoke, dead batteries and fried speed controllers almost every match and a host of other really bad problems. If any of these are happening to your robot please ask for help. We are bound by "gracious professionalism" to give assistance.
In the area of battery chargers, while it is ok to draw large currents it is not ok to charge at large currents. Battery manufacturer says "thou shalt not charge any higher than 4.2 amps" Using this charger on a cool battery that was fully discharged would take about 5 hours to full charge. Since no team can take a battery down to full discharge (robot controller reset at 8v.) most teams using a 4 amp charger should be up to full in 2-3 hours. 1 amp chargers will take 10-12 hours normally (over night) or 18-20 hours for fully discharged.
Any more questions, just ask. I will be going off-line while at Great Lakes starting tomorrow PM.

Re: The Ruling from the Buckeye Regional
 

Do you have a copy of the official answer in writing that we could bring to the regional, just in case?

Overnight charging???
 

OSHA wouldn't like this at all...

So FIRST is actually ADVISING that teams charge batteries overnight, eh? If you know anything about lead-acid batteries, you know that they release combustable gasses when charged. Well, lets make a situation here. There are regionals with sixty or so teams, right? Lets pretend that each of those sixty teams charges both of their batteries overnight, shall we? I'll bet dollars to doughnuts that the pit areas are swept by the college's janitorial staff, and I bet some of those staffmembers smoke while they work. OSHA, SAE, ASE, and many other organizations and almost all lead-acid battery manufacturers will tell you NEVER smoke near a battery, due to their gas discharges while being recharged. With all that combustable gas from 120 charging motorcycle batteries in the air, you can get a really not cool looking explosion. I'm sorry, but FIRST really should think before they print something as ludicrous as this.

Just a few words of sense...

I do believe that SLA batteries have a gas resevior (hence the sealed part), so no gas is actually released into the air.

Gas
 

The SEALED in sealed batteries means that you can't add fluid to the battery without damaging the case, it doesn't mean that gas cannot vent. The gas must vent or the case will rupture while charging. Trust me, an automotive service technician, they vent combustable gas into the air while charging.

Thanks for your thorough reply, Al. It reinforces my understanding of the situation:

A team relying solely on the two batteries and single one amp charger supplied in the kit this year is likely to have problems keeping their robot adequately powered throughout the two days of Qualifying and Elimination matches.

Dodd