From the 2005 Q&A
ID: 1217
Q: Can we use previous years’ batteries? Having only 2 batteries (which last only 1 or 2 rounds) will not last the entire day on Friday. The batteries take hours to fully charge-2 aremn
**A:**You may use batteries from prior years as long as they are the same make and model as the competition batteries. Note: EX18-12 and ES18-12 batteries are equivalent batteries.
We ran 4 CIMs, 1 motor for the turret, and 1 motor for the pulley to raise our lift, and all the accompaning electronics without and issue. battery life was pretty long in terms of robots (15 minutes or so).
Ken,
The answer to your questions…
The kit batteries are more than enough to run a robot in our competition for a two minute match. Efficient drive systems, those with a well designed transmission and reduction (translates to about 8-12 ft/sec no matter the number of motors used) and low losses in steering/turning and the ability to push without stalling the motors should be fine. Things that eat batteries are robots designed for high speeds, robots that encounter extreme currents in turns, robots that stall multiple motors regularly and robots that run almost continuous on the air compressor.
There is no difference in the ES or EX battery. They are the same amp hour and output voltage rating. However, there are some manufacturer/suppliers that will tell you their battery is a equivalent, cross reference or a replacement. Do not accept substitutes, the only allowed batteries are the ES or EX battery.
You should have no problem running a four motor, kit transmission, (all Chalupa/CIM) RWD robot provided you do not try to couple large diameter wheels or design a step up gear ratio between the wheels and the transmission. Provided you do not expend a lot of energy on the rest of the robot systems.
Its much more closer to 1 amp for 18 hours. If you look at the specsheet you linked, the 18AH of capacity is rated for a 20 hour discharge. According to the specsheet, for an 18A discharge, you only get about half that capacity.
Interestingly enough, I’ve been running some tests on a fixed 1.2 ohm resistor (so approximitally a 10A discharge rate, average), and getting about 85-90 mins of run-time before the circuit automatically shuts off and re-charges at 10V. The spec-sheet only shows about the same time (maybe even a little less), so I’d say those numbers are pretty good (or atleast the 10A discharge rate #). I haven’t bothered with lower discharge rates, and I need some bettter cooling for my resistor before I can go to greater discharge rates.
In this case, I would have to say that the wheelchair people don’t know how little current a FIRST robot can draw. An example: 330’s 2005 robot has 8 motors (4 CIMs, 2 FPs, and 2 window motors). We ran it for 45 minutes on two batteries, and the only reason we stopped was because it was getting hot under the lexan sides. Need I say more?
For the drive, the CIMs are probably your best bet. Four small ones will do the trick, with power to spare. If you can do four, do four. If a motor in the drivetrain fries suddenly, you want a backup (even though it’s hard to fry a CIM).
Feels good to be so thouroughly vindicated - thanks a lot, you really helped, all of you. I’ll bring it up again at tomorrow’s meeting, we’ll have to start trying these things ourselves…
Sanddrag et All,
I’ve performed continuous 50A load discharge FIRST KOP 18AH SLA Battery tests to ‘exhaustion’
The discharge Vbatt Vs time characteristic for this years is different in shape with less time at a lower Vbatt for 2006 Exides - perhaps slight different electrolyte implementation ? (requiring P/N change?) Ca Vs Pb?
(2003-5 Ca were 13.30v fully charged Vfloat Vs 13.00 to ~31.10 for 2006
else lot deviation on batts we got though BOTH of this years show this.
Specifically, with a 100% Midtronics charge
(achievable only by overnight charging i.e.
[fast charge to ~80% capacity+several hours capoff for remaining 20%]
(100% not likely achieved during multi-chg-dischg during competition =~80%)
Tests results:
**9 min life with Vbatt@term 13.10v noload, @50A load quickly down
11.61v then gradually ending at 9.72v at 9 min,
Vbatt@term drops roughly linear, ~2mV/sec**
note: Rload is ~constant so current gradually decreased with time, I=V/R
R=11.61v/50A = .22ohm (note: add .5A @11.61v from fan load)
Equipment:
DMM direct to batt terminals,
Two each **Harbor Freight 12v 100A Battery/Load testers in Series ** (~$15ea)
(50A load for longer test time and match typical 2005/6 robot drain)
Used large 12V .5A KOP fan over vent holes to extend test time over
mfr 10 sec limit
(keeps resistor element loads from burning bright red over 9 min test)
Vbatt@terminals taken ea 10 sec & imported to excel spread sheet & graphed
I don’t have the data & graphs on this computer but will post, if requested.
BTW this method/data was presented at my FIRST Advanced Electronics 2005 Workshop at Calif State Univ Northridge.
Dale(engr294] (TRW / Northrop Grumman)
Dale.Hall@earthlink.net 310 374-8323
Al, I beg to differ…
We compared the capacity of the then new EX battery to year old ES batteries last year, and found the year old batteries to have 50% more capacity at a constant current 20 amp load. This year’s milage may vary, but the batteries were quite different in performance when checked last year.
Eugene
They are right in one sense, if you stall all 4 CIMs, you won’t make it anywhere near one match. However, you can also design everything very conservatively (like we did last year and EricH mentioned) and have them last forever.
Design for 1/4 stall torque and your motors and batteries will thank you.
There is another possibility. When we did some tests a few years ago, we found significant variation from battery to battery even among those that are nominally the same model number.
While I don’t think that it is likely to be the factor that moves a robot from good to great, using the “good” batteries over the “medium” ones can’t hurt.
Joe J.
Well, FIRST disagrees with you. Use that to your advantage 
there are definate variations in capacity from battery to battery from the same year. Every once in a while we got one that was so poor at holding a charge we wrote “BAD” on both sides with a sharpie. They would only last about 3 or 4 minutes, while a good battery would last 12-15m, and they were new!
Gene,
I will test our batteries, I have a CBA battery analyzer from Mountain Radio. I know that there was a manufacturing defect in some batteries last year. When discharged, at some point in time the output voltage suddenly dropped by 2.1 volts. Then later there was another sudden drop. Indication was that cells were shorting or of vastly reduced capacity. I will post when I know more. There should be no reason for a 50% difference between production runs.
Dale, I have to take issue with your methodology. The load testers are for short term testing. When you load to capacity over a long period of time the resistance changes with temperature. It is possible the load testers were changing value to a lower resistance.
ah yes! battery temp is a hugh factor for lead-acid/jel cells.
They can loose as much as 50% capacity from 0F to 150F (across their temp curve)
usually the warmer the battery, the more capacity. Im not sure if it drops off at higher temps?
Last year when we did these tests, we checked two S ones from the prior year and two X ones from the current year. The two S ones were very close to each other in capacity, and the two X ones were very close to each other (within a few percent). One can’t draw conclusions from a sample of two, but the moral of the story is that you need to test the capacity of your batteries and qualify them for competition use. A good battery does not make good robot great, but a bad battery will definitely make a great robot marginal.
We were not seeing sudden drops when we did our testing and our load was a constant current. We will pull the tester out of the attic in the next week or so and check all of our batteries for this year. It will be interesting to compare notes…
Well we checked one of our 2006 batteries and compared it to graphs from previous batteries. This one battery is very similar to the previous years batteries. I am still working on getting test data locked down between a very old battery and the fresh ones this year.
In most cases a battery appears to produce better when warm but there is a diminishing return. The chemical reaction progresses better at higher temperature but the internal resistance changes with temperature as well. It is for this reason that some folks turn their headlights on for 20-30 seconds before trying to start the car on a cold day. For our use, batteries at room temp are just fine, they will heat up during competition but will cool off in a short period of time. The temperature changes also affect charging and the way the charger senses the state of charge on the battery. For this reason I don’t recommend putting a battery on charge immediately following a match. Give it 10 or 15 minutes and the charger will more accurately assess the condition.
I know one reason some engineers might think that the batteries weren’t capable of supporting the current draw of a first robot. A battery of the form factor that FIRST uses cannot put out the amperage that the FIRST robots use. While robots have a 120 amp breaker they will often draw in excess of(i’m guessing here based on how often my team’s robot stalls out) 200 to 300 amps for short periods of time. I checked the battery spec sheets a while ago and they were labeled by exide as being a special type of “Deep Discharge” battery. That basically meant that they were able to under heavy loads be able to output more amps than a regularly configured battery. I can’t rememeber what the written max load was, I think it was around 240 amps, but I am not sure. The batteries as other people have stated have quite a bit varience between them so I’m sure that some of the batteries can put out 300 amps easily with less resistance(and thus for longer periods of time) than other batteries. Anyhow my 2 cents.
Now I have this image stuck in my head, of a white haired guru teaching the batteries at Exide
"when you are sent out into the world there may come a day when you are called upon to give all that you possibly can. Do not draw your power from up here, by your terminals. You must reach down deep, all the way to the foundation of your plates, and pull the energy from your entire being!
You must prove yourself worthy. He who offers up his deep energy freely shall be recharged 100 fold, but he who keeps his energy to himself shall be cast aside
torn asunder, and recycled, according to the law!"
(sorry! lost my mind just for a moment :^)
A tale of 7 batteries:
I finally got around to firing up the load tester on our batteries.
I list the results below:
Constant Load Current: 20.3 amps (checked with fluke current clamp)
Cutoff Votage: 11 volts
All of the batteries were charged with a
three stage battery charger just prior to testing.
The test is to record the time in “EDU main loop ticks”
that the battery takes to drop below 11 volts while under the
constant current load of 20.3 amps.
“Bat Num” Model Year “EDU Main loop ticks”
Battery 1 EX18-12 2006 77559
Battery 2 EX18-12 2005 62519
Battery 3 EX18-12 2005 35849
Battery 4 ES18-12 2004 79576
Battery 5 ES18-12 2005 82548
Battery 6 EX18-12 2006 95907
Battery 7 ES18-12 2004 109930
Battery 3 is one of the kit batteries from last year, as is battery 2.
The low performance of Battery 3 was double checked and a
second test produced 34377. This reflects the repeatability of
the measurement. It is unlikely that we would want to trust
Battery 3 for more than a single match before charging.
Battery 4 spent some time powering an electric fence
during the off season, which I think did it some harm.
It matched the capaciity of battery 7 when it was checked last year.
I have been digging for the time of an EDU main loop
clock tick, I remember it to be 0.0172 seconds, but
I have not found it in the legacy documentation yet.
2004 was, and still is, a very good year for batteries…