Some members of the drivetrain team went down to a company that makes motorized wheelchairs a while back, and came back saying that we can’t run four CIMs on the battery, alone without electronics, and still have enough battery life. I contend that last year, several teams ran more than four motirs, plus electronics, the compressor, etc, and still ran it. And since the battery is pretty much the same 12v-18a power supply ((Can anyone clarify the difference between the ES-18-12 and this years EX-18-12?)), we should be able to run ours. DOes anyone have any adivce on this? I think we should at least run two motors for each transmission on a RWD system, but they keep saying it wont go…
Also, what motors do you recommend for the drive system? On a RWD I mean.
In terms of the company that makes wheelchairs, no it wouldn’t have enough battery life. We ran 4 CIM’s, 1FP, pneumatics and electronics last year. Worked fine, and we had a battery life of maybe 10 minutes (while driving the robot around constantly, but not stalling the motors)
In 2005 we used 4 CIMs for the drivetrain, used three pneumatic cylinders (one large and two small), and another big motor for our arm joint. A battery charge would last for two matches easily. Given the maximum current draw through the fuse/breaker block, I don’t think you could run out of battery power in two minutes.
Our 2005 robot had a compressor running a lot, 4 cims on the drive train, and 2 FPs on the tetra elevator. The drive train was geared for torque and the elevator was geared for speed, so we drew a lot of current. Our effective battery life was about 4 minutes, but who cares with a round that is 2 minutes long.
Using the two new cims along with all the other motors WILL murder your battery. So my humble opinion is, try not to use 11 motors. My team is using 7 this year, along with the compressor, and WE’RE worried about power issues.
Don’t build a battery killer. Please, think of the batteries.
Sometimes you really have to question what professionals tell you if it just doesn’t seem right. The battery life probably would not be sufficient for a wheelchair but most definitely would be for an average FIRST robot. Also, the battery is rated as 18 Amp-Hours capacity, not 18 amps supply. Roughly roughly speaking, it means you can continuously pull one amp for 18 hours or 18 amps for one hour, etc. You should look at the spec sheet http://www2.usfirst.org/2005comp/Specs/batex.pdf for a more specific answer.
Plus, twice the motors doesn’t automatically mean twice the current draw. What it does automatically mean, is almost half the loading each, which would mean approximately half the current draw (maybe a little more) each resulting in the total current draw of the drive system being only slightly higher than with only two motors. It is kind of the same concept as walking in snow with stilts or walking in snow with snowshoes. You weigh the same no matter what, but the snowshoes distribute the load better, because they have more area. For a robot, more motors, more load distribution. Less current draw per motor. Only slightly more current draw total.
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 FIRSTKOP 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.
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.
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.
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.
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.
