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TheAnsweris42 20-01-2006 11:45

Motors/Battery life
 
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.

devicenull 20-01-2006 11:48

Re: Motors/Battery life
 
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)


It's definitely possible.

Rick TYler 20-01-2006 11:55

Re: Motors/Battery life
 
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.

Veselin Kolev 20-01-2006 12:04

Re: Motors/Battery life
 
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.

sanddrag 20-01-2006 12:09

Re: Motors/Battery life
 
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.

Katie Reynolds 20-01-2006 12:36

Re: Motors/Battery life
 
Quote:

Originally Posted by TheAnsweris42
((Can anyone clarify the difference between the ES-18-12 and this years EX-18-12?))

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.

KTorak 20-01-2006 14:55

Re: Motors/Battery life
 
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).

Al Skierkiewicz 20-01-2006 15:38

Re: Motors/Battery life
 
Quote:

Originally Posted by TheAnsweris42
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.

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.

BrianBSL 20-01-2006 16:34

Re: Motors/Battery life
 
Quote:

Originally Posted by sanddrag
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.

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.

EricH 20-01-2006 17:41

Re: Motors/Battery life
 
Quote:

Originally Posted by TheAnsweris42
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 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 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).

TheAnsweris42 20-01-2006 18:29

Re: Motors/Battery life
 
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...

Dale(294engr] 20-01-2006 21:00

Re: Motors/Battery life
 
Quote:

Originally Posted by sanddrag
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.

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

eugenebrooks 21-01-2006 02:02

Re: Motors/Battery life
 
Quote:

Originally Posted by Al Skierkiewicz
There is no difference in the ES or EX battery.

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

Joe Ross 21-01-2006 10:49

Re: Motors/Battery life
 
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.

Joe Johnson 21-01-2006 10:56

Re: Motors/Battery life
 
Quote:

Originally Posted by eugenebrooks
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

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.

Manoel 21-01-2006 11:29

Re: Motors/Battery life
 
Quote:

Originally Posted by eugenebrooks
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

Well, FIRST disagrees with you. Use that to your advantage ;)

KenWittlief 21-01-2006 11:50

Re: Motors/Battery life
 
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!

Al Skierkiewicz 21-01-2006 12:04

Re: Motors/Battery life
 
Quote:

Originally Posted by eugenebrooks
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

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.

KenWittlief 21-01-2006 12:38

Re: Motors/Battery life
 
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?

eugenebrooks 21-01-2006 13:54

Re: Motors/Battery life
 
Quote:

Originally Posted by Joe Johnson
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.

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.

eugenebrooks 21-01-2006 14:00

Re: Motors/Battery life
 
Quote:

Originally Posted by Al Skierkiewicz
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 1.2 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.

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...

Al Skierkiewicz 21-01-2006 19:10

Re: Motors/Battery life
 
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.

Ianworld 23-01-2006 12:45

Re: Motors/Battery life
 
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.

KenWittlief 23-01-2006 12:53

Re: Motors/Battery life
 
Quote:

Originally Posted by Ianworld
I checked the battery spec sheets a while ago and they were labeled by exide as being a special type of "Deep Discharge" battery...

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 :^)

eugenebrooks 05-02-2006 23:50

Re: Motors/Battery life
 
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...

Al Skierkiewicz 06-02-2006 07:58

Re: Motors/Battery life
 
Gene,
We are running tests on all of our batteries (some going back to 2002). Using the CBA II at 7.5 amps, we test down to 8 volts (the cutout for the RC). The computer then allows us to overlay the results and I will be printing those up shortly. It takes a few hours to run the test so we don't want to leave a battery on test when no one is present. Interestingly enough, we have another battery that has diminished cell capacity on one cell that obviously reduces by 2 volts during the test. The CBA also calculates amp hour ratings from the test.

Gdeaver 06-02-2006 12:12

Re: Motors/Battery life
 
Unfortunately, many teams do not have constant current robots. The batteries are subjected to huge inrush currents constantly and a occasional 30 sec grudge push match. A more realistic test would be to cycle the batteries at 20 amp with a periodic surge. In 2003 our bot ruined the batteries from constant high loads. I use some gel cells in some portable lifting equipment that are designed to supply short term surges as opposed to constant load. They are of the wheel chair form factor. This battery design would be more appropriate for first uses but they are much more expensive than the battery design we use.

Al Skierkiewicz 06-02-2006 13:50

Re: Motors/Battery life
 
G,
We are more interested in veryifying that the batteries we own match a published curve and can be compared to other batteries. We are not trying to predict run time, just looking for batteries that don't meet spec or are past their prime. What we have determined thus far, is most of the batteries fit the curve and that older batteries are showing reduced capacity.

eugenebrooks 07-02-2006 01:49

Re: Motors/Battery life
 
The battery tester we built, controlled by an EDU RC controller, can test with varying loads. One can program any desired load profile from 0 to 63 amps, in steps of 1 amp, and simulate a match if you want (with current limited at 63 amps). One could upgrade to support higher currents if one wanted. We have only used the load tester for a fixed load, however, to certify the capacity of our batteries against the published curves. It pays to identify any batteries that are not measuring up and relegate them to practice use, or single match use.

Al Skierkiewicz 07-02-2006 18:08

Re: Motors/Battery life
 
1 Attachment(s)
Well,
Here as promised is a few of the battery test runs. The CBA II can draw down 7.5 amps and gives a visual display of amp hour capacity. It at least will allow us to compare to published curves for the batteries. Please note that one of the old batteries is slumping quite a bit and will suffice for while longer in practice at home. Please also note that one of the newest and unused batteries exhibits a reduced cell (drop of 2 volts) but in general the curve still fits the other batteries. It is interesting to view the slightly reduced capacity of the older batteries which may have gone through several hundred charge discharge cycles in their lifetime.
Although this method does not represent real world current draw on your robot, it is representative of the health of your battery compared to new. I believe these tests can be further modified by using a shunt (a parallel load resistance) to accelerate the current draw. However, the tested curves do match the published curves fairly well, at 7.5 amps the amp hour rating is reduced to at least 12 AH and the batteries are testing at almost 16 AH when new.


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