Batteries Required??

[thinkpad]

Hi,

Can some of the teams who have already competed in this years game (Especially the teams who played till the finals), give an estimate of how many batteries you used up during the entire competition? How many is enough?

Any assistance is welcome.

The more the better....

Generally we had 3-4 batteries ready while competing. Obviously if you make the elimination rounds you can't charge them all in the short time the elimination rounds occur, so we always asked fellow teams if we could borrow batteries and almost always they would oblige (you know, that whole GP thing)

[jdiwnab]

Well if you bring you charger, then 2-3 is plenty. We brought our charger, and 8 batteries. We only used this year's batteries though. We swaped off batteries before a match and put the older one on the charger. You should always have a charged battery. Even if you don't swap you batteries every match, one should last 2-3 matches easly.

[sanddrag]

Originally Posted by jdiwnab

Even if you don't swap you batteries every match, one should last 2-3 matches easly.

How can you make such a general comment like that? It depends on the robot's drive system, efficiency, driving style, pneumatic system, load on and frequency of use of arm actuation motors, etc, etc, etc. Please, no one just take anyone's word for things like this. "Your results may vary" Something like current draw is not a standard measure between different robots and different operating conditions.

Our 2003 robot completely drained a full battery every match.

[jdiwnab]

Originally Posted by sanddrag

How can you make such a general comment like that? It depends on the robot's drive system, efficiency, driving style, pneumatic system, load on and frequency of use of arm actuation motors, etc, etc, etc. Please, no one just take anyone's word for things like this. "Your results may vary" Something like current draw is not a standard measure between different robots and different operating conditions.

Our 2003 robot completely drained a full battery every match.

Our robot was very heavy on motors (4 CIM, 2FB, 1VD) and batteries would last 2-3 matches. Given that we were being pushed around like crazy and were draining our battery as fast as we could, I think that most people would do OK. All the experience I have had is that 2-3 matches per battery is the norm. But maybe I haven't asked enough.

[Venkatesh]

Lets take a look at the datasheet for the battery for a minute...

The battery we use in competetion is the Exide EX18-12, which is very similar to the commonly available Exide ES18-12. I couldn't find the datasheet for the EX18-12, so the ES18-12 will have to suffice for now.

The ES18-12 is billed as an 18 amp-hour battery. This is true, but only at relatively low discharge rates. In FIRST robots, current tends to be very high, in the highest band of data available for the battery. The datasheet tells us that we can only get 9.0 amp-hours from the battery if our discharge rate is 18.0 amps and has a maximum current output of 230 amps. The usable energy at such high current levels will be lower than 9.0 amp-hours.

Each robot is limited to drawing 120 amps by its main breaker. Even though there have been reports of the main breaker not tripping at notably higher levels for long periods of time, we should ignore those to make these calculations simpler. Let us estimate a robot uses an average of 100 amps throughout an entire match. A match is 2 minutes. This is equal to roughly 3.3 amp-hours per match.

This would lead us to think that a battery can be used for two matches without any issues at all.

However there are more factors to think about - not all robots will drain an average of 100 amps per match. If your robot has a very undemanding drivetrain, a limited number of motors, and never runs motors near stall, you will be able to use one battery for 3 matches (or more) without any issues at all, as more energy will be available at lower current levels. Big, burly robots or ones which put motors in stall conditions often will have to deal with less available time from a battery.

The estimate of 100 amps continuous is probably very high, but can be used as a guide. If you want to swap batteries every two matches, having three batteries would be fine. A fourth could act as a spare when matches are too close together.

Another thing to keep in mind - the charger. Most teams use the kit-provided 6 amps charger, which is relatively quick at charging. We used the 2001 4 amp charger, which took longer somewhat longer. If any teams are using 2 amp automotive chargers or the 2002 1 amp trickle charger, think about having more batteries on hand.

The loads that we place on these batteries are large. Do not count on old batteries to provide as much energy as modern ones, particularly at high current levels.

Draining an entire battery in one match is relatively unlikely. We would need to drain at 200 amps (an entire match stalled, with all motors on, and the main breaker well-cooled) to take us down to those levels of performance. Batteries abused in such a manner will not last more than one or two seasons.

And too many batteries can never hurt.

[slickguy2007]

We drain our battery very quickly because we are running 4 cims, one VD and an onboard compressor. We kill a battery every match. If we run on a low battery the robot doesn't function properly. We have 6 batteries and I think that we probably need 8-10 to be safe. How much do they cost anyways?


GO 1403!!!

[Hokie4Life]

When we go to competition, we always bring 4 batteries. We bring the two provided with the kit, as well as two from the prior year. We bring the newest chargers, where we modify the charging cords and put them into the Anderson power connectors, allowing us to charge without worrying about whether or not the connections are interfacing.

Of the two chargers we take, they both always charge the two competition batteries we use. We always keep both batteries charged, and immeadiately after each match we have someone responsable for taking out the competition battery, and replacing it with a battery from a prior years for testing purposes in the pit. We always have both competition batteries ready for use at a moments notice. This is particularly important because much of our autonomus phase is based on dead reconing, and it is important that our battery has a full charge.

This is my $0.02.

[Mike Betts]

We have done FIRST for 11 years now... Trust me...

You should get 3 to 4 matches from a battery even if your robot uses every motor in the kit.

I usually limit my team to 2 matches to a battery to insure they are performing at peak power.

If you make it to the finals, you will find that 4 batteries leaves you wanting. However, as stated by others in this thread, other teams will gladly lend you batteries to make it through...

As a veteran team, we usually have at least 8 batteries and 4 battery chargers in our pit at a competition.

Regards,

[Tristan Lall]

Let's look at this from a slightly different perspective--what work do your motors have to do? If you have, for instance, motors running near their free speeds, you'll tend to waste less energy, and consequently your battery will last longer. Since most of the kit motors don't spin at a very convenient speed on their own, selecting gear (or chain, or lever) ratios that encourage the motor to spin as close to its maximum-efficiency speed as possible can be very useful for keeping power consumption down.

As a concrete example of this, let's compare 188's last two (i.e. 2004 and 2003) robots, Blizzards 5 and 4, respectively. Both employed six drive motors, with very high top speeds (>16 fps and >13 fps, respectively, under ideal conditions). Blizzard 4, with a total of nine motors on board (six of which were used continuously) required a fully-charged battery for any match where significant pushing was undertaken; this is because it forced the motors to operate at too low a speed, and consequently drew more power (refer to one of FIRST's motor spec. sheets, where the power vs. torque curve is vaguely parabolic--this robot was often forced to operate to the right of the parabola's vertex, in a zone where current is high, and consequently, so is electrical power consumption). In fact, power consumption was so high that during on particularly memorable pushing match, the robot controller reset several times due to insufficient voltage (there was no backup battery that year), and the OI was (intermittently) registering less than 6 volts after a 2-minute round, from a previously fully-charged battery. Don't do this; it's fun, but it's bad for the robots (to the tune of over 200 A).

By contrast, Blizzard 5 added a low gear (ideally 4 fps), which enabled pushing contests to be undertaken at lower intended ground speed, while still operating the motors at an efficient speed; this moved the robot's power consumption well to the left side of the vertex (referring to the same graph), and enabled better use of the limited battery capacity, even though Blizzard 5 also had nine motors (including the compressor). We still changed the battery after every match, but the option to prolong their use by an extra match or two was now viable, thanks to the reduced demands on the always-busy drive motors.

So what's all this off-topic lecturing about? Basically, depending on how you selected your mechanical components (and this applies to any motorized apparatus, not just the drivetrain, and excluding the compressor), you'll end up with some level of power consumption, to achieve a given output power level. This is obviously a huge factor in estimating how long a battery will last.

As I hope I've illustrated, there is no one expected value for a robot's endurance--every robot is different. As design criteria, it is helpful to consider several factors in addition to the above: how many batteries can you afford (not just the cost--can you charge 6 batteries at a time in the pit), how much mechanical power does your robot need to use to play the game (and consequently, how much electrical power is drawn), etc.. When you have the luxury of designing from scratch, you can sit down and say, "I want three matches per battery, at a reasonable pace, or one match, flat-out-pushing everything around."

Since your robot probably can't be changed much from here on in, give us some specifications regarding what motors it uses, what reductions it uses, and the intended functions and duty cycles of those motors, and we can give you a more concrete example of what to expect. For example, does it (or perhaps, should it) quickly lift a stack of 4 tetras, 16 feet in the air, three times per match, or does it just flip the corner tetra off of its magnet once in autonomous mode--you can do both with a pair of F-Ps, but you don't have to care quite so much with one of the two implementations.

One other factor: there are different perceptions of when the battery is due for recharging. Some people don't care if their battery is showing 9 V on the OI; others try to put it back on the charger as soon as it drops below 11 V (on the OI). Personally, I tend toward the latter, rather than the former!

So, you should probably design the next robot for two matches without difficulty, with the option of stretching to a third. Fortunately, most robots end up with this sort of performance, without even trying. Since you don't have the benefit of these estimations for the current robot, you should probably attempt to change the battery after every match, replacing it with a fully-charged one. (A good practice, anyway.) This means, figure out how many matches you'll play at a regional, and factor in (partial) charging times, then see if you can afford the number of batteries and chargers that you come up with. Since you're from Canada, I'll assume that you're coming to the Waterloo regional or the Greater Toronto regional. Expect lots of matches in quick succession at Waterloo, because of the small number of teams there--you should therefore bring many batteries. 188 and 1114 both tend to have around 8 batteries at a competition, but these are both relatively well-equipped teams, and they usually have robots with higher-than-average power consumption needs.

If you wanted a short answer, it's buried in there, somewhere.

[Al Skierkiewicz]

Guys,
The question is "how many batteries will we need?" The simple answer is this...a robot that can run 3-4 matches on a battery charge can get by with four.(Ask around for teams that could supply a spare if needed.) A robot that drains a battery in one match will need 3 to 4 times that number, either owned or borrowed.
The 3-4 match rule of thumb is for designers at the beginning of build. A robot that runs a battery down in one match is dangerously close to running out of power before the match ends, i.e. no reserve. As to actual current drawn and trying to predict what the battery will deliver, there is no way to predict. Each match is different and there is so many other variables it is impossible to try. Suffice it to say that these are the real world constants you should consider...
1. The battery can deliver over 400 amps but not at 12 volts.
2. The 120 amp breaker can sustain 600% over current for a few seconds without tripping. (It is a temperature controlled device.)
3. The 18 amp hour rating is at less than two amps load current. When drawing in excess of one hundred amps expect that to exponentially decrease. I estimate that one match robots pull the 18 AH down to let's see 36 amp minutes.
4. The amp/hour rating is based on terminal voltage of 8 volts. When a heavy use team draws high current, the battery will allow robot function down to about 6-7 volts before the controller quits sending PWM. When the terminal voltage rises again, the robot works for a little while longer.
5. Most high current robots, draw max current in turns, so if there is little turning, the current is much lower.

I will try to cover more of this in a presentation at nationals. Let me know if there is anything else you want me to discuss.
Al

[Mike Hendricks]

We used 5 batteries this year, and 3 chargers. We always went out with a fully charged battery. However .. we dont have pneumatics or a 6 motor drivetrain .. so the load wasnt as much as some other teams.

[Denman]

the best thing to do, is to get a battery reader thing. We borrowed one that tells you the voltage, the amp hours left and the temperature of the battery. it means you can tell which re the most charged...

[DougHogg]

Originally Posted by Denman

the best thing to do, is to get a battery reader thing. We borrowed one that tells you the voltage, the amp hours left and the temperature of the battery. it means you can tell which re the most charged...

We bought two different types of battery tester this year, but neither of them shows the "amp hours left". What was the make and model of the "battery reader thing"?

I suspect that your tester was basing its amp-hours-left reading on the battery voltage. One possible problem: if you start charging a battery and then test it part way through the charging cycle, the battery will have a surface charge which would throw off the tester.

What I am saying is, watch out for half-charged batteries. The voltage on a battery will register high if you take the battery off a charger in the middle of the charging cycle (as the charger was putting a high voltage on the battery to charge it and thus creating a surface charge on the battery), but the charging process won't be complete, so the battery could die during a match if your robot uses more current than the battery got from the partial charge. (We experienced this first hand in the third match of the finals at the S. Calif. Regional this year.)

[Denman]

i'll find out. It is pretty nice tester....