Batteries

[Dave Hurt]

Now that my FIRST years are over, I've been having fun with other forms of R/C things, mainly planes. (It's actually quite fun adding the 3rd dimension of control in there) One of the things I've been playing around with is electric planes. Although my electric Spitfire isn't anywhere the size of some of the usual 130 pound robots, I have to look at the difference in the batteries.

I recently listened to a lecture on electric planes. He talked a little bit about how the batteries for these things are now quite powerful. And to prove that he had setup a 10 cell pack of nickel cadmium batteries could repeatedly turn over the V8 engine in his car, and even turn it over faster then his regular car battery.

So where am I going with this? Look at the difference in weight and power between nicad's and the cells we use. Look at the charge time it takes to fully recharge the current batteries. Most nicad's or nickel metal hydride's can be fully recharged around 20 minutes, depending on how many cells you use. They may not hold a charge as long as a car battery, but for how long they hold a charge they will put out more power. The only downside is the price. For top quality cells, they can cost as much as $15 per cell, and if you have a 10 cell pack that's $150 per pack. But those are 5000 mah cells.

Like I said, there's a small difference in size between my airplane and our robots, but would it be possible to use nicad's instead of car batteries?

[Elgin Clock]

I think the main reason that we have the batteries that we do is the sponsorship end of the deal, EXIDE is going to donate the least expensive batteries that they can. But if we do ever progress to NI-CAD why not take it a step further and go with NickleMetal Hydride. Or hey what about fuel cells???


Sorry It's the Alternative fueled vehicle club I'm in that puts these crazy ideas into my head.


Come see #18 Kineticar @ Tour De Sol 2k2 from N.Y.C. to Washington D.C. Whooo Hoooo!!!!!!!

[BSMFIRST]

The only arguement here would be that NiCds somehow allow teams to build better/more entertaining robots which isn't really a factor. NiCds and NiMH batteries are used in about 50% of BattleBots and 75% of winners. NiCd and NiMH batteries are almost twice as efficient (when compared to their Ah rating) as SLAs when drawing very high current.

The benefits of NiCds are totally lost on FIRST robots.

-The faster charging time is moot since most robots don't need to charge after every match--this is only sometimes an issue near the finals. NiCd and NiMH chargers are expensive as well.

-All performance characteristics are negated when every team has the same battery. There's no real advantage if FIRST gives you lighter, or higher current sourcing batteries. Perhaps the playing field is "lower" but all the teams are still on it.

Dan

[Jon Lawton]

I don't think using NiCads on a robot would be ideal. NiCads have a nasty habit of forming "memories," meaning it's best to discharge them most of the way before recharging them. NiMH would be cool, LiIon would rock. I'm not sure about the charge rate for LiIon, but I've read the ideal charge rate for NiMH to be .1C, with a charge rate of 1C causing a few hundred cycle reduction in life. Hydrogen fuel cells would grab attention, but they are quite impractical at present *shrug* I really think the Lead-Acids do the job just fine.

[Dave Hurt]

Actually, the whole battery memory thing is a fallacy. NiCd and NnMH do not form "memories." What does happen, is that the batteries discharge at different rates. With most peak chargers, it detects when the battery pack has hit the peak of it's charge. If you let the cells sit for a little while, 1 cell will probably have a higher charge then the rest. When you're charging them, that 1 cell will reach it's peak before the others and set off the peak charger, stopping the pack from getting a full charge. But that only happens if you let the cells sit for several days. You could go for a couple days without having to discharge the cells, but it's best to totally discharge them when you're done for the day. Other then that you don't have to worry about that problem. And it's relativly easy to fix that problem as well if it does happen, you just put the cells on a slow charge overnight and it puts an even charge on all the cells.

Perhaps it may not be useful for the general matches, but I had the finals in mind when I was writing this. I can remember a couple finals matches where we could've used new batteries, but we couldn't get them because our other packs were low as well. And our robot didn't perform as well as it could've on a full battery. Whether that effected the match or not, who knows, but it would be nice to have that option to fully recharge batteries between those close matches.

[Dan 550]

Lets get one thing straight: we use Exide 6-cell sealed wet batteries, not automotive batteries. An automotive battery produces 12.6 volts and 740 amps through the reaction between electrolyte and a lead mesh, and is not top-sealed except in racing applications. The sealed Exide 6-cell produces 12.6 volts and fewer than 200 amps (Can't find exact, it's on the side of the battery.) from the a lead-acid setup. Take it from an automotive technician; you won't start any car easily with that battery. Also, I bet you anything that that guy tricked you by using an inductor between the starter and Ni-Cad. 200mah will not start any car. It will produce grinding sounds from the starter because it doesn't have enough energy powering it to move quickly. Also, try touching the battery casing from your Ni-Cad in your plane after an hour's flight -- it's too hot to touch, right? That's the amount of strain four drills would put on it during ten seconds of operation in standard tank drive with frequent herky-jerky movements. No chance it would survive a match. A huge Ni-Cad array is required for the robots using them on TV, to make sufficent amps. We're using our Exides for longer than you might want to think, because they're good and reliable.

[Don]

I believe someone told me that it is a motorcycle battery.

[Dan 550]

Our Exide battery is used only in small machinery, such as our robots. The only other application I know of is a remote-control submarine that uses it because it is reasonably compact and cheap and provides enough amps to continuously power its lights, cameras, and other equipment. No, its not a motorcycle battery; they are never sealed, it's the law.

[BSMFIRST]

This isn't on-topic but I didn't want to make a new thread.
I saw an IBOT wheelchair today (in Pittsburgh) as I was walking to school. It was being loaded into a FedEx "Custom Critical" truck. I was 10 minutes late for a test, otherwise I would've given it a thorough looking.

I guess I can make this kind of on topic...the IBOT uses NiMH batteries (lots of them as you'd suspect.) Dean was at a trade show talking about it and noted that, after heavy research, they found Sanyo cells and specifically the cells used for cordless tools were the best. The R/C hobby cells were actually inferior despite their higher cost.

Dan

[Kit Gerhart]

Those of us who have been around FIRST long enough to remember when our robots used NiCads really appreciate how good the currently used sealed lead-acid batteries are.

Before '98, the robots used two NiCad drill batteries in parallel. We had much less run time than with the current batteries, and it was not uncommon for the batteries to fail within a few months of robot use. It is true that NiCads have very low internal resistance and, thus, provide very high current for their size, but there are shortcomings. Most important is that they do not like being reverse charged, and to avoid this happening, a multi-cell pack cannot be completely discharged. Also, the power density is not all that high, though it is higher than lead-acid. For the robots, using a fairly big lead-acid battery that we have seems to work very well. Out batteries are big enough to provide the current we need without too much voltage drop. You just need several batteries and chargers in order to conduct all day practice sessions.

Nicads definitely have their place, and are hard to beat for running the radio equipment in my R/C airplanes. You never run them completely down, avoiding the reverse charge problem. Also, even though lightness is a virtue for anything in an airplane, the fact that you only need 600mah batteries for the R/C equipment, NiCads' having low power density is not a problem. In our robots, battery weight is not a problem at all, as long as we are all building to the same weight rules. If I remember correctly, FIRST went for 120 to 130# max when they switched to the current batteries.

I have been surprised at how well the current batteries hold up to deep discharges, knowing that only a few total discharges will destroy a car battery.

[echos]

I wish Hawker would sponser first. Especialy with something like their Genisis series. We use those on are BattleBots and they probally the best lead acids you can get. They are so durable you can charge a 18Amph battery in 15 min. Plus they can definatly put out.

I was also wondering what kind of cells I should use for my airsoft gun, weather it be NI-CAD, NM-HD, or LI-ION cells. I'm looking for 8 1.2 volt cells that can pack a punch as far as instantainious current draw. They need to be about the size of a AA cell. I would also like to have a large amount of amprage.

[Kit Gerhart]

Originally Posted by Dan 550

Our Exide battery is used only in small machinery, such as our robots. The only other application I know of is a remote-control submarine that uses it because it is reasonably compact and cheap and provides enough amps to continuously power its lights, cameras, and other equipment. No, its not a motorcycle battery; they are never sealed, it's the law.

The battery we use for our robots is, in fact, a motorcycle battery. I'm not sure what bike(s) use that particular one, but I have a motorcycle and a scooter, both of which use sealed batteries like the ones used by our robots. My scooter, a Honda Helix uses a somewhat smaller one and my Suzuki SV650 uses one similar in size to the robots.

[Kit Gerhart]

Originally Posted by echos

I was also wondering what kind of cells I should use for my airsoft gun, weather it be NI-CAD, NM-HD, or LI-ION cells. I'm looking for 8 1.2 volt cells that can pack a punch as far as instantainious current draw. They need to be about the size of a AA cell. I would also like to have a large amount of amprage.

If all you need is high current for a short time, nicad are the best in this regard. Nm-hd and lithium batteries have higher power density than nicad, but nicads have lower internal resistance, thus higher available instantanious current. You can probably pull 30-40 amps from AA nicads for a (very) short period of time. As far as capacity, AA nicads are now 600-800 mah or something like that. (A few years ago they were around 550) NiMHd are close to twice the capacity of nicad. Lithium ion batteries have about twice the power density of NiMHd. They are higher voltage per cell, so you use fewer cells in series and sometimes series-parallel them in motor packs for electric RC airplanes.

[KenWittlief]

The batterys that FIRST gives us can be used for lots of things. I believe they are used in electric wheelchairs, for emergency lighting, for un-interuptable power supplys

and I bought one a couple years ago to power the running lights on my Hobie Cat sailboat - it will keep the lights on for about 8 hours.

The big difference between the various battery chemistries is Amp-hour capacity vs weight and size. The batterys we use are somewhat heavy and large - a nicad or Nickle metal hydrid pack would be smaller and lighter, but far more expensive

weight is not a primary concern for our bots - we almost never need high acceleration or speed, or cornering ability - so the sealed lead acid bats work well. If they gave us $200 batterys that were half the weight, you would hardly notice the difference in the performance (speed and handling) of our robots

but you take 6 or 8 pounds off a radio controlled airplane, and it makes a huge difference in its glide slope. Thats why RC pilots spend the extra $$$ for the more expensive batterys.

[sanddrag]

As for the issue of starting a car, it is possible to do with a FIRST robot style battery. http://www.harborfreight.com/cpi/cta...emnumber=39954 I have one of these things and you can jumpstart a car at least 4 times before it doesn't have enough power to do it again. Also, what about those things you plug into the cigarette lighter to jump a car. How do they work and what batteries do they have?