TerraBreak Forums

Hey guys, I'm the tall dreaded guy on VT's team. This is my first post on here, but I kinda HAD to ask.

WHY CAN'T WE USE LITHIUM BATTERIES!! LiFePO4 batteries (A123) are so safe....but we still can't use them. Why not? They're safer than NiMH's because of their great discharge rate and they're stable on top of that. It'd help SOO much!!!

I agree. At some point in the near future it's going to get very hard to find quality nimh cells in the quantities most teams need.

-Adam

I can see them saying that we can't go over the rated amp draw on LiPo's...even if we have to use another fuse or something to gauge our amps. Like if a Lipo is rated to 20C, 30C burst and is 2000mAh....make the fuse blow at 60A, but not a slow-blow fuse.

I just don't think that lipo's are as dangerous as they make them out to be, and the benefits are huge.

Can you explain the above to me in more detail?



I can think of a couple reasons right off the bat. These are my reasons alone, and should only be taken as 'semi-informed opinions'

1) New Teams
NiMHs and NiCads are dangerous enough. Add in a chemistry that could runaway, and/or has special handling requirements and the possibilities of someone getting hurt go up. Have you even checked to make sure that you are allowed and/or equipped to keep LiPo cells of any variety in your lab? I've heard engine test stories to make you fall over from the stupidity. Discharge isn't the most concerning aspect either. Charging LiPo cells is just as dangerous... and considering the lengths students go to 'for maximum performance', I can see why 'the powers that be' are hesitent.

2) They are Too Easy to Design for



I believe your skirting the issue with these two comments. Basically, your complaining that it's too hard to design a propulsion system using NiMHs that completes all the missions. That the performance of LiPo cells would make it easier to design a propulsion system that would meet the requirements. Correct me if I'm wrong.

However, the inverse of that is what's actually going on. The design goals are made conflicting on purpose to see what kind of compromise your team will design to accomplish all the missions. Allowing LiPo cells won't make it any easier.

If they allowed LiPo cell use, I can see 1 of 2 things happening. 1) Significantly shorter takeoff distances (~50'?). 2) Smaller a/c.

#1 would likely drive teams to use a larger prop size. The only problem with this is creating an adequate mission to test the endurance of the pack selected, that runs less than 10minutes. I see #2 as being the more likely of the two. In case you hadn't noticed, a/c size & battery pack limits dropped when the contest switched from 'NiCad only' to 'NiCad & NiMH'. In the 'NiCad era' most packs were on the 4-5# range, and a/c had median wingspans of 8-10ft, with contest winning empty weights of ~7-9#s. These days most wingspans are in the 5-7ft range, with empty weights around 4-7#s and pack weights in the 0.9-2# range.

[quote="AdamatMIT"]At some point in the near future it's going to get very hard to find quality nimh cells in the quantities most teams need[/qoute]
This is probably the most poweroff argument for the use of LiPo cells, since the contest directors are conscious trying to keep costs down (which I think was part of why NiMHs were allowed in the first place).

I'd suggest we require the teams to use 'Off the Rack' Lead Acid cells (Energizer Bunny Anyone?), but that be a lot of waste.

Can anyone else tell I was tired when I posted the first one? Anywho, A123 cells are rated for up to 30C continuous and 60C burst. Pulling the "40" (everyone knows it's significantly higher than that) amps continuous is possible with lower capacity batteries or farther from the limits than with similar-sized battery packs. A123 cells have a higher energy density, and everyone knows that increased weight increases drag, which increases thrust needed, which increases amps drawn. For those reasons I think that they'd be at least as safe as NiMH/NiCd. If you watch this video, you'll see that A123 packs aren't terrible. The specs shown in the description are for off-brand A123 cells, for the record.

Charge times are also very low (much lower than with Nickel-based cells), therefore there would be even less tendency to overcharge them. I mean, most chargers can't output enough am that will cause perage to come close to damaging from over charging. Teams would be less pushed to fly with hot batteries, which is something that's never safe.

New teams seem to not have the need to push the limits as much as the seasoned teams. Also, complacency has got to be the worst thing for safety. It's gotten me into a lot of trouble. In fact, I crashed one of my planes as well as the competition plane (AT competition) due to complacency. About LiPo's in our lab, not only does one team power an SUV from Lithium power. The team we share our lab with have a ton of LiPo's just sitting around for no apparent reason.

As for being too easy to design for? I'm really not convinced about that. You stated that with no back-up information. One of the things that LiPo's would help the most with is the fact that high-quality NiMH cells are harder and harder to find...and the ones we can find don't come in as many varieties as LiPo's. An example, this year we wanted to step up from 2000mAh cells to 2200mAh cells. That's a 10% increase in power. We had to suffer a 25% increase in weight....so we went with the lighter cells. Since I knew we had very marginal capacity batteries, I pushed the limits of minimal airspeed and the tightest 360 I could. The plane tip-stalled. LiPo's just give a more continuous spectrum of capacities and there's no "step" in LiPo's that will cause one of those terrible barriers we had to face this year.

I don't think that smaller aircraft are necessarily a requirement with lipo's. This year put a fun twist on limiting the minimum wingspan on aircraft. A heavier payload would also push wingspans back up (not that I'm recommending anything over 1gal of water....please don't do that again ) All jokes aside, that stupid tank gave me nightmares. The tank and the box. Anywho, a powered-glider style mission requirement would also lead to increased wingspans. As the pilot, I'd push for a HUGE wingspan on our plane in that case. I don't think that LiPo's will make planes smaller, but they will allow for a larger range in planes. If the RAC stopped being geared towards rewarding smaller, lighter planes, or if the scoring system were totally changed, people wouldn't push for those tiny planes. In the real world, as in DBF, smaller and lighter translates to more money. If the judges were cost-conscious, they would do away with rules that rewarded fully-composite aircraft and boxes (MIT and RPI). I don't mean that small planes cost more, what I mean is that a smaller plane that can achieve the same goal as a larger plane will cost more.

You wanted a well thought-out answer, I can't deliver....but this is about as close as I can get :

I think the only valid argument is the availability of quality and quantity of NiCd and NiMh cells. As far as design, it is a restraint giving by the judges just as wing span, take-off distance, etc.

I remember a few years ago that at the AUVSI contest a team burned up their hotel room with LiPos.

Dan

I don't think this is exactly true. OSU's planes we predominently standard rc plane contstruction. The wings and tail were balsa covered with monocote. Our fuselages were composite. Our box was made out of coroplast, cheap. In fact our prototype boxes were made out of signs from the OSU sign shop that were going to be thrown away. Made for interesting boxes.

I don't remember MIT's box really but their plane looked to be balsa/ply with film covering. If you look at the planes a few years back almost everyone had carbon and kevlar planes. I think that the rules driving the weight down on the planes made many teams realize that on the scale we are working with, balsa and covering may be the lightest way to go.

Dan

Yeah the box was just pink foam with some glass and strategic kevlar.

I'm personally a fan of the smaller planes, makes for easier design/build/crash/repeat processes, and certainly keeps propulsion system costs down. I think the best rule to ever show up in past years was the RAC=wingspan*weight. That year had some of the most varied and interesting designs.

-Adam

I meant MIT's plane and RPI's Box. And about the AUVSI team, that really sucks....but LiPo's have really progressed in the past few years. Did anyone watch the LiFePO4 video I posted? That gave me a lot of confidence in those cells. Plus, a lot of the constraints that DBF is attempting to put on the battery packs don't really apply to us. The only constraint that affected us this year was that we thought we couldn't have greater than 1:1 thrust-to-weight ratio....and that wasn't even a rule. We were well under half the maximum weight limit on the batteries.

OSU really impressed me this year. Both teams did a great job of using very simple techniques and materials to get their stuff done. It really takes engineering and thought to achieve those results with nothing more than standard building techniques.

This is what I agree with most. Availability decreases every year for acceptable NiMH cells, and NiCd are all but dead. Cost is in favor of LiPo, A123, etc. as well. Anybody try to get performance data from the manufacturer/distributer for a NiMH cell? I was told that they wouldn't even bother looking up any data they had for me, because the demand was just not there. Not so with newer chemistries.

Allowing the new cell types makes sense to me from the availability standpoint. Sure, the performance benefits are huge, but when it is a performance benefit available to every team, it is really no new benefit at all as far as the competition goes. For example, if the rules next year required BRUSHED motors to be used, the playing field would not be affected, as every team must use a brushed motor. You would adapt and move on.

However, I cannot agree with the statements about the safety of NiMH vs. LiPo, etc. Abused, any cell chemistry can be dangerous. The statement that 'charging LiPos is dangerous as well' is not true, in my opinion. Improper charging is dangerous, for all chemistries.

My .02C

Jonathan

{removed critical fail in posting}

What was that massive quote for?