Lipo in heavy

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Originally Posted by graeme

We used one of nippers packs in little flipper at the uk champs pulling 500A and there wasnt a problem with the lipos, also ls4 pulls over 6000w at 37v spinning up fast and either packs could run a heavy so if we werent allowed them in heavys then the feather rules should also be looked at as they have less armour to protect them. Big nipper has 300A fuses on each lipo pack and 200A on the motors so if a motor stalled it would blow, so the 1000A lipos would never be stressed. No problem.

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OK so you have a 1000A lipo and a 300A fuse, all seems very good with a more than adequate safety ratio.

I had a look up to see if I could get some reasonable time/current characteristics on line. Weren't extensive so i settled for these http://www.littelfuse.com/data/en/Data_ ... aSheet.pdf

If you take a look at page 5 you will see the time/current characteristics of this particular 300A fuse.

Follow the 300A line up and you will note that this is quite capable of holding a 450A load for about 1000seconds.

A 1000A load would take 20seconds to blow this fuse

Now I don't know what fuses you are using but I would suggest that the fuses are similar in terms of their actions. To corrctley size a fuse you need to know what the fault current is for a dead short accross the battery therminals and the time you need to clear the fault in.

I would like to see a set of hw lipos set on fire by whatever means over charging or shorting, on a table and physically see what happens.
How much would a pack cost?

Exposed LiPo packs vs large spinning masses, powerful flippers, axes and sharp edges

is something we really want at our events?

It's something we've had at almost every RL and RR event since they started, with the Antweights, I know what we're talking about here is heavyweights but it was AWS9 in Aylesbury that LiPos were first used, we've had punctures and fires since then and we've learnt to deal with them.

Some of the things we've brought in would be difficult to scale up, I'm not sure I could get all the packs I would require to put in Tanto in the largest LiPo, being worst case scenario that all would need dealing with, but we can do other things. There are 2 issues I see with LiPo's:-

1)The way in which they are stored in the robot - they would need to be secured properly and not left to bang around inside the robot where they could be ruptured.

2) Controlling a potential fire - they don't burn like NiCads or NiMH burn how ever it's not like its not controllable, we're lucky that we don't hold that much flammable materials in our robots or in the pits. I would like to point out I know there are flammable materials, but I refer mainly to Formula 1 and a like where LiPo's are used in conjunction with high octane fuel.

If we can cover the main problems I don't see why we can't see them used.

In late ,2010 as part of my masters, I made a set of motors (4) that we're a unique design, in testing them I had 4 packs of LiPos powering the test rig. We did a full load test to prove the high end of the power output, while we ran this test one of the LiPo packs started to swell, upon noticing this I stopped the experiment but it was too late, the pack burst and caught fire in my hands. We didn't have the equipment on hand (although we did have the equipment) to deal with this fire, I had to rush it out and put it in a safe place to let it burn itself out. Cause of the fire was a current limiter failure (wasn't programmed correctly). The pack was one of these (equivalent)

http://www.hobbyking.com/hobbyking/stor ... ouse_.html

At Folkestone 2004, we had a NiCad fire in TX-108 (cheap cells) neither experience was fun, but I've been on the end of fires of our primary power source. The way we deal with them is whats important, we need to develop new avenues of technology or we'll be stuck in the past. Maybe we should look into a specific safety procedure for LiPo before they get totally dismissed.

A 36v 10Ah pack would cost around the £400 mark if you buy the top end sets which are the only batteries I would want to run

Something like these http://www.flightpower.co.uk/index.php? ... cts_id=189

I should also say I'm not interested in running lipos for ragging the arse off them like I do in feathers. Two bosch 750s and a couple of linear actuators will draw bugger all in comparison. My reasoning is purely the fact that you can run a much lower capacity because of the higher energy concentration.

Just an idea but flightpower may be able to give you a discount on a lipo pack if you explain to them that it's for testing to see if they're suitable for robot use as I'm sure they'll do everything they can to sell a load of large lipo packs to robot builders and if they do become the norm as in feathers then that's a lot of £400s for flight power!

I have had problems with lipos in my rc car, in small configurations the cells seem to run down the same amount, but when I'm on 22.2 v 6 cells I am sometimes overdischarging a cell sometimes to under 1 volt, they do swell and don't Wont to charge again. With propably 7 in series minimum we will see more problems even with voltage cut offs set correctly they require serious attention when charging and Inspection after use , any low cells would make the whole pack unusable. I think we should stick to a123s.

Well it's standard practice in featherweights to remove the cells and inspect them before charging and you have to use a balancing charger to keep the cells balanced. Any swelling or damage means that the pack has to be chucked.

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Originally Posted by terry

I think we should stick to a123s.

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I actually disagree. The main technical issues I see are the internal resistance of the battery and correct fuse sizing.

The internal resistance of a battery increases over time. It can 'fall of a cliff' and still be within the tolerences of a lipo charger so it should be measured at each event to see if it shows a significant deterioration in resistance. It would be useful for the nipper team to do this during their testing so we have something to compare with.

Fuse sizing can either be done theorhetically using time/current curves or we could simply do some destructive testing and short a few batteries to see when the chemical reaction is initiated. This would also give Jonno his wish to see one go up (Hopefully)

Taking on board everything else which is already done then they are probably as safe as they can be.

Then it comes down to how you deal with an 'explosion' should it happen. OK, it'll be contained anyway, the amount of smoke is an issue as is accessibity and removability of the pack to put it out. As an example it took us about an hour to remove hydras batteries when they caught fire so an easily removable panel and a battery holder which can be cut out with a set of bolt croppers would not go amiss. That said risk assesments for that are Jonno and Alans bag as they'll be the ones dealing with it.

I've been trying not to comment but its so difficult. :lame:
Huntsman went to compete in Robogames with LiPo's. The drive was powered by a 6s 5000mAh 40c pack and the axe was powered by 10s of the same batteries. Huntsman wasn't the only heavy to run LiPo's, but the majority of the big name robots used A123s because they didn't want to spend the money to change when they already had some perfectly good batteries. Mostly it was new build robots with the LiPo's, such as my Huntsman, most of the collage teams and I think Touro Maximus aswell. The Robogames procedure during the safety inspection was just to declare if your robot had LiPo's or not, with no extra checks after they ticked the box. Fires inside and outside the arena were handled the same if it was LiPo or just electronics burning. A clear walk way was kept between the arena and the rear building exit. During a fire it was loudly called out around the pit and the walkway cleared. The dedicated fire marshalls went in when it was safe (usually after it stopped smoking) and covered the robot with a fire blanket and rushed it to the exit and fresh air. The arena also had an extraction fan on the pit side to remove smoke. A fire really only slowed the show down by about 5 minutes but the audianced usually enjoyed it. I checked the skip after the competition and most of the crispy batteries were A123s. The LiPo's seemed to have a better survivabilty rate.
As for resrictions on chargers, fuses, armour and that sort of rubbish they didn't really seem to care. With 650+ robots at the competition it was mostly down to common sense by the builders and firmly putting the boot up anyone who stepped out of line. The restrictions I heard mentioned here wouldn't work if more then 10 robots turned up at a competition. Its more down to the EO knowing and preparing flr what will happen if there is an accident (as opposed to guessing what will happen, big difference) rather then imposing rules on the builders that are too restrictive. Spending 15 minutes to check each robot, pit table and procedures would be too much. And 50 rules on batteries won't be followed. Robogames was a great event because the EO's and arena marshals knew what was going to happen and exactly how hard to kick. I only got kicked a little two or three times, but my collage mates across my pit table got a big kick for their stuff up and shown the door.

And if you want to try being difficult with LiPo's trying taking 7kg of them in a back pack through several international airports. Ahhhhh, fun.

On a side note we just had a promotional day here at a big shopping centre that resulted in the death of 4 or 5 LiPo packs in feather weights. We had one fire in the electronics of one robot (a dead short) but not one bit of smoke from the LiPo's. They got hot, pregnant and fruity but we couldn't get them to pop.

This is exactly the info we are looking for , this is the point of the discussion :-)