arkmundi
10 MW
liveforphysics said:
"LiPO", another "be careful" warning...
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10 kW
liveforphysics said:
Agreed on confinement; do not make confinement gas proof. You could get a bomb which is more hazardous than a fire.
Until the LiCoO2 itself decomposes, all you have is the gases from the polymer electrolyte burning and the rate of combustion is limited by the amount of atmospheric oxygen that can get to the fire. Until this point, you can put the fire out relatively easily. However, when the electrode decomposes, you will quickly move into a thermal runaway mode where you can't put the fire out easily with a conventional fire extinguisher because it is catalytically producing its own oxygen (see Diego Lisbona and Timothy Snee, Process Safety and Environmental Protection, 89 (201 ), p. 434–442). It could burn in a completely anoxic environment, theoretically underwater. The only way to stop this fire is to wait for it to burn out or find a way to remove heat faster than it can be produced. This can happen with any oxide-containing electrode I know of, but in LiCoO2, it happens at a lower temperature.
If a battery reaches 150 deg C, something has gone wrong, clearly. However, as with any system, the more safety built into the system, the less hazardous. So the first priority is making sure the battery does not fail, possibly due to an internal short, very high discharge rate due to an external short, or over voltage situation. How ever this is accomplished, it is a desired characteristic. However, a well-designed system will accommodate a failure of this system. My opinion is that this is where RC LiPO falls down. I have heard of situations where resting LiPO has more or less spontaneously caught fire. I will not address this situation, although if this happens with any frequency, it is a very hazardous material, clearly. What I believe to be the case is that RC LiPO is much more likely to burn when any of the possible modes of failure above are reached. There are people whose opinions I value highly on here who advocate BMSs as the solution. There are those who are adamant that they are worse than no BMS. I believe that a well-functioning BMS is a reasonable part of the solution, but is not, in of itself, the only precaution.
If there were BMS available from from sources with proper QC/QA, I'd feel more confident about them as a reasonable sole solution. However, I don't think that is the case. I think they are being made with very little/no QC/QA standard and with completely unknown failure rates. (If I am wrong, let me know here). I think that as long as they are being purchased and not returned at a rate that results in no profit, there is very little incentive to track failure rates. In addition, product liability issues for equipment purchased directly from a factory in China and shipped outside is non-existent. There are a lot of people working on making all lithium chemistries safer. Until that happens, I think an abundance of caution is warranted. I see no problem with using LiPO appropriately but I think that includes storing it and charging it as though it could catch on fire at any time. As far as riding with it, you can just ditch the bike, so no problem there. I would have no problem at all with riding a LiPO bike all over the place. I may work on a LiPO build, but: 1) it will have a BMS (this is for some protection of the pack, not for my protection), and 2) I will never ever store or charge it where it can cause anything to catch on fire. Certainly not inside a house. If I have a problem, all I will lose is a bike.
Not trying to start a fight here. Just want to be clear where I am. The above seems to make sense to me. Right now, I store no batteries sourced directly from China inside. I am doing literature search on if this is, in my mind an appropriate requirement or not. This is not a slam on China, I have several friends and colleages from there. However, the lack of product liability is a very serious issue. All the incentive is to the lowest price with very little concern for product safety.
I have been thinking about this a lot lately. I'm done now. You can agree or not. Worth what you paid, unless it saves your life, then worth a lot more.
RoadWrinkle
100 W
I am guessing LIPO chemistry has the best power 
output to weight ratio and that is why they are so popular despite the fire risks? Is this true?
output to weight ratio and that is why they are so popular despite the fire risks? Is this true?
10 kW
RoadWrinkle said:I am guessing LIPO chemistry has the best power
I'd say that is a fair statement, based on my experience. You get a lot of C rating for your money also. Just be careful.
Ykick
1 GW
pdf said:RoadWrinkle said:I am guessing LIPO chemistry has the best power
I'd say that is a fair statement, based on my experience. You get a lot of C rating for your money also. Just be careful.
You do get a fairly powerful cell, reasonable interconnect materials/practices and a source to buy in the first place. You also reap benefit of scalable pack configuration(s) and general education about battery pack construction, care and feeding.
But it is NOT for the uninterested user! These little fireballs require technical skill, curiosity, interest, patience, test/charging equipment and an investment of time in order to implement RC Lipo. Even then, it isn't for everybody.
In this day and age of genuine Automotive grade pouches finally beginning to reach our workshops, I wouldn't trade good RC Lipo cells for much else. But thanks to RC Lipo when the time comes, I'll know how to handle whatever battery pack the future brings.
