RC Hobby Explosion: April 2014

Written by Michael Roberts

Hey everyone. I wanted to take a minute and relay a story that could be very beneficial to those of you who are either new to using LiPo batteries or are thinking about switching from NiMh batteries to LiPos to get more power and punch from your brushless motor and ESC systems.

About a week ago I was helping out with the phones as we were very busy and luck had it that I took a call that was one of those one-in-a-million conversations. The customer (who will remain anonymous. We'll just call him "Bob" here) began asking a series of questions about what would cause a LiPo battery to catch on fire or explode. I was about to launch into a script that I've got etched in my brain about the dangers of overcharging or overdischarging LiPo batteries when my gut told me that this was more than just mere curiosity or a casual inquiry. There was something in his tone and the phrasing of his questions that assured me there was a primary agenda he didn't want to reveal.

So, I did begin by saying that LiPo batteries (and almost any lithium content battery for that matter) did have certain sensitivities that make them vulnerable to fire if not handled correctly. But I quickly turned from responder to inquisitor as I was anxiously curious to see what Bob was really looking to find out. So, I took a slight gamble and just made the assumption that he had a LiPo that had caught fire. I went with a simple either/or question to get him to commit. I asked him, “Did your battery catch fire in a vehicle or while on a charger?” After a moment, and with some almost audible hesitation, he admitted that he had two batteries that had exploded while being charged. Okay, now we were getting somewhere. As we talked he began to open up little by little and eventually explained how he had done a number of things in charging his LiPo batteries that violated just about every rule out there regarding the handling and charging of LiPos. The end result: he nearly burned down his garage!

What I found out from Bob was that he had bought two higher mAh LiPos for an R/C truck. In addition, he bought a really high-end multifunction charger that could charge up to 4 batteries at once (I didn't even know that anyone was making chargers that could charge 4 batteries at the same time). Anyway, so Bob didn't follow the other golden rule, which is always read the instructions. He proceeded to plug his LiPos into the charger and with sure certainty of what he was doing he started charging the batteries on the NiMh setting at 5 amps. I hope that didn't just slip right by you - yes, I said he had the charger set to "NiMh". It gets better, though.

Bob, I suppose, decided he had to run to the store to pick up some beer, cigarettes, bread, or a fire-retardant hazmat suit. I never really found out why he left, but Bob made the decision to go run errands while his 2 brand new LiPos were charging in his garage. I'm guessing that Bob made one too many stops on his shopping excursion because it probably didn't take too long for those LiPos to reach full capacity. Once the lithium cells were full and the charger kept pumping current into them - BOOM! They probably didn't explode at the exact same moment due to variations in the starting current levels in each battery, but I would guess that the second battery blasted off within a few minutes of the first. Bob arrived home with both the cigarette in his left hand and his garage burning.

Fortunately for Bob his entire house didn't burn down. According to him there was extensive damage to the garage and of course the batteries were a total loss. The by-product of burnt lithium has absolutely no value whatsoever. I don't think I ever asked him about that fancy charger and whether it survived. I seriously doubt it. Considering Bob didn’t use a LiPo safety bag or container, I would say he’s lucky he wasn’t in the garage when the fireworks went off. He could have been burned, blinded, or frightened into a near-fatal coronary. Who knows?

What I do know is where Bob screwed up. Here are my thoughts on how one mistake proliferated into major property damage:

1. Always, Always, Always Make Sure your Charger is Set to the Correct Battery Type.

Bob had a multifunction charger that could charge multiple battery types (Life, Lion, LiPo, NiMh, NiCd, etc.) and could charge each of these at varying amperages. To complicate matters more, it could charge multiple batteries at different settings at the same time. For some beginners, this might have been a case of information overload - too many options and not enough understanding of each and the interrelationship between them.

Unlike some battery types such as NiCd and NiMh, which are fairly resilient and tolerant of overcharging, LiPo batteries must follow certain rules or the implications can be severe. The individual cells in a LiPo battery cannot be overdischarged or overcharged. In Bob’s case, he had 3-cell 4000mAh LiPo batteries. Each cell in each of the batteries has a nominal voltage of 3.7 volts for a total nominal voltage for the pack of 11.1 volts. A fully charged LiPo cell can safely reach a maximum of 4.2 volts in most cases. If the charger continues to charge the cell at high amperage after it has reached 4.2 volts, a whole world of unpleasant possibilities opens up - that is, fire, explosion, and a garage that closely resembles Bob’s.

If Bob had chosen the correct LiPo setting, plugged his battery into the charger, and then connected the small white balance tap on the battery to the LiPo balance board on the charger, he probably would have been fine. When charging began, the charger would have began dispensing current to the cells and the balancing function of the charger would sort of act as a traffic cop directing the flow to each cell as needed to make sure that each cell had exactly the same level of charge (4.2 volts) at the end of the charge cycle. This would result in a fully charged and balanced battery of 12.6 volts that would have been ready for use in Bob’s truck.

What actually happened was much different. By setting the charger to NiMh and not using the balancing function, the charger only knew to keep charging until the battery reached what it detected as its peak level. If the charger’s circuitry couldn't correctly determine what the battery’s capacity was because of the incorrect setting, then it might have kept charging well past the limit of 4.2 volts per cell. Or, even if the charger could sense it correctly, a battery that started out of balance was never brought into balance and individual cells were charged past the limit of 4.2 volts.

To further exacerbate the situation, Bob charged the battery at higher than the normally recommended 1C rate. The C-rate is calculated by dividing the charge rate times 1000 by the mAh rating of the battery. In this case Bob charged a 4000mAh battery at 5 amps (5x1000/4000), or 1.25C. An easier way to determine a 1C rate for a battery is to simply divide the mAh rating of the battery by 1000. In this case, 4000/1000 = 4 amps = 1C. Note that there are many LiPo batteries on the market that can charge at 2C rates or higher. This will be noted on the specifications of the battery, so always pay attention to this.

So what we have is that fully charged 4.2 volt cell continuing to be charged at a higher level of current than the cell can handle. This keeps going and going, building and building, until at some point the cell becomes unstable and a chemical reaction occurs that causes the battery to explode or catch fire. Ouch!

The Solution:

--Use the LiPo setting on your charger or buy a charger specifically designed for LiPos.

--Use the charger’s balancing function

--Charge at the appropriate C-rate.

2. Never, Never, Never Leave a Charging LiPo Battery Unattended

Because of the potential dangers inherent with lithium batteries, you never ever go walk the dog, make a beer run, or depart for any destination outside the confines of your home while a LiPo battery is charging. If you have to leave, then just unplug the battery from the charger and complete the charging process when you return. I’m not even going to say anything else about this one because it’s a total and complete no-brainer.

3. Use a LiPo Safety Bag or Charging Container

Okay, this one may be open for debate. I must admit that probably over 95% of those using LiPo batteries don’t own any type of LiPo charging safety bag or container. They are a nice invention if you don’t like leaving things to chance or if you are charging at really high amperages, though. The fact that most hobbyists don’t have any protective enclosure for charging LiPos combined with the fact that LiPo fires are extremely rare actually shows that LiPos are not something to be feared. When you have good electronics and use them correctly, overcharging and overdischarging are not likely to happen. For the record, I have to say that we do recommend them and especially for new hobbyists using lithium batteries.

4. Read the Instructions and Specifications for your Charger and Battery

Another no-brainer. Trees were cut down to provide the paper on which your wonderfully crafted instruction manuals were printed. Don’t let those trees die in vain. Respect them by taking the time to learn of the correct usage, capabilities, and limitations of your R/C products. Although I have never read the manual to Bob’s fancy charger, I would bet a week’s salary that it addresses every single issue I’ve outlined above. If Bob had read the manual, I will bet his home insurance premiums wouldn’t be going up next year.

My Summary

I have been in this industry for most of my life. I was there when the first LiPos were released to the market. In all that time, I’ve never actually seen a LiPo battery explode or catch fire. I have had a couple of older LiPos that I neglected puff up from falling too far below the low-voltage level, but that’s it. Exploding LiPos are not common. In fact, most LiPo battery manufacturers will tell you that in their testing they will overcharge batteries, charge them at too high of a C-rate, and try everything they can to get the batteries to fail and it is not easy to get them to explode. So, feel confident that if you correctly use and correctly charge your LiPo batteries, they will serve you well for a long long time.

In the end, I think Bob was trying to find out one or two things by calling us. The first thing he was looking for was whether he had a chance at getting replacement batteries from the manufacturer. Well, I don’t know of any manufacturer that will replace LiPo batteries under the warranty that were incorrectly charged. So, I don’t think I made him feel better about that possibility. More importantly, though, I think Bob was trying to find out how much of this was his fault. He was looking for some comfort or solace in this misery of misused LiPos. I had no choice but to be somewhat blunt with him about his responsibility. You can’t really sugar-coat that one too much and doing so wouldn’t have benefited him, I believe. A little tough-love was in order to make sure he learns from this experience and doesn’t put himself, and possibly others, in jeopardy in the future.

I was able to end the conversation on a slightly upbeat note, though. I told him I have an uncle who fixes garages for a really good price. Hopefully, that will be his next call.

Michael Roberts

President

RC Hobby Explosion
www.rchobbyexplosion.com

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