Understanding LiPo Fires 101
 

In the lithium ion/lithium polymer battery industry – the severe battery failures and battery fires are very well characterized. There are a lot of companies that make a living from failure analysis of batteries. I pick their brains on a regular basis and I get to plagiarize what they have taught me. All severe lithium ion/polymer cell failures can be attributed to one of the follow root causes:

1. External Thermal Damage
2. Overcharge
3. Overdischarge
4. External Short Circuit
5. Internal Short Circuit
6. External Mechanical Damage

1. EXTERNAL THERMAL DAMAGE – LiPo cells will get damaged by external heat. Most manufacturers recommend keeping the cells under 60 deg C or 176 deg F. In my experience, a cell will start to balloon up and the layers start to delaminate once the temperature rises above 90 deg C (194 deg F). If the temperature is extremely severe – the cell will go into thermal runaway and you will have a flaming mess. The thermal volatility is directly related to the cell chemistry. LiCoO2 is the industry standard and unfortunately, the most volatile. LiFePO4 (used in most power tools) is the least volatile and LiMnO4 Spinel (used by Apogee) is somewhere in between. Many of the responsible LiPo manufacturers are using mixtures and additives to LiCoO2 to decrease volatility at the expense of voltage output. Unfortunately, most manufacturers still use pure LiCoO2 since it seems to make the most power.

2. OVERCHARGE – LiPos are extremely non tolerant to an overcharge condition. A standard charge profile is CC/CV to 4.200V. Drastically overcharging a cell just once is a sure way to send a cell into thermal runaway. Overcharging a cell slightly but repeatedly is also extremely bad for a cell. For example, it you charge a cell to 4.300V, the lithium ions start plating on the electrodes forming lithium metal. Lithium ions are not flammable, but lithium metal is. Every slight overcharge cycle will plate more and more lithium metal resulting in a battery that is very prone to igniting.

The best way to prevent overcharging is to charge through a balancer and to avoid chargers that do not charge with the standard 4.200V CC/CV charge profile. At this point in time I do not recommend passively balancing the cell (matching the voltages while the cell is not charging) – I posted my reasons for this in another thread somewhere. There is an Integy charger with a 4.25V/Cell setting which allows for a slight overcharge. In my opinion, that charger should be banned.

3. OVERDISCHARGE – overdischarging by itself is not dangerous, but it will destroy the cell. Overdischarging below the recommended cutoff voltage will cause the copper to start dissolving in the electrolyte. The dissolved copper will then start plating on electrodes which may start an internal short circuit within the cell. The safety of the cell is compromised once this plating starts and the next charge/discharge cycle will be of concern (see internal short circuit).

Don’t store you cells completely discharged. All cells have a small self discharge when left alone and if the self discharge takes the cell down below its minimum voltage, then the cell will be destroyed. It is also a great idea to disconnect the battery from all electronics (remove from speedos, disconnect lipo receiver packs from regulators etc) since most electronics have a small current drain even in the “off” position.

4. EXTERNAL SHORT CIRCUIT – lipos have current capabilities that would embarrass NiMh. When these cells are shorted out, the extreme current drain will cause the battery to overheat. As soon as one section of the cell overheats – then the cell will go into thermal runaway resulting in a nice cozy fire. As far as I know, Kokam is the only lipo that designs the negative tab as a fuse.

5. INTERNAL SHORT CIRCUITS – this is mostly caused by contaminants getting into the cell. You would be amazed how many manufacturers operate in a nearly open air environment. One of my overpaid consultants uses a very simple tool – a magnet – and waves it over the exposed electrodes (prior to cell assembly) and it’s amazing all the metallic contaminants that little magnet picks up. Contaminants can poke through the separator over time. If you are lucky, the cell will just have a high self discharge rate. If you are unlucky, the microshort will cause a localized heat buildup and create a thermal runaway condition – and thus another cozy fire. Another source of internal shorts is the punching process the manufacturer uses to stamp out the anode and cathode electrodes. Most of the low end manufacturers use a low cost steel rule die that cost maybe a couple hundred dollars. The better manufacturers use a high end die that cost a couple orders of magnitude more. The problem with cheap steel rule die punch is that it tends to leave burrs on the electrodes. These burrs have a tendency to puncture the separator and create microshorts. This microshort will create an area of localized heat. In most cases, this will cause the cell to puff up. In bad cases, this localized heat may be enough to ignite the cell. Every time you charge a cell, the cell will expand about 5% in the thickness dimension. This expansion/contraction may cause the burr to eventually rub through the separator. The vibrations and shock from an RC car also rubs the burr against the separator. Most of the lower end manufacturers don’t perform a simple high pot test (apply a high voltage and check for current leakage) to check for small internal shorts during the manufacturing process. The big Sony recall was largely attributed to burr type contaminants as well as Dell’s non-standard pulsed fast charge technique during the CV part of the charge cycle.

What can YOU do to minimize the risk of this type of failure? - Not much actually. It’s not like we are able to audit every cell manufacturer and even if we did, most of the RC cell distributors hide the manufacturer information. Of all the factories I visited, Saehan Enertech had the highest level of quality control and cleanliness followed by Kokam. Enerland is passable though they lag behind Saehan and Kokam. I don’t think I am allowed to list some of the factories I visited that are downright dangerous.

As a distributor – if a lipo cell manufacturer claims that they can produce a custom size cell tooling for you under $2K, then it’s an indication that their electrode punching process is dangerous.

6. EXTERNAL MECHANICAL DAMAGE - it doesn’t matter if you have the highest quality cells in the whole world if you dent your pack. In order to understand how mechanical damage affects a lithium polymer battery – you need to understand how they are mechanically built. A lipo battery is made up of 20-30 layers of a very thin sheet copper anode, a thin plastic separator and a thin aluminum cathode. A dent will can create a microshort by making the stiff metal anode or cathode poke through the soft plastic separator. As I mentioned above – this microshort will create an area of localized heat. If you are lucky – the cell will puff up. If you are unlucky – you will have a fire hazard. Another repercussion of a dent is that some layers of the cell will become delaminated and thus inactive. This means that the working layers will need to work harder to provide current and thus generate more heat in a localized area. I’ve been a huge proponent of hard cases to protect the physical integrity of lipo cells. In my professional opinion, a hard case for RC cells should be mandatory. I cannot believe that there is at least one lipo battery distributor that is spreading propaganda that hard cases for lipos are not good. In my opinion, this is absolutely irresponsible. Toshiba had a recall on laptop batteries when they found out it would not withstand a drop test. The soft-shelled lipos don’t stand a chance in a drop test.

I’ve seen many postings on how XXX user has a soft shell pack that’s been ejected from the car and run over by another car and there was no fire and therefore hardshells are not required. Great – lucky you! Most of the time nothing will happen – but you don’t want to be around when that 1% of the time when something bad does happen. I’m surrounded by thousands of lipos everyday, but I wouldn’t pit next to a racer running a soft cased lipo.

What can you do? If you are a track owner/operator – insist on hard cased lipos.
As a governing body like ROAR – start specifying rules for the hard case. For example, the hard case needs to have a seam so that in case the cell expands, the case will pop open so that the cells do no build up any more pressure. In my opinion – Orion did a fabulous job designing the case (except that they flopped the + and – going from the 4800 to the 3200 battery) and I hope others copy it.

Great info! As hobbyist many of us are of the mentality of wanting to understand how things works. This information is great, and easy to understand.

for those that might be questioning this "linger" guy's credibility. he does this for a living. working with lipo batteries everyday for a medical company. :nod: :nod:

ling.. no pictures of hot women in your thread? FAILURE! :lol:

very interesting! Thanks for writing this.

THANKS!
amazing information. this thread should be made STICKY.

linger vs maxamps,smc,etc
 

Who's opinion do you want to trust? I think the simple and professional presentation of facts makes this point obvious. I for one, will only buy lipo's in a case (today, that is orion-> i like the 3200 case design)

Hey Ling

Thanks for the info. I need to talk to you about disposing of a suspect Lipo or maybe you can post to let everyone know what a proper safe way of doing this is. Know of any place that take these batteries in the B.A.?

Great to see useful information for a change

As always great info and well needed. :nod:

Edit: Hey linger you should repost that very informative piece you did on lipo quality control as a new thread. It must be buried in another thread and it would make it easier to find for anyone interested. Ling's greatest hits thread anyone? :lol: