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Failure Models of LiPO

There are a few basic failure modes for lithium batteries

 

1. due to external heat source

 

2. high rate operation (causing heat)

 

3. internal short circuit (hot spot)

- if there are any impurities between the anode and cathode layer this may create a micro short. localised heating may form and accelerated temperature rises may ignite the unstable material in the cell

- internal short circuits are a byproduct of either poor manufacturing process or failure of the seperature material

- internal short circuits are almost impossible to discover when batteries are coming off the production line.

 

4. external short circuit

 

5. over charge/over discharge

- undervoltage causes the electrode layers to thin and possibly short circuit. upon charging again localised heating may occur and gases generated

- over charge causes heat and HF gases to be formed. See below for more detail

 

So as you can see its all about HEAT HEAT HEAT

 

Failure modes are always difficult to predict pre-event or decipher post incident due to lack of usable material

 

The above scenarios may cause excessive temperature above 400degC, venting, leakage and fires

Why do lithium cells burn?

How can I tell a good LiPO from bad one?

Why do packs bloat?

How does temperature effect my battery?

How do I look after my batteries?

In the RC game this is very difficult.  The manufacturing process for RC lipo cells and say for example an OEM iPAD battery pack are very different. Although the chemicals be the same, the number of steps taken to release a cell from the factory varies amongst manufactures.  The lesser the number of quality control steps the more likely a cell internal fault may occur.  Of course more steps in the process costs more money.

In some way you get what you pay for so use a trusted brand where you know the cells have gone though a more stringent QC process and the dealer you buy them from won't disapear overnight

If in doubt..........don't do it.

 

 

 

Heat is the general cause for batteries swelling or bloating.  It can be caused by over charge or over powering the cell causing excesive heat.

High cell temperatures induce cracks in the electrodes making way for the electrolyte to react with the surface. The interaction with the cathode released H+ ions and ultimately HF gas

 

- LiF + H+  > Li+  + HF

 

The HF reactes with Li2Co3 present in the electrodes causing fragility of the SEI protective film

 

- Li2Co3 + HF > 2LiF + Co2 + H20

 

Depending in the intensity of the current and cell temperature gas in the form of CO2, CH4, C2H6, and H2 are released due to electrolyte contact with the electrodes.  These gases are explosive and any ignition temperature will cause combustion.

 

Ambient temperature effects your battery greatly

 

1. in cold wather lithiumION migration within the cells is more difficult

 

2. at higher temperatures (30 - 40degC) lithium cells have the best energy and power performance but be careful that you don't everheat the battery duing use as the critical thermal limit could be reached quite easily due to the already high ambient temperatures.

 

Lithium batteries also change in its resistance as you charge or discharge. Within the 'zone of temptation" individual cell IR's increase quite quickly as will the temperatures generated.  So try stay above these levels.

 

Operating Li Po batteries. Motors, and ESCs is not unlike operating a model engine.

 

Confusion and danger has come about because too many do not understand or misinterpret the rating criteria for Li Po cells and quality standards of cells/packs have fallen. Here are the facts:

 

1. Cell capacity and discharge rates are specified at 0.2 times the rated capacity for the cell (0.2C) as the discharge rate. That is, if a cell has a nominal capacity of 1000 mAh, that capacity is measured at a discharge rate of 0.2X1000 = 200mAhr. This is by international standard. The scheme is not too useful for high current drain unless the curves are available for the expected operating range.

 

2. Li Po cells deliver a nominal 3.7V when discharged at 0.2C. Note that this is a nominal level, The average voltage for the particular cell above is almost 3.8V. This will vary some what from cell to cell but is the statistical average of many, many cells.

 

3. The discharge curve shows lower voltage and lower capacity as discharge rate is increased. The industry has drifted into the habit of specifying discharge rate as a multiple of the nominal C for a cell. This is because C is in

mAh or Ah, not amperes. This misclassification has created confusion.

 

4. That confusion created the “discharge rate race” in which manufacturers have begun to claim ever higher “Times C” rates. The maximum discharge rate is NOT the rate at which cell capacity is specified. Rather, it is the maximum, or “REDLINE” discharge that must not be exceeded and above which cell damage will almost certainly occur.

 

5. When using Li Po packs, we recommend that you:

- Set the cut-off above 3.0V, or nearer 3.3V in most situations.

- Avoid designing your pack so that it has to operate at the maximum allowable discharge rate.

- Stay out of the “zone of temptation.”

 

Now we can see from many of the answers on this website that heat and overvoltage kills the cells.  So here are a few tips.

 

1. If you have a charger that has extended life battery charge mode... use it.  This means the charge voltage per cell will be limited to say 4.1 - 4.15V and this lessens the stress on the anode and cathode material and will extend the life of your battery pack. Don't worry about the packs not being full as the extra energy (mahr) in the pack by charging up to 4.2V would be only increased by ~5%.  RC Hobbists tend to stay above 3.5V upon discharge too, so you can limit the operation use of your pack from low end of say 3.3 to high end of 4.1, not only will you have a longer play time but also a safer pack as you won't get close to overcharging any of the cells either.

 

2. HEAT.  Heat kills cells thus paying a little bit more for true higher C rated packs with lower IR will mean they will run cooler.  The cooler your packs the longer they will last.

 

3. Charging.  Try not to charge immediately after a flight. Packs are already warm and you'll just heat them up further. Give them an hour at least to cool down.

 

 

 

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