My cousin recently came to me with a battery problem. He got a drone for Christmas, and he had not unboxed it for a while. Last Christmas, right? I asked. Apparently, it was the Christmas before (i.e a year and a half of being in a box). Uh-oh, I go. I almost knew what was coming. The drone obviously did not work, and I got to see a picture of a very bloated Lithium Ion pouch cell.
Your LiPo battery does need some attention from you every few months in the form of a periodic recharge, or you will end up with a cell that looks like this. My colleagues have brought me plenty of gadgets with the plastic case popped open from a bloated battery.
Lithium-ion cells can swell due to many reasons, but somehow the issue of deep discharge swelling and the related hazards with charging such a battery seem to be widely underestimated.
Common causes of battery swelling include:
- Overcharge conditions which accelerate parasitic reactions between the electrodes and electrolyte, with release of heat and gases.
- Poor cell quality and design with low anode to cathode stoichiometric ratios, particulate contamination and other manufacturing defects
- Mechanical damage to electrodes induced either during cell assembly or from the product application
- Excessive temperatures (Do not leave your cell phone inside your car)
- Deep discharge of cells
Anything that increases the self-discharge rate of a battery is bad (including poor manufacturing quality, and exposure to heat) and allows your battery to go into deep discharge quicker.
The CT scan below shows severe jellyroll distortion from gassing due to deep discharge. As you can imagine, a battery such as this one (Figure 1) would not work very well.
Figure 1: CT scan of battery from deep discharge swelling
A deep discharge condition can also create safety hazards. When a lithium-ion cell goes into deep discharge, it is in a highly de-intercalated state. (The word Intercalate means to insert between layers in a crystal lattice). In a lithium-ion battery both the anode and cathode have ‘cubbyholes’ for the lithium-ion to shuttle back and forth. When the battery is deep discharged, the protective passivation layer on the anode called the Solid Electrolyte Interphase (SEI) layer decomposes, with new electrode and electrolyte surfaces coming in contact and new SEI layer is formed. All of these reactions lead to gas formation.
Figure 2: Mechanism of Deep Discharge in a Lithium-Ion Battery (Reproduced under Wiki Commons license from https://www.nature.com/articles/srep30248)
The copper current collector on the negative electrode also starts to dissolve as shown in Figure 2. Upon recharge these copper ions deposit on electrode or separator surfaces potentially leading to a copper short and the risk of thermal runaway (Battery folks call this an event, while normal folks call this an explosion).
Such a deeply discharged battery should not be recharged and a deep discharge voltage cutoff and mitigation must be implemented in the battery protection system by the device manufacturer. This can include a trickle charge till a certain threshold voltage is reached within a given time and ramping up charge subsequently. Failure to reach the voltage threshold in the given time, would classify the battery as damaged and shut it down, preventing it from being used.
What can you do as a consumer to prevent a bloated battery situation? Do not buy cheap batteries, where you are not in a position to assess cell manufacturing quality. Do not buy aftermarket chargers, where protection adequacy is unknown. Do not expose your batteries to high temperature. And lastly, if you do get a drone (or other electronic gadget) for Christmas don’t wait for a year and half to open it – your battery does need a periodic recharge.