Safety Tips for Using Lithium Polymer (Li-Po) Batteries

All remotely piloted vehicles will have at least a battery for the control and some kind of power source for propulsion. The power can come in many forms from rubber band power to gasoline to rocket motors, however, lithium polymer (Li-Po) batteries are far and away the most common energy storage platform for drones and UAVs. Batteries are an easy thing to take for granted. Almost everything we use requires some kind of battery. Many battery types are very safe and pose little risk and minimal maintenance. The LiPo battery does not fall into this category. Lithium-Polymer batteries require special care and maintenance to keep them working well and to keep you safe. Let’s take a look at why the LiPo different and what you can do to keep your wallet happy and the fire-department from making an unscheduled visit.

Lithium Polymer Batteries

LiPo batteries are chemically the same as Lithium Ion batteries used in cell phones and laptops. The main difference in the battery types are the housing. What RC model and drone enthusiasts refer to as LiPo batteries are really lithium Ion batteries in a polymer casing vs. the hard shell one typically finds in other lithium cells. This makes LiPo batteries advantageous for several reasons. First of all, LiPo batteries have one of the best charge to weight ratio of all batteries on the market today. Secondly, LiPo batteries have a relatively flat voltage curve as they discharge. What that means is the voltage remains fairly constant under load, up to a point. While the chemistry of the battery gives us a favorable capacity to weight ratio and a high discharge rate, there are a few issues to be aware of when using LiPo batteries. LiPo batteries produce hydrogen (a very flammable gas – see Hindenburg Disaster) as a result of discharge. Under normal conditions, the amount is minimal and poses little danger. However, damage to the battery can easily result a fire. Over charging, discharging , and physical damage can all cause internal shorts facilitating rapid discharge of the battery’s stored potential, resulting in a buildup of heat and hydrogen: a recipe for disaster. (Take a look at this link to a youtube search for Lipo Fires for some samples of the carnage). Special care is required for extending the life of your batteries and precautions should be taken to minimize the risk when using LiPo batteries.

Maintenance, Storage and Safety Tips

The first rule for LiPo batteries is to never leave them unattended while charging. An battery left unattended while charging can easily overcharge and catch fire. Secondly, a LiPo battery should always be charged and stored in a LiPo bunker. A LiPo bunker is a fireproof bag or box that will contain any battery fire, should one occur. LiPo batteries should never be completely discharged. This can cause the battery to short internally and/or cause it to fail to charge. Never leave the battery charging unattended. Overcharging is a sure way to cause problems: At best, the battery can be rendered useless, at worst the battery could cause a fire, and many have. Never use a battery that is puffy. The puff comes from the cells breaking down internally and can release flammable gases. A ‘puffy’ LiPo has a reduced capacity to store charge and can result in your remotely operated vehicle losing power and crashing or catching fire. Never use a battery that has visible external damage. Anything that punctures the cells can cause internal shorts. The high charge density of LiPo batteries means that the cell can rapidly discharge across that short, generating intense heat and releasing toxic fumes. Always balance charge multi-cell LiPo batteries. Doing so will extend the battery life and reduce the potential for individual cells to over-discharge and short internally. A good balance charger is a relatively inexpensive step to take to minimize the risk to you, your equipment, and, most importantly, to other people around you.

Safety Tips

• Charging LiPo batteries on surfaces made of cement, ceramic, stone, or steel are good ideas. These cannot catch and spread fire. Carpeted floors and wooden tables, on the other hand, are not safe charging surfaces.
• If you see that your LiPo is swollen or swelling, immediately stop charging it. Close it up in a safe container and observe it for 15 minutes. Your LiPo is now a potential firecracker that could go off any minute but could un-swell and return to a safe setting.
• Store your LiPo pack in a non-flammable container. These include ceramic, metal, and fireproof cases made especially for LiPo batteries. Keep these containers away from flammable objects.
• If you mistakenly allow the positive and negative battery leads to come into contact with each other, be ready to expect a fire or explosion, and on a milder note cell ballooning or cell damage.
• The settings for your lithium polymer charger should be ideal and compatible with the LiPo battery it is charging. Current settings and cell count has to be considered. As a general rule of thumb, you should charge LiPo batteries no more than 4.2 volts per cell or discharge them no less than 3.0 volts per cell.
• Lipo charging safe bags are an outstanding safety option. Use these to hold your LiPo’s during charging.
  Never keep LiPo’s inside a vehicle, even an open-air one.
• When disposing LiPo batteries follow the set of safety rules that will render the object non-lethal to the environment. Disposing in fire or heat is a terribly dangerous idea.
• You should invariably store your LiPo’s partially charged. This will encourage them to maintain their performance levels over the long term.
• There really is no need to cycle them unless they have been stored for more than 3-6 months and you are bringing them back out for use again.

Conclusion

Taking care of your batteries will save you in the long run. You will save money, in terms of extended battery life, but also, you will save yourself from increased risk of damage to property, equipment, and human life. Drones and remote control aircraft are a fun and rewarding hobby, but proper precautions are essential to keep yourself and those around you safe.