With appealing attributes such as low weight, high energy density, and ever greater discharge rates, Lithium-Polymer (LiPo) batteries have transformed all facets of RC. The emergence and continual improvement of these batteries has provided a significant performance boost for RC cars, boats, airplanes, and helicopters, while also paving the way for new vehicles such as multi-rotors. All of this electric goodness does not come without a cost. If not handled and utilized properly, LiPo batteries can quickly become damaged or even catch on fire. Today, I'm sharing some of the basic things you should know about making the most of LiPo batteries. I will also provide techniques to mitigate the risks that these batteries pose.
Understanding the LiPo Lingo
When talking about a LiPo, the primary characteristics to understand are the battery’s voltage and capacity. This is typically noted in a shorthand such as “4S-2200”. In this example, “4S” denotes that the battery has four cells in series. The nominal voltage of each cell is 3.7 volts (4.2v fully-charged), so the total pack voltage is:
4 cells x 3.7v = 14.8v.
When talking about a LiPo battery, the primary characteristics to understand are the battery’s voltage, capacity, and discharge rate.
The second number denotes the capacity of the battery in milliamp-hours (mAh). A fully charged 2200mAh pack is rated to provide a current of 2200 milliamps (2.2 amps) for one hour before it is fully discharged. This capacity value is completely independent of how many cells are in series. In simple terms, the capacity value allows you to estimate how long a battery will provide useful power in a given application. In practical terms for RC use, the capacity rating is typically only helpful for rough comparisons of different batteries. i.e. a 2S-5000 battery will provide about double the run time of a 2S-2500 lipo in the same RC car.
While it was quite common 10 years ago, is now rare to find a RC LiPo battery that uses cells in parallel. Let’s look at an example in case you happen across one. A 4S2P-2200 battery would consist of two 4S-1100 batteries wired in parallel to provide a total 2200mAh capacity. All other things being equal, you would care for and use this battery the same as you would the previous 4S-2200 example (which is really a 4S1P-2200, but we ignore the 1P). There may be a difference in physical size, but a 4S-2200 and a 4S2P-2200 are functionally equivalent. The differences will really only matter to the guy at the factory who has to assemble the battery.
Another important aspect to understand about LiPo batteries is their discharge rate. This value allows you to determine how many amps the battery can output continuously without becoming damaged. Since many RC applications have high amperage requirements, the discharge rate of a battery can be a very significant factor in the overall performance of the vehicle. The discharge rate is expressed as a multiple of C. The internet can’t seem to agree on what “C” really means. Whether it is coulombs of charge, the capacity of the battery, or the one-hour charge rate isn’t really important for our purposes anyway. The end result is the same.
Going back to our example of a 4S-2200 battery, let’s say it is rated for 20C discharge. To calculate the maximum discharge capability of the battery, we multiply 20 by the second number:
20 x 2200 = 44000 milliamps = 44 amps
The calculation tells us that this battery can safely be used in a system that is expected to continuously pull 44 amps or less. It also means that a fully-charged battery will last about three minutes at that current draw. Using the same logic, a 4S-2200 30C battery would be valid for applications requiring up to 66 amps of current, with a two minute duration.
For aircraft models, the common practice is to select a battery with a discharge rate that is at least equal to the maximum current draw of the aircraft.
It is rare to have a RC vehicle that pulls a continuous amperage value. The current draw will more likely be all over the map during a race or flight. For aircraft models, the common practice is to select a battery with a discharge rate that is at least equal to the maximum current draw of the aircraft (i.e. “full-throttle”). If you don’t know the current draw of your aircraft, it can be measured or predicted.
With RC cars and boats, hard acceleration can cause huge short-term current spikes, which complicates things in terms of selecting a proper battery. When in doubt, choose a battery with a higher discharge rate. The performance of the vehicle will only benefit from it. The only downside is that you’ll almost always pay more to get a higher discharge rate. Sometimes the batteries with higher discharge rates are slightly heavier as well.
I’ve not yet heard of a LiPo that burst into flames during storage. All of the fire incidents that I’m aware of occurred during charge or discharge of the battery. Of those cases, the majority of problems happened during charge. Of those cases, the fault usually rested with either the charger or the person who was operating the charger…but not always.
The first lesson from all of this is to get a good quality charger and learn how to operate it properly. Many chargers on the market can charge LiPo and several other battery chemistries (which is very handy). These chargers often have built-in checks to make sure that they are configured properly. Those safeguards are helpful, but it is ultimately the burden of the user to make sure that things are kosher. Is the charger set for the correct number of cells? Is the charge rate within the battery’s limit? Is the balance plug connected? All of these are things that should be in your mental (if not written) charging checklist.
Most multi-cell LiPo batteries feature two power connectors. The primary connector provides the full series voltage of the battery to whatever device it is intended to run. You will also find a balance plug which enables you to tap into individual cells within the battery. The voltages of the cells in a battery pack may not stay in synch as they are cycled. The balance plug allows the charger to ensure that each cell within the pack is charged to the 4.2 volt maximum and no more. Most other battery chemistries don’t require this level of precision when charging. It matters with LiPos, so don’t overlook it.
Even when you’ve done everything properly, things can go wrong while charging. The best way to prepare for this circumstance is to fireproof your charging area. I have my charging bench topped with ceramic tiles. I also place my batteries in a fireproof container while they are being charged. Neither of these precautions will prevent a fire, but they will serve to mitigate the effects if a fire occurs.
There are several fireproof containers on the market that are designed specifically for LiPo batteries. One of the most popular is the LipoSack by WoodWorks Products. It is a soft pouch made of fireproof material. LipoSack is a sacrificial piece of equipment, but replacement is free if it ever does fulfil its fire-squashing destiny. Like most good ideas, there are cheap knock-offs of the LipoSack that may or may not work as well. The stakes are pretty high here, so why risk it?
To manage charging risk, I have a smoke detector just above my charging area.
Everything you read will warn that you should not leave a charging LiPo unattended. You want to be present to recognize any early signs of failure (puffing, smoking, venting) before things get out of hand. While I agree that this is sound advice, I am also enough of a realist to admit that it is impractical to keep a continuous watch (or even to always be in the same room) every time I charge. To manage this risk, I have a smoke detector just above my charging area. I also have a baby monitor set up. I never leave the house while charging, so I should hear the smoke detector regardless. The baby monitor lets me hear any warnings emitted by the charger or even strange sounds from the battery itself. My system is not as good as keeping a continuous watch over the battery, but I think it’s the next best thing.
Emptying the electrons from our LiPo batteries is the payback for tolerating their charging idiosyncrasies. This is where we get to plug in and enjoy the performance benefits of LiPo. Unfortunately, even here we must be mindful of some limitations to avoid problems in the near term, as well as down the road. We’ve already talked about verifying that your battery must be chosen to withstand the amp draw of the vehicle that it is powering. It is worth mentioning again since it is so crucial. Using a battery that can’t deliver the amps will result in lackluster performance and quite likely an early grave for the battery. Generally speaking, the less that batteries are pushed to their max (that goes for charge and discharge), the more cycles they will provide.
Being kind to your LiPo batteries also requires that you prevent over-discharge. This means that you should not let the battery voltage drop below about 3.5 volts per cell. Most modern Electronic Speed Controls (ESC) will automatically warn you when your battery voltage is getting near the worry point. It may pulse the power to the motor, shut the motor off, or give you some other signature that it is time to wrap things up. Make sure that you are aware of how your ESC will notify you -- and listen when it talks. You can also monitor the voltage of your battery with a small and inexpensive voltage checker, which will buzz a warning when you reach below a specified threshold.
To prevent over-discharge you should not let the battery voltage drop below about 3.5 volts per cell.
Over time, or perhaps from just one traumatic event, a LiPo battery may get “puffy”. Depending on how much it expands, it may or may not be ok to continue using it. A little swelling is usually ok as long as the battery will still physically fit in the vehicle. Once you notice a degradation in performance from a puffed battery, it’s time to retire it.
LiPo batteries intended for use on RC cars are usually encased in a hard plastic shell. This helps to protect the fragile cells from damage during the roughhousing that it will inevitably endure. Most other LiPos, however, have nothing more than a thin layer of heatshrink tubing encasing them. They are prone to accidental puncture with tools or knives. They are also quite likely to be damaged if they are involved in an aircraft crash. Be sure to thoroughly inspect any crashed battery for damage and keep an extra close eye on it for the next several charge cycles. Most RC racetracks and flying clubs keep buckets of sand handy to safely quarantine suspect batteries.
You may be able to mitigate crash damage to your battery by analyzing the physics of a potential crash. Perhaps the battery is located behind a bulkhead that has fasteners poking through it. If crash energy throws the battery forward, those protruding fasteners could very well puncture the battery and cause much greater damage overall. Do what you can to address the most likely scenarios.
LiPos have great self-discharge characteristics; they will hold their charge for a long time when they aren’t in use.
Crashes definitely take their toll on batteries, but I’ve seen at least as many LiPos that were ruined due to absent-minded mistakes. The most common error is leaving the battery plugged into the ESC. Even if you aren’t operating the vehicle, the ESC still pulls a small housekeeping current that will drain your battery completely if left unchecked.
LiPos have great self-discharge characteristics; meaning that they will hold their charge for a long time when they aren’t in use. For maximum longevity, it is advisable to set batteries to a storage state (about 3.8v per cell) if they are going to be unused for more than a few days.
End of Life Planning and Proper Disposal
Some recycling centers will accept Lithium-based rechargeable batteries, and that’s always a good choice. Unlike many other battery chemistries, however, LiPos are considered safe to discard in your regular trash once they have been fully discharged. The key is making sure that they are actually fully discharged. You want the discharge to be a gradual, controlled process, so nails and pistols are considered poor tools for the job. I usually discharge a trash-bound battery by connecting it directly to a large DC lightbulb. I also do all final discharges on a large pad of concrete with nothing else around. I’ve had several batteries get hot and swell during this process, but none have caught fire…yet.
I let the battery drain down to 0 volts before disconnecting it from the lightbulb. I then allow the battery to cool to ambient temperature. Once it is stable, I repeat the lightbulb treatment just to make sure there is no residual energy. Lastly, I connect the positive and negative leads of the battery together to create a short circuit and prevent and sort of energy recovery. After sitting for a day or two in a fireproof container (to make sure it is stable), I’ll put the battery in a Ziploc bag and place it in the trash can.
Hopefully this article has not scared you away from using LiPo batteries. While they do carry significant risks, being aware of the dangers is the best way to combat them. As long as you follow the basic operating guidelines and treat these batteries with respect, they can be an irreplaceable energy source for your RC fleet.
Terry spent 15 years as an engineer at the Johnson Space Center. He is now a freelance writer living in Lubbock, Texas. Follow Terry on Twitter: @weirdflight