Safely using LiPo Batteries

Today, we will be taking a detailed look at the safe use of LiPo batteries. Used properly, these incredible sources of energy can power up your radio-controlled or robotic projects, no matter how large or current-consuming.

However, improper use of LiPo batteries can severely shorten their lifespan, or worse, result in a deadly LiPo fire. And we certainly want to avoid that at all costs!

Let’s see how to safely use LiPo Batteries.

Introduction

LiPo, or Lithium-Ion Polymer Batteries, are one of the great technical advances to come around in the last 20 years. These powerful energy sources are used in mobile devices, radio-controlled aircraft and vehicles, electric vehicles, and, of course, robotics.

LiPo batteries have a very high “specific energy, or mass-to-energy ratio. In simple terms, they pack more power for their weight than any other type of battery, including other Lithium-based power cells.  

In terms of cost, LiPo batteries may seem expensive, but when you factor in the amount of power they provide and the weight they save they are pretty cost-effect.

However, LiPo batteries require more tender loving care than regular batteries. They have very stringent requirements for charging, discharging, storage, and disposal, and not observing these requirements can result in poor performance, reduced battery lifespan, or even a fire or explosion.

But, when used properly, LiPo batteries can be perfectly safe and can allow you to build projects that you couldn’t power up otherwise.

Let’s learn more about LiPo batteries.

LiPo Batteries

You’ve probably seen a number of batteries that use “Lithium” technology. So how do LiPo batteries fit in? Learning a bit about how they were developed can help answer that question.

LiPo History

All rechargeable Lithium cells or batteries (a “battery” is a collection of cells) are based upon the principle of the reversible storage of ions, a technique pioneered in the late 1970s.

In 1980 the very first lithium cell was developed at Oxford University.  These were Lithium-Ion cells, and they used a liquid electrolyte to hold the lithium ions.

The first commercial lithium-ion, or Li-ion, battery was produced by Sony in 1991, for use in some of their camcorders.

In a LiPo battery, the liquid electrolyte is replaced with a gel polymer. The first LiPo battery was released by Ericsson in 1999, for use in their mobile phones.

One very popular use these days for LiPo batteries is in radio-controlled aircraft, quadcopters, and ground vehicles. Their incredible power to mass ratio allows for a long operating time while minimizing weight, which is especially important in aircraft.

How LiPo Batteries Work

A LiPo battery, or more accurately a cell in a LiPo battery, is made up of a container that holds a polymer gel electrolyte. Each end of the container has a current collector, one end is the anode and the other the cathode. There is a separator in the middle of the cell, dividing it into two sections.

Lithium ions are deposited in one of the sections of the cell.  As to which section, that depends on if the cell is charging or discharging.

If the cell is discharging, supplying power to a connected load, then the Lithium ions are released from the anode side and travel to the cathode side.  Electrons travel through the load from the cathode to the anode.

If the cell is being charged, then the movement of the Lithium ions is in the opposite direction, from the cathode to the anode.

Understanding LiPo Specifications

In order to select the proper LiPo battery for your application, you’ll need to know how to read and interpret its specifications.  If you have used other batteries, then some terminology will be familiar, while some other terms may be confusing.

Let’s end that confusion, shall we?

Voltage and Cell Count

A LiPo battery, like any battery, consists of one or more cells wired in series. It is also possible to have cells in parallel, as we will see in a bit.

The total output voltage of a LiPo battery is therefore dependent upon the number of cells wired in series. However, that voltage is not constant.

The voltage specified on a LiPo battery is the total value of the Nominal voltage of each cell. Nominal voltage is the average voltage of the cell and is defined as 3.7-volts per cell. So a two-cell LiPo battery will have a nominal voltage of 7.4-volts, a 3-cell battery will be 11.1-volts.

The fully-charged voltage on a LiPo cell is 4.2-volts, so the two-cell battery will have a total output of 8.4-volts and the 3-cell LiPo battery will read 12.6-volts when fully charged.

The minimum safe voltage per cell is 3-volts. Most users never let their LiPo batteries get down to that level.

The number of cells in series within a LiPo battery is defined by its “S” number:

• A 2S LiPo battery has two cells.
• A 3S LiPo battery has three cells.
• A 6S LiPo battery has six cells.

So a 6S battery will have a nominal output voltage of 22.2-volts (3.7 x 6) and a fully-charged output voltage of 25.2-volts.

LiPo cells can also be wired in parallel, although this is not that common. The “P” specification determines how many parallel cell banks there are.

Battery Capacity and C Rating

The capacity of a LiPo battery is measured in milliamp hours, or mAh.  This is the amount of current that a fully-charged LiPo battery can be expected to continuously deliver in one hour before its cell voltage drops to the minimum acceptable level, which is 3-volts per cell.

So a LiPo battery rated at 5000 mAh could deliver 5 amperes continuously for one hour before it needs to be recharged.

But the same battery can actually deliver more current, just for a shorter period of time. If you were to draw 50 amperes continuously, the battery would only last for a tenth of an hour, which is six minutes.

You would think that this would mean that the battery could also be relied upon to deliver 300 amperes of current for only one minute, but that may not necessarily be true. This is where the “C” rating comes into play.

The “C” rating specifies how many times the rated current capacity can be multiplied by before the battery will be in danger of thermal runaway, or catching fire.  You multiply the “C” rating by the specified capacity to determine the maximum amount of current you could safely draw from the battery.

Some examples:

• A 5000 mAh LiPo battery with a 50C rating – You can draw a maximum of 250 amps from this battery. At that rate, it will only last for 1 minute and 12 seconds.
• A 2500 mAh LiPo battery with a 30C rating – You can draw a maximum of 75 amps from this battery. But you had better do it quickly, as it will only last for 48 seconds!

Normally you would never draw the maximum C-rating, it is really there to handle peaks. So if your device consumes 4 amperes but occasionally requires 20 amperes (such as a motor when it is starting or when it gets stuck) then a battery with a 4000 mAh rating with a “c” rating of 5 or higher will work.

Another use for the “C” specification is when charging the battery. Some LiPo batteries can be quick-charged, and the maximum current you can give them is also defined with a “C” value. So a battery that is rated at 3000 mAh with a maximum charge rating of 3C can be quick-charged with a current of 9000 mA, or 9 amps.   However, as we will discuss when we talk about charging, it is generally a good idea to charge at a rate of 1C, 1.5C maximum.

Internal Resistance

Another very important measurement when working with LiPo batteries is the Internal Resistance, or IR, of the cells within the battery. 

This value is rarely specified on the batteries label, although you can usually find it in a spec sheet (if you can obtain one).  This is because the value changes over time.

The internal resistance is measured in milliohms, and the higher the value is, the less efficient the cell is. Excess resistance within a cell will be dissipated as heat when current flows through the cell.

The internal resistance will rise as the cell ages. In addition, the operating temperature can affect this reading.

You require special test equipment to measure internal resistance, you can’t use an ohmmeter or multimeter. Many advanced chargers have a function to measure total resistance, which is the combined internal resistance of all the cells in the battery.

It’s a good idea to measure internal resistance (if you have the equipment) when the battery is new, and check it periodically. If you see a sudden rise in the resistance value, then it could be an indication that the LiPo battery is approaching the end of its useful life.

Balancing Outputs

You’ll find two sets of leads emerging from your LiPo battery. 

The output leads are the thicker wires, a red and black one, that are used to power the target load. 

There is also another set of wires going to a smaller connector, the number of wires depends upon the number of cells your battery has. These are the balance or monitor outputs.

The balance wires contain a lead running to the output of each of the cells in your battery. They can be used for a number of purposes:

• To monitor individual cell voltages.
• To charge cells independently.
• To balance the voltage levels in the cells, so that they all match.

We’ll discuss balancing a bit later on, as it’s one of the more important tasks you’ll need to perform to keep your LiPo batteries healthy.

LiPo Connectors

The main power output of a LiPo battery is brought out on two low-gauge wires, terminated with a connector. There are several types of connectors that you’ll find on LiPo batteries:

Deans Connectors – Probably the most popular LiPo connector, used in model airplane and quadcopters as well as RC vehicles

XT-60 Connectors – Becoming very popular, easier to solder than Deans. There are also larger XT-90 and smaller XT-30 connectors.

EC3 Connectors – Made for Horizon Hobby, popular for RC aircraft.

Traxxas – Very popular with RC Cars and Trucks, very easy to assemble.

Anderson Power Poles – An older style connector, originally designed for HAM radio. Not used too often due to their size.

Tamiya Connectors – Essentially these are Molex connectors, not used much anymore as they have poor performance.

You can also buy adapters to adopt one type of connector to another one.  However, it’s best to choose a battery that matches the connector on your device, if at all possible.

LiPo Battery Accessories

There are a number of accessories that you can purchase to make using and maintaining LiPo batteries a lot easier. You can pick them up on Amazon or at any hobby shop.

Electronic Fast-Acting Fuse

One very handy accessory to have in your kit is an electronic fast-acting fuse. This is something that you would temporarily insert at the output of your LiPo, between the battery and the load you are powering.

The purpose of this device is to make certain that your load isn’t presenting a short circuit to the LiPo, which could result in a deadly LiPo fire.  You only need to use this fuse during testing, after which you can connect your battery directly to the load.

LiPo Monitors

These inexpensive devices are used for testing, but they can also be permanently included in your device. I’m using a couple in my Outdoor Rover project.

These monitors have an LED numeric display, and they connect to the balance or monitoring leads from your LiPo. They cycle through the cell voltages continuously.

Another feature they have is an alarm, a loud buzzer that can be set to trigger when any of the cell voltages drop below a preset threshold. This is a very useful feature, as it will let you know that it’s time to remove the LiPo and recharge it.

Capacity Controllers

A Capacity Controller also can be used to monitor the output voltages from each of the cells, and it is more accurate than the LiPo Monitors are. It can also be used to balance the cells as well.

This is a great device to have in the field for checking up on the health of your battery.

Splitters, Joiners, and Taps

While you can purchase silicone-insulated wires, heat-shrink tubing,  and connectors yourself to make your own wiring adapters, it’s often a lot simpler just p purchase premade ones. They are quite inexpensive, so making your own doesn’t really have much economic benefit. Plus, they are professionally made and properly insulated.

In the video accompanying this article, I demonstrated a few of these devices. One was a splitter that lets you power two devices from one LiPo battery, a very common requirement.  Another performed the opposite function, putting two LiPo batteries in parallel to increase current capacity.

Another one was a tap, it had some higher gauge wires connected to a Dupont connector that you could use to tap the output of the LiPo to drive a low-current device, or to simply monitor the battery output.

The devices I illustrated all used XT-60 connectors, but you can get them with other styles of connectors as well.

LiPo Charging

You’ll need a LiPo-specific charger to charge up your LiPo batteries. This is due to the strict charging requirements these batteries have.  You can also use a multi-use charger, as long as it can be set to LiPo or Lithium Polymer.

LiPo Charging Basics

To charge a LiPo battery, you need to bring the cell voltages up to 4.2-volts. Do not exceed this voltage.

Make sure to set the number of cells (the “S” rating) on the charger before you start. Many chargers will detect this automatically, but even so, it is always best to confirm that the detection was correct.

There are actually two methods to charge the battery:

• Charging the whole battery, through the output cable.
• Charging the cells individually, using the balance cable.

It is always better to use the balance cable, although it can take a bit longer to charge the battery this way. Many advanced chargers use a combination of both methods, which is fine as they monitor the battery during charging.

The safest charging current is 1C, the rated capacity of the LiPo battery. You can also go to 1.5C if you are in a hurry, but OI wouldn’t recommend exceeding this, even if your r battery is rated for a higher charging current. If you do decide to exceed that rating, make sure you check the temperature of the LiPo during charging, reduce the current if the battery gets too hot.

If your battery is new, you might consider “breaking it in” by charging and discharging it five to eight times before you use it. This can increase the longevity of the battery.

LiPo Charging Safety

There are a number of rules to follow when charging a LiPo battery.

The number one rule when charging a LiPo battery is to NEVER leave a charging LiPo unattended! Not even for a second. If you have to leave the charging area, then stop the charge.

The majority of LiPo fires happen during charging, and the deadly ones are usually a result of leaving the battery unattended. 

 Here are some other safety rules:

• Always check the connections before starting the charger.
• Charge outdoors, or in a well-ventilated area.
• Always have a fire extinguisher nearby when charging.
• Never charge the LiPo inside the device it is powering. Always remove it and place it in a safe area.
• Always charge a LiPo on a non-flammable surface.
• Never charge a damaged or ballooned LiPo.
• Never short circuit the output of a LiPo at any time, but especially during charging.
• Always have a fire extinguisher handy.
• Keep children and pets away from the area you are using to charge the LiPo.
• Keep away from flammable liquids or objects when charging.
• Never leave a charging LiPo battery unattended (I know I already said this, but it is so important that it is worth repeating).

When charging your LiPo it is a good idea to have it inside a battery bag or other protective enclosure. And check the battery periodically for excess heat, it’s not uncommon for it to get a bit warm, but if it gets too hot to hold, then stop charging.

Also, observe the battery while charging and look for signs of ballooning. If you observe this, then immediately halt the charging process.

Never charge to a level above 4.2 volts. It’s strongly recommended to balance the cells at the end of the charging process, many chargers will do this automatically.

LiPo Discharging & Balancing

The whole purpose of having a LiPo is to power something up, which discharges the battery. Unlike other types of batteries, it’s not a good idea just to run a LiPo until it dies.

Monitoring During Discharge

As with petty well every aspect of using LiPo batteries, it’s a good idea to monitor the battery while discharging.

In the discussion about LiPo accessories, I showed you an inexpensive LiPo monitor, the one with an LED display. These are very useful to make sure that your battery is healthy, and is not excessively unbalanced. And they will also sound an alarm when the battery approaches a critical low cell voltage.

The Capacity Controller can also be used to periodically test the LiPo cells, one mode it has is to display the tidal voltage difference between the highest and lowest cells. It also displays which cells have this difference, although on a 2-cell (2S) LiPo that would be pretty obvious!

Balancing a LiPo Battery

Another great use for the Capacity Controller is to balance the LiPo.  It accomplishes this by determining which cell has the lowest voltage and then applying a resistive load across the other cell(s). The load will drain the cells until they match the lowest-voltage one.

You can also purchase a separate device that is exclusively made to do this, it operates similarly to the capacity controller.

If you are near your charger check to see if it has a balancing function, if, so you can use that.

LiPo Storage

Unlike “regular” batteries, LiPo batteries have pretty specific storage requirements. In an ideal world, you wouldn’t store them in your home or garage, due to the fire risk. However, in the “real world” this isn’t always possible.

Let’s see how we can safely store and transport our LiPo batteries.

Storage Voltage

You should never store a fully-charged LiPo battery. Instead, it should be brought down to “storage voltage”, which is 3.8 to 3.85 volts per cell.

There are a few ways you can achieve this. 

If the LiPo cells are currently under 3.8-volts then you can charge them up to this level. Most good LiPo chargers have a Storage function, which will raise the cells to 3.8-volts.

If the cells are still charged above 3.8-volts, then you will need to drop them down to that level. Again, many chargers have a function to do this, but it can take a very long time to reduce the voltage, especially if they are near a full charge.

Another method is to put them back in your device and run them down to that level, or just below it, so you can top them up in the charger.

You can also use a resistive load to drain the batteries, a 20 – 40 ohm power resistor is a good choice. The resistor should get warm, but not too hot, and the battery should remain cool. Depending upon how charged up they are, it may take some time to complete the drain down to 3.8-volts.

If you do need to bring them up or down to storage voltage, be sure to observe all the safety tips from the charging section, and never leave them unattended.

LiPo Battery Bags

If you search for LiPo storage, there is a good chance that you’ll come up with a number of “LiPo Bags”. 

These bags purport to be both explosion and fire-resistant, and on the surface, it sounds like they would be all you would need to safely store your batteries. But the truth is, they aren’t all that they are advertised to be.

These bags are made of cloth, usually with a metallic compound woven into them. They generally have either a zipper or a Velcro fastener, and they come in a wide assortment of sizes.

But despite their somewhat grandiose claims, they really can’t withstand the heat of a LiPo fire. At best, they will slow it down enough to allow you to grab your extinguisher. But left alone during a fire, they will quickly break down.

Now, don’t get me wrong, these LiPo bags do have their use.  I use them when I’m charging a LiPo, an operation that is always supervised and for which a fire extinguisher is on hand.   And I also use them to supplement my other storage solutions.  And they are great for transporting a LiPo from its storage area to the device you are powering.

But, on their own, they don’t offer adequate safety, and shouldn’t be the only thing you rely upon to protect you from a LiPo that experiences a thermal runaway.

Using a Cinder Block

A very popular method of storing LiPo batteries is to use a cinder block. These are pretty inexpensive and are made of cement, so they can provide a lot of fire protection.

In practice, you just place the LiPo batteries inside the “hole” in the cinder block. For added protection, you can place the batteries inside a LiPo bag before inserting them into the cinder block.

You can also cover the cinder block with a piece of steel or drywall for added security.

Using an Ammo Box

One excellent and inexpensive method of storing LiPo batteries is to use an ammo box. These containers are made of steel and, as such, offer exceptional fire protection. They are available at sporting goods stores, as well as at Amazon.

Before you use the ammo box, you need to make one modification. These boxes have a rubber seal on the lid, whose purpose is to render the box both air and watertight. But for a LiPo storage container, having an airtight enclosure is the last thing you would want. LiPo batteries can vent gas, and if the gas can’t escape, the pressure could build up to a dangerous level.  If it ignites, then you have yourself a bomb!

Always remove the rubber seal from an ammo box before using it as a LiPo storage box!  The seal is generally very easy to remove, I used a screwdriver to pry it off the box I’m using.

Some users also drill some small holes in the box (you need a good sharp drill to penetrate steel) to allow for more airflow, but taking off the seal should suffice.

You can also put your LiPo inside a battery bag within the box for added protection.

My Homemade LiPo Storage Box

I built a storage container for my LiPo batteries, you can see the details in the video that accompanies this article.

I used a steel toolbox that I picked up on Amazon, you may even have an old one already. The most important factor is that it is made of steel, which is great for resisting fire.

I lined the toolbox with drywall (sometimes called Sheetrock), as it also has excellent fire-resistant properties. I lined the bottom and sides of the box with the drywall, and also made a drywall “lid” to cover the batteries. 

Once again, I placed the batteries inside battery bags for additional protection.

You can assemble a similar storage box yourself, it’s great for situations where you need to store or transport several batteries.

LiPo Fire Extinguishing

I sincerely hope that you never experience a LiPo fire, it is a fast-burning and very hot fire that can be extremely dangerous, especially as it is often accompanied by a toxic outgassing of burning chemicals.

But even if you take every precaution, it can happen, due to an accident or a defective LiPo. In that situation, you need to be prepared to extinguish that fire as quickly as possible.

LiPo Fire Risk

A LiPo battery can experience thermal runaway, a condition in which the heat continues to increase unabated. This can be caused by several things:

• Charging a battery that has an excessive Internal Resistance in one or more cells.
• Charging a damaged, defective, or ballooned battery.
• Exposing a battery to excessive heat. Try and keep it under 60 degrees Celsius.
• Excessive shock or pressure, such as in a crash.

Note that the majority of times thermal runaway occurs during the charging process, so it’s important to be prepared for a fire when charging your LiPo battery.

Another risk is that a LiPo that is experiencing thermal runaway may vent toxic gas, which can also explode if ignited. This is why you should always charge a LiPo in a well-ventilated area, preferably outdoors.

Extinguishing a LiPo Fire

Despite its name, a Lithium-ion Polymer Battery actually has very little lithium inside it. The small amount of lithium will react with water, but it is usually burned off within seconds at the start of the fire event. By the time you realize you have a fire, the lithium will likely be gone.

Because of this, it is NOT necessary to obtain a Class D fire extinguisher, which is the type normally recommended for a lithium fire. They are expensive and hard to use.

Instead, a standard Class ABC extinguisher can be used. This is something you should have around your workshop anyway, so you probably already own one. If not, they can be obtained and just about any hardware store.

You can actually use water during a LiPo fire, but it is best used to hose down the area around the burning battery to prevent it from igniting. Often the biggest danger from a LiPo fire is the items surrounding the battery that catch fire. Another great reason to charge your LiPo away from such items!

A great way to quickly extinguish a LiPo fire is with sand. A bucket or bag of sand is pretty easy to obtain, and you can smother the fire with it.

The accompanying video has some scenes of LiPo fires, to give you an idea as to what you would be up against if you had one. After watching it, you should have a new appreciation for the necessity of preparing for a LiPo fire!

LiPo Disposal

All batteries have a limited lifespan, and LiPo batteries are no exception. If handled correctly, these batteries can last for 350 to 500 charge and discharge cycles, but eventually, they will require replacement. 

When they do come to the end of their useful life, they need to be discharged and properly disposed of.

Determining when your LiPo needs Replacement

If you’ve taken care of your LiPo batteries then they should last a pretty long time, as stated above you should be able to get at least 350 uses out of them.

But if you have abused them by not keeping them balanced or stored properly (at 3.8-volts per cell), or if they have been damaged, then they may not last as long.

If you’re tracking the internal resistance of the cells and notice a sudden increase, then it’s a sign that the LiPo is near the end. If the battery gets warm under normal use, if it is ballooned, or if it fails to hold a charge or stay balanced, then it’s time to replace it.

Discharging a LiPo Battery

Before you can dispose of your LiPo battery, it needs to be completely discharged. Even a “dead” LiPo can hold a lot of energy, and it can catch fire if crushed in a garbage truck or trash compactor. 

Many quality LiPo chargers have a “Discharge” or “Destroy” function that will completely drain the battery. The procedure can take several hours, or even more than a day, depending upon how much energy is left in the cells. 

Another way of draining a LiPo is to use an automobile taillight or a resistive load, leave it on the LiPo for a day, and then measure the output voltage. If it is more than zero, then put the load back on for a few more hours.  Keep going until no electricity can be detected.

One method that many RC hobbyists use is to soak the entire cell in a saltwater bath for a day or two.  You can also do this after draining the battery with one of the above methods, to ensure that it is completely depleted of energy.  Table salt will work fine, you need to saturate the water until it can’t hold any more salt.

Disposing of a LiPo Battery

After discharging the battery completely, you can dispose of it.

In many municipalities, you can simply throw it in with the rest of your trash, as it no longer contains any volatile or dangerous chemicals and has very little lithium. But in some areas, you will need to take it to an authorized disposal facility.

Make sure you check with the proper authorities to determine the proper disposal method for your area. In my community, we have a bi-annual hazardous goods disposal at the city hall, and they take all batteries, including LiPo batteries.

Conclusion

When it comes to using LiPo batteries, nothing is more important than safety. Fortunately, it doesn’t take a lot of effort to observe the proper safety precautions, minimizing the chance of a dangerous fire or outgassing.

LiPo batteries are powerful devices that need to be treated with respect and handled with care. If you observe proper handling, charging, balancing, and storage techniques, these batteries can last a long time and provide a virtually unlimited source of power for your projects.