Batteries were once heavy, awkward things, delivering only a limp amount of current for their size and weight. Thankfully, over time, technology has improved, and in 2020, we’re blessed with capable, high-power lithium polymer batteries that can provide all the power your mobile project could possibly need. There are some considerations one must make in their use however, so read on for a primer on how to properly use LiPos in your project!
So Many Types!
With the first commercial lithium-ion battery entering the market in 1991, the (nearly) 30 years since have seen rapid development. This has led to a proliferation of different technologies and types of battery, depending on construction and materials used. In order to treat your batteries properly, it’s important to know what you’ve got, so paying attention to this is critical.
Lithium-ion, or Li-ion typically refers to the overarching technology of rechargeable lithium batteries, but also specifically refers to the traditional cells built in cylindrical metal bodies. The venerable 18650 is one such cell, but a large variety of sizes and types exist. Their stout casings make these cells popular for rough-and-tumble vehicle use.
Lithium-Polymer, or Li-Po refers to a lithium-ion battery that uses a polymer electrolyte instead of a liquid electrolyte. This enables the construction of pouch cells with different geometries. This flexibility of design makes lithium-polymer batteries useful in applications like smartphones and tablets, where a high-capacity battery is needed and a flat form factor is desirable. They’re also commonly used in radio-control models, where their lightweight construction is a huge benefit for flying vehicles.
Lithium-HV, or High Voltage Lithium are lithium polymer batteries that use a special silicon-graphene additive on the positive terminal, which resists damage at higher voltages. When charged above 4.2V, most lithium batteries exhibit significant capacity loss and reduced lifespan. However, by using this additive, cells can be charged to 4.35V without exhibiting these negative effects. This extra voltage provides up to a 10% gain in energy density over conventional lithium polymer batteries.
Lithium-Iron-Phosphate, or LiFePO4 batteries are an altered lithium-ion chemistry, which offers the benefits of withstanding more charge/discharge cycles, while losing some energy density in the tradeoff. They operate ideally between 3.0V-3.65V, instead of the more typical 3.0-4.2V range of a standard lithium-ion chemistry. This, combined with a very flat discharge voltage curve, makes them ideal replacements for 12V lead-acid batteries in many applications, where four cells substitute for the original six. They’re generally more stable, with lower rates of self-discharge and capacity loss over time.
Respect The Limits
Moreso than most battery types, lithium cells are not tolerant of mistreatment. Discharging cells below their low voltage limit leads to the formation of copper dendrites, which can reduce cell capacity or short circuit them entirely. Overcharging cells causes damage to the anode by lithium plating out of solution, creating lithium dendrites, often leading to a short circuit or full thermal runaway of the battery, leading to a release of smoke and flames. Each cell in a pack must also be kept at the same voltage as its neighbors, to avoid cells getting damaged prematurely.
It’s important not to charge lithium cells too quickly. Ambient temperatures also play a big role in battery performance. Lithium batteries don’t appreciate being taken down below freezing, particularly when they’re already fully charged. Below 0°C, charging is impractical, as metallic lithium can electroplate at the negative electrode, causing major damage or even short circuiting the cell. Between 0-5°C, charging is possible, but must be done slowly. Damage will tend to occur when batteries are charged at temperatures above 45°C, too.
Working outside these parameters will quickly lead to a dead battery at best, or a fire and explosion at worst. They also tend to swell up, outgas, and just generally become unseemly to deal with. On the surface this can seem like a lot to deal with. Thankfully the battery-electronics complex has worked hard to solve these issues. With the proper hardware and precautions, it’s possible to use lithium batteries safely and effectively. But anyone working with these chemistries should familiarize themselves with the hazards. Bob Baddeley published a great article on Li-Ion safety back in November.
For applications working with bare cells or packs, such as when using LiPo batteries in RC models, simply using a lithium-ready charger is enough. The balance leads should be hooked up during charging, particularly when the battery has been taken to a fully-discharged state in use. Ensuring that a smart charger is used with the correct voltage limits (particularly when using LiFePO4 and HV packs) will make sure you get the most out of your batteries. Make sure you’ve got some method to stop discharging the batteries when voltage gets low, whether by a warning light, buzzer, or automatic shutdown.
If you’re producing a device that needs a permanently integrated battery, protection and charging circuits are just the ticket. Off-the-shelf modules and ICs exist to take the hassle out of managing a lithium-ion battery. A wide variety are available, from those that act as a simple low-voltage cutoff to complete charging and protection solutions. Companies like Adafruit sell modules that are a great starting point for those eager to integrate a neat charge and battery solution without having to spin up PCBs themselves. However, since these designs are open source it will be easy to integrate the circuit design into your own PCB in the future.
For larger applications featuring custom-built battery packs, a battery management system is a good choice. Basically, a BMS is not much different from a battery protection IC or similar, simply being designed for larger applications. A BMS is typically used on packs of 10 cells and up, used in transport applications like electric bikes and other rideables. The BMS is soldered directly to the battery pack, including a connection to each individual cell. Its purpose is keeping the cells balanced, limiting the maximum discharge current for safety reasons, and of course controlling the recharging process. Experienced pack builders will often integrate a BMS inside the battery’s housing or covering, leaving simply a discharge port and a charge port accessible. This allows the end user to easily drop a battery into a project vehicle without having to worry about handling protection themselves.
If your application is particularly critical and needs to withstand environmental extremes, you’ll want to monitor battery temperature. Keeping an eye on cell temps, particularly during the charge process, is a great way to protect your battery against damage. High-feature protection chips and battery management systems have provisions to monitor pack temperatures in order to achieve this. At this level, you’ll likely be building custom packs, thus allowing you to install thermocouples at precise locations during the build. For high-power installations, temperature management is mandatory, with virtually all e-bikes and electric cars containing hardware to monitor battery temperatures and control systems accordingly.
Lithium-ion batteries can bite, but used properly, they offer great performance and are more than safe enough for most applications. The key is to use the correct hardware, to make sure you’re avoiding crossing voltage and temperature limits that can lead to disaster. Hopefully, this guide serves you well as you seek to integrate lithium power into your own projects. And, in the unlikely event you do have an amusing battery mishap, be sure to do a diagnosis and hit up the tipsline. Happy hacking!