Maker Meeting March 2, 2021

What Every Maker Needs to Know About Rechargeable Lithium-Ion Batteries

Check out the March 2, 2021 Workshop 88 Maker Meeting here:

Introduction

If your next project involves portable electronics, this is what you need to know about batteries. Here are slides from Bob Van Valzah’s presentation on rechargeable lithium-ion batteries.

Lithium-Ion Battery (PDF Presentation Slides) from video above

Rechargeable Focus

Portable projects running on single-use batteries don’t face some of the complexities you run into with rechargeable batteries. Lithium-ion batteries can burn your project to a cinder if they’re mistreated.

Cautionary Tales

Basics

You’ll need to know the vast range of sizes available for lithium-ion batteries. Manufacturers use units like mAh milliamp hour and Watt hour to describe their batteries.

Comparison with Other Rechargeable Batteries

Lithium-ion batteries outperform all other battery technologies in power density and energy density. Moreover, they can be designed to cover a huge range of power and energy densities.

Charging, Discharging, and Temperature

Voltage, current, capacity, and temperature are all related as a battery charges and discharges. Curves are presented to show the relationships.

What’s Inside a Cell?

Lithium-ion batteries don’t behave like other batteries and are downright counterintuitive in many ways. Understanding just a bit about them at the level of molecular physics and chemistry helps explain some of the mysteries.

Battery Packs

When your project needs more voltage, current, or capacity than a single cell can provide, you need a battery pack of many cells. You can make your own pack or buy them pre-made. Or 3d print adapters for commercial battery packs:

The Ubiquitous 18650

By far the most common lithium-ion cell in battery packs is the “18650.” Every maker should know why they have that name and be aware of the pitfalls of buying them.

Safety

It’s easy to make mistakes in designing and building your project that end up “mistreating” a lithium-ion battery. In particular, there are many ways to go wrong building serial strings of batteries in the quest for more voltage. Protection circuitry or “brains” watch for and avoid the most common forms of electrical mistreatment. In extreme environments like the surface of Mars at night, batteries must be kept warm.

Battery Voltage Monitoring

If your project has a user interface, the user might want to know how much charge is left in the battery. We show typical circuitry for an MCU to read the battery voltage, along with the charge and discharge curves.

MCU Project Integration

You could charge your batteries by removing them from the project, but that’s often impractical. Modest-sized projects can be charged via USB and built-in charging circuitry. It only takes a two chips if you want to do this yourself.

Battery Holder MCUs

A nice alternative to DIY integration for many MCU-based projects is to use a “battery holder” MCU. Several varieties are available on AliExpress. In addition to holding the battery, these boards provide USB charging, a USB serial interface for programming, and a power switch.

Not Quite Lithium-Ion

Lithium-ion batteries are well suited to personal electronics where small size, light weight, and long runtimes are prized. But other chemistries are better suited to solar, marine, and other applications.

Mistakes and Summary

I learned a lot from the projects I’ve done with lithium-ion batteries. You can easily avoid some of the mistakes I made. Slides summarize the key knowledge points about lithium-ion batteries and advice for makers.

Chargers and Interconnects

Jim Williams describes more details of lithium-ion battery charging and how he built a generic LiPo charger. It uses a current meter from a cheap “Charger Doctor” and a TP4056 chip. Jim replaced the fixed current programming resistor with a pot for setting the optimal charging current for the battery at hand. He used HF Alumiweld to fabricate a custom heatsink and managed to fit the whole project into a palm-sized case.


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