Is It Dangerous to Connect Lithium Batteries in Parallel?

Is it really dangerous to connect lithium batteries in parallel?

How can we understand this?

Lithium batteries are ubiquitous in our daily lives, powering devices ranging from smartphones to electric vehicles. However, when it comes to connecting them in parallel, concerns arise regarding safety and performance. This article explores the issue and introduces a simple method I tested to verify the safety and balancing of lithium batteries using the INA3221 chip.

All the information you are about to read can be found in this video on my youtube channel.

Have fun reading!

Materials

INA3221 module

Arduino Uno

I2C LCD Module 16x2

Modular cables

Small breadboard

Dupont jumper wires

Tools

Multimeter

T12 soldering iron

When connecting multiple lithium cells in parallel, it is crucial that they share similar characteristics in terms of capacity, voltage, and internal resistance.

Even a small difference could create a high current flow, potentially causing overheating, accelerated degradation of one or more cells, or even safety risks like fires or explosions.

In this short experiment, I used identical batteries salvaged from disposable e-cigarettes.

They have a capacity of about 310 milliampere-hours, and unfortunately, I couldn’t find any documentation or datasheets on the specific features of these batteries.

Initially, I attempted to measure the balancing current using a standard multimeter. To do this, I connected the batteries in parallel, set the multimeter to measure current, and placed its probes between the positive poles of two batteries.

From the start, I noticed that the detected currents were very low (between 1,5 and 5 milliamperes). I immediately suspected that my budget multimeter was not suitable for measuring well currents.

Additionally, the process was slow as it required frequently disconnecting and reconnecting wires to obtain readings from different cells.

Test failed!

Since my multimeter couldn’t measure the current correctly, I decided to use an INA3221 module. I had never used it before, but I believe it’s the most affordable method for taking reasonably accurate measurements without investing in expensive equipment.

The INA3221 is a three-channel current and voltage monitor that can simultaneously monitor up to three cells or separate circuits. Each channel measures both the voltage of the cell and the current flowing through it.

Naturally, to make it work correctly, you will need a microcontroller (I used an Arduino Uno, but you can use any open-source microcontroller from the Arduino, ESP, or Raspberry families) and a 16x2 LCD display to view the results in real time.

For wiring, you can refer to the image above, and you can find the code and all files on my GitHub.

Be careful, as I found a small bug in the library I used. You can fix the bug by reading this forum or watching this video.

After correctly connecting the INA3221, Arduino, and the display, I first tested the voltage measurement and then the current measurement with one battery at a time. The measurements seemed consistent.

Next, I connected three batteries in parallel and, finally, added a resistor to simulate a small load connected to the battery pack (see image).

As expected, the measured currents were quite high, ranging from 400 to 600 milliamperes.

These currents don’t seem excessively high, and the batteries didn’t overheat, but considering that the maximum charging current for these batteries should be around 100 milliamperes, we can say with certainty that the balancing current is very high and potentially harmful to the cells or even dangerous!

To safely and simply balance the cells, you can use basic resistors.

Just connect a small resistor (e.g., 5 or 10 ohms) between each cell. This helps distribute charging and discharging currents evenly among the cells. As you can see in the photo, the currents decrease...

By using 10-ohm resistors, the current drops to about 4 milliamperes, which is certainly a low and safe current!

Of course, given the low currents, the pack will take several hours to balance.

From this brief experiment, I learned that:

1. Budget multimeters often don’t measure current well. Not everyone can afford expensive tools, so it’s essential to test your instrument thoroughly before making critical measurements.
2. The INA3221 can be a good tool for measuring voltages and currents without spending much!
3. Never connect different batteries in parallel or similar batteries with different voltages. Doing so could compromise battery reliability or lead to explosions or excessive overheating.
4. Before soldering a battery pack, it’s best to charge all cells to the same level or balance their charge by connecting them with small resistors. This limits the balancing current and prevents cell damage.

Thanks for getting this far!

If you want to learn more about this topic, watch this video and subscribe to my channel!

Your engagement and support means so much and motivates me to bring you more DIY electronics projects.

Dario