how to charge custom battery packs

When it comes to charging lithium batteries correctly, I’ve learned that it not only extends their lifespan but also keeps them safe. Using the right charging methods is very important, especially since different types of lithium batteries—like lithium nickel manganese cobalt oxide (NMC) and lithium iron phosphate (LFP)—have different charging recommendations.

For instance, I usually fully charge LFP batteries regularly, while NMC batteries don’t need frequent full charges. Following these guidelines keeps the battery in optimal condition.

Understanding a Lithium Battery Pack

Before charging, it helps to know what a lithium battery pack actually is. A battery pack is made by processing, assembling, and packaging individual lithium battery cells—a process called PACK.

A pack can be a single cell or a series-parallel combination. Typically, it includes:

• Plastic shell
• Protection board
• Battery cells
• Output electrodes
• Connecting tabs
• Insulating tapes, double-sided tapes, etc.

Knowing the pack structure is important when charging series or parallel-connected batteries.

How Lithium-Ion Charging Works

Charging a lithium-ion battery uses an external power source to push lithium ions from the cathode to the anode. Electrons move in the opposite direction externally, from the anode to the charger, and into the cathode.

During discharge, lithium ions move back to the cathode while electrons flow through the external load.

In simple terms:

• Charging: Lithium cobalt oxide releases ions at the cathode, and they embed into the graphite anode.
• Discharge: Ions flow back to the cathode, generating usable current.

Internally, ions cross multiple interfaces: cathode → cathode-electrolyte interface → electrolyte → separator → electrolyte-anode interface → anode bulk.

Series Charging of Lithium Batteries

I usually use series charging because it’s simple, low-cost, and easy to implement. However, individual cells may differ in capacity, internal resistance, or self-discharge. The smallest capacity cell can fully charge first, and continuing series charging may overcharge that cell, so monitoring is important.

Parallel Charging of Lithium Batteries

When charging batteries in parallel, balancing is key. Each cell must be equalized to prevent performance and lifespan issues. Common balancing methods include:

• Constant shunt resistor
• Intermittent shunt resistor
• Average battery voltage balancing
• Switched capacitor balancing
• Buck converter balancing
• Inductive balancing

Lithium-Ion Charging Stages

Charging lithium-ion batteries involves four main stages:

1. Trickle Charge (Pre-charge): For very low battery levels, charge at about 1/10 of full rate until reaching a threshold. Some chargers split this into pre-charge and trickle charge.
2. Full Charge: Constant current charging begins once the voltage is high enough. Battery voltage rises slowly.
3. Taper Charge: When voltage hits the maximum, maintain constant voltage while current gradually decreases. This prevents overcharging.
4. Cut-off/Termination: Charging stops when current drops to 1/10–1/20 of full rate. Avoid floating charges, as they reduce battery life.

Important Charging Considerations

• Use manufacturer-specified chargers to match the battery model.
• Avoid high temperatures: Never charge above 40°C; high heat degrades capacity.
• Charge promptly: Don’t wait for deep discharge each time—regular charging extends battery life.
• Optimal environment: 25°C is ideal; extreme temperatures can damage the battery.
• Partial discharge: Avoid frequent full discharge or over-discharge, which can be catastrophic.

For high-quality batteries, the typical charging range is 0°C–45°C. Cheaper batteries may charge safely from 8°C–45°C. Some allow up to 60°C, but with a reduced charge rate.

For parallel-connected lithium batteries, follow manufacturer instructions closely. Protection chips help, but proper charging practices prevent potential damage.

By keeping these points in mind, you can charge custom lithium battery packs safely, maximize their lifespan, and maintain reliable performance in all your devices.