Small lithium-ion batteries power our phones, headphones, watches, and medical tools. But even the best batteries can fail.

The most common faults in small lithium-ion batteries¹ include overcharging², over-discharging, short circuits, thermal runaway³, leakage, capacity drop, and internal resistance increase.

If you’re a product designer, engineer, or importer, knowing these faults helps you choose safer batteries⁴, avoid complaints⁵, and protect your users⁶.

What happens when a lithium battery is overcharged?

Overcharging is one of the most serious and dangerous issues in lithium batteries.

Overcharging causes overheating, gas buildup, electrolyte damage, and in extreme cases—fire or explosion.

How it works

When voltage goes above 4.2V per cell, lithium ions continue to move into the anode even though it’s full. This causes:

• Lithium plating⁷ on the anode
• Internal gas⁸ buildup from decomposed electrolyte
• Swelling or rupture of the battery pouch
• Thermal runaway⁹, if unchecked

Symptom	Risk
Battery feels hot after charge	Internal overheating
Swelling	Gas from electrolyte decomposition
Voltage > 4.2V	Likely overcharging

Always use a BMS and certified charger to cut power once the battery is full.

Can over-discharging also damage small batteries?

Yes—and it’s more common than people think, especially in wearables and toys.

Over-discharging drops voltage too low, damaging electrodes and reducing the battery’s lifespan permanently.

What happens

If a lithium battery is drained below 2.5–3.0V:

• Copper ions¹⁰ may dissolve into the electrolyte
• Battery can’t hold charge¹¹ properly after
• Cycle life drops¹², sometimes by 50%+

Voltage Level	Action
3.6V–3.0V	Safe discharge
3.0V–2.5V	Risk zone
< 2.5V	Damage likely

To prevent this, I always recommend low-voltage cutoff via the BMS.

How do short circuits happen in small lithium batteries?

Short circuits are dangerous and usually come from physical damage or internal flaws.

A short circuit creates a sudden high current, leading to heat buildup, smoke, and even fire.

Two types

1. Internal short¹³ – from contamination, faulty separators, or lithium dendrites¹⁴
2. External short¹⁵ – caused by punctures, crushed casings, or bad connections

Cause	Effect
Sharp object punctures cell	Immediate short
Internal defect in cell	Slow failure or sudden fire
Faulty soldering on PCB	External short

Always inspect for dents or leaks, and never reuse a physically damaged battery.

What is thermal runaway?

This is the failure that causes headlines—and real danger.

Thermal runaway is a chain reaction of heat and chemical breakdown that can end in explosion.

Why it happens

• Heat builds up due to charging faults or internal shorts
• Heat breaks down electrolyte and separator
• Flammable gases are released
• Ignition can occur when gases meet heat or sparks

Trigger	Consequence
Overcharge	Internal heating
Short circuit	Instant heat spike
External heat source	Pushes battery past limit

Use thermal protection films, auto shutoff circuits, and flame-resistant casings if you work with embedded batteries.

What does electrolyte leakage mean?

If the pouch fails or internal pressure breaks the seal, liquid can escape.

Electrolyte leakage causes corrosion, capacity loss, and electrical shorts in circuits.

What causes it

• Overcharging → swelling → rupture
• Manufacturing defect in sealing
• Physical impact

Leakage is especially dangerous in devices worn on the body or near sensitive electronics.

Inspect batteries for:

• Bulging
• Wet marks around edges
• Chemical smell

Why do lithium batteries lose capacity over time?

Even healthy batteries lose power as they age—it’s called capacity degradation.

With every charge and discharge, battery materials wear out, slowly reducing how much energy they can hold.

Main reasons

• Cycle aging: from repeated use
• High temp: speeds up chemical breakdown
• Deep discharge: more wear on electrodes

Usage Pattern	Effect on Capacity
Full charge/discharge cycles	Normal aging
Keeping at 100% charge long-term	Accelerated aging
Storing in hot conditions	Faster degradation

Always advise your customers to avoid deep discharge and heat if they want longer runtime.

Why does internal resistance increase?

As a battery ages, it gets “slower” to deliver power. That’s because of internal resistance.

High internal resistance means less efficient power output and more heat during use or charging.

How to detect it

• Device resets during load
• Charging takes longer than before
• Battery warms up faster

This is a silent killer. Use internal resistance meters for QA before shipment. Many of my clients test every batch now—it’s worth the effort.

Are all cells in a pack always equal?

No—cell imbalance is a real issue in packs with multiple cells.

Cell inconsistency means one cell might overcharge or undercharge, creating risk even if the others are fine.

Solution

• Add cell-balancing circuits¹⁶ in the BMS
• Use matched cells from the same batch
• Check voltage of each cell¹⁷ regularly

If your product uses multi-cell packs, don’t skip balancing. It’s the key to longer battery life.

Do lithium batteries discharge on their own?

Yes—even unused, they slowly lose charge. That’s self-discharge.

If stored too long without charging, the voltage can drop below safe levels, damaging the cell.

Storage tips

• Charge to 40–60% before long storage
• Store in cool, dry place
• Check every 3 months

Condition	Self-discharge Rate
Room temp, 50% charge	2–5%/month
Hot environment	5–10%/month

Always inform end users about storage rules—most returns I see from distributors are just dead storage issues.

Conclusion

Small lithium-ion batteries are powerful but sensitive. By understanding the most common faults—overcharge, deep discharge, shorts, swelling, and imbalance—you can prevent failure, reduce complaints, and build safer products.