This article explains in simple terms:

• how a lithium-ion battery functions
• factors that contribute to its premature aging
• steps to maximize its life expectancy.

 

The Principle Behind Lithium-Ion Batteries

Just like lead-acid batteries, lithium cells are made of positive plates, negative plates, and electrolyte. The plates are porous, and they completely absorb the electrolyte so that there is no free moving liquid inside a lithium-ion battery.

In a lead-acid battery, charging and discharging are chemical reactions that transform battery components. In a lithium-ion battery, it is the lithium ions that move from one plate to the other.

 

Charging: Lithium ions migrate through the electrolyte from the positive electrode (LiFePO4) and insert themselves into the graphite structure of the negative plate. The positive electrode loses lithium, and when the battery is fully charged, only the ferric phosphate (FePO4) crystalline structure is left.

Discharging: Lithium ions leave the graphite structure of the negative plate and insert themselves back into the ferric phosphate crystals to recreate LiFePO4.

The constituents of a lithium-ion battery are highly stable, and under the right conditions they are little altered by this “insertion” process. However, other phenomena may occur that can cause the battery to deteriorate over time.

 

Excessively high or low voltage or temperature
will reduce a lithium-ion battery’s life expectancy

 

What factors shorten lithium-ion battery life?

Aging reduces battery capacity. Once capacity has fallen below 80% of nominal capacity, the battery has reached the end of its useful life.

 

1 – High temperature

When the battery is manufactured, a chemical reaction is triggered to build an interface layer (called SEI) between the plates and the electrolyte. This layer helps stabilize the battery. Over time, the SEI layer continues growing to the point where it becomes detrimental. High temperature is one factor that is responsible for accelerated build-up of the SEI layer.

 

 

2 – Quick Charge, Overcharging and Excessively High Voltage

“Lithium plating” occurs during charging when lithium ions cannot insert themselves into the structure of the graphite electrode. The lithium ions are transformed into metallic lithium and no longer contribute to the charging/discharging process, thus reducing battery capacity. Below are some situations in which that occurs:

• Rapid charging occurs when the current is higher than the graphite absorption rate. Charge current should not exceed 0.3C (60A for a 100Ah battery).

• Overcharging: 3.4 volts per cell (13.3 volts for a 12V battery) is sufficient to fully charge a battery. Anything above that will overcharge and damage the battery even if the charge current is very low. In practice, a charge at 3.45 volts maximum per cell will rapidly charge the battery close to 100% SOC.

• Float voltage occurs above the nominal voltage (3.2 to 3.3 volts per cell) when the battery is full and there is no more space for the lithium ions to insert themselves into the graphite.

 

 

3 – Low Temperature Charging

The graphite absorption rate decreases as temperature drops. “Metal plating” will occur when the charge current exceeds that absorption rate.

 

4 – Excessive Voltage

Voltage above 4.3 volts per cell (17.2 volts for a 12 volt battery) causes the electrolyte to decompose into gas. Internal pressure will increase, causing the battery to bulge, crack, or even explode.

 

5 – Excessive Discharging

If a cell is discharged below 2.0 volts (8 volts for a 12V battery) its polarity will reverse itself and the anode (positive copper electrode) will dissolve into the electrolyte. If that battery is charged again, the copper will precipitate on the cathode in the form of sharp crystals that can short the plates.

 

6 – Short-Circuit

The short-circuit current of a lithium battery can reach 20 to 30 C (2,000 to 3,000 A for a 100 Ah battery). Although manufacturers claim this will not ignite a battery in good condition, it will not help extend battery life and your electrical installation can incur major damage.