How your battery will die?

Lithium battery principle

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 in a lithium battery.

In a lead-acid battery the charge and discharge correspond to chemical reactions that transform it components, where in a lithium battery it is the lithium ions that move from one plate to the other.

Charge: The lithium ions migrate in the electrolyte from the positive electrode (LiFePo4) to insert themselves in the graphite structure of the negative plate. The positive electrode loses lithium and when the battery is full only the iron phosphate (FePo4) crystalline structure is left.

Discharge: The lithium ions leave the graphite structure of the negative plate and insert themselves back in the iron phosphate crystals to recreate LiFePo4.

The constituents of a lithium battery are quite stable and in the right conditions do not get much altered by this “insertion” process.  But other phenomenons may occur that will deteriorate the battery over time.

What reduces the life of a lithium battery?

Aging is seen as a reduction in battery capacity.  Once the capacity is below 80% of the nominal capacity the battery has reached the end of it’s 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 contributes to the stabilization of the battery.  But over time the SEI layer keeps growing to the point where it is detrimental.  High temperature contributes to speed-up the build up of the SEI layer.

2 – Fast charge, over-charge and voltage too high

“Lithium plating” happens during charging when the lithium ions cannot insert themselves in the structure of the graphite electrode.  The lithium ions are transformed into metal lithium and no longer contribute to the charge and discharge process, hence reducing the battery capacity.  Some of the situations when that happens:

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

• Over-charge.  To fully charge a battery 3.4 volt per cell (13.3 volt for a 12V battery) is sufficient. Anything above will over-charge and damage the battery even if the charge current is very low.  In practice a charge at 3.45 volt max per cell will rapidly charge the battery close to 100% SOC.

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

3 – Charge at low temperature

The graphite absorption rate decreases with the temperature.  If the charge current is above that absorption rate then “metal plating” occurs.

4 – Excessive voltage

A voltage above 4.3 volt per cell (17.2 volts for a 12 volt battery) decomposes the electrolyte into gas.  The internal pressure increases leading to battery bulging, cracking and even explosion.

5 – Too deep discharge

If a cell gets discharged below 2.0 volt (8 volts for a 12 volt battery) it’s polarity will reverse itself and the anode (positive copper electrode) will get dissolved into the electrolyte.  If that battery is charged again the copper precipitates 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 (2000 to 3000 A for a 100 Ah battery).  Although manufacturers claim this will not ignite a battery in good condition, it will not contribute to extending the battery life and the damages to your electrical installation can be important.