How your battery will die...

battery explosion

This article explains in simple terms how a lithium battery functions, the factors that contribute to it’s premature aging and the steps to maximize it’s life expectancy. 

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.

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Voltage too high or too low as well as high or too low temperature
will reduce a lithium battery’s life expectancy

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 contribute to speed-up the build up of the SEI layer.

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TAO.bms monitors the temperature of each cell,
warns if the temperature gets too high,
can activate a fan or a cooling system,
and records those situations data to estimate life expectancy

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)
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TAO.bms will warn you when the charge current is above a set value

  • 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.
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TAO.bms will warn you when the voltage is above that value and will disconnect or stop the chargers

  • 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.
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It is of the upmost importance to stop charging when the battery is full
even if the voltage is within safe limits
Avoid float charge above 3.3 volt per cell (13.2 volt for a 12V battery)

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.  Charge should be avoided at low temperature (close to and less than 0°C).  If you have no other choices, you need to greatly reduce the charge current.

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TAO.bms warns you when the temperatue is too low and can stop the charge

4 - Excessive voltage

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

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To avoid the bulging and cracking of the cells
they must be compressed between strong plates on their larger surface
Straps are not enough!

5 - Too deep discharge

If a cell gets discharged below 2.0 volt (8 volts for a 12V 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.

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Never recharge a lithium cell when it’s voltage as gone below 2.0 volt
It could create an internal short-circuit and a risk of fire

6 - Short-circuit

The short-circuit current of a lithium battery can reach 20C to 30C (2000A to 3000A for a 100Ah 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.

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Install a good quality and properly sized fuse as close as possible to the battery
The battery connections must be protected under a cover to avoid that the fall of a metallic object creates a short-circuit

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