Lithium batteries

Why lithium?

Lithium batteries are the standard for Electric Vehicles and portable equipment where charge and discharge currents are important compared to the battery capacity. But for an off-grid application is the extra cost justified?  What are their benefits and disadvantages when currents are much lower and performance over a long period of time is more important?

Benefits of a lithium battery over lead acid / AGM / gel batteries

1 – More cycles (*)
Lithium

Manufacturers claim 4000 cycles or more when discharging the battery down to 20% SOC*.  Real life tests over many years show that after 750 cycles the loss of capacity is negligible.  It seems therefore realistic to expect at least 2000 cycles.

Lead acid

The best in class claim 400 to 500 cycles.  I have never seen a battery in real use situation (where it is seldom recharged to 100%SOC) survive so many cycles before it’s capacity is reduced drastically.  200 to 300 cycles seem to be a realistic expectation.

2 – More usable capacity
Lithium

Can be safely discharged down to 20% SOC with a voltage that remains above 12 volts.

Charge rate is constant up to 90% SOC allowing complete charge at each cycle.

Usable capacity: between 70% and 80% of nominal capacity.

Lead acid

A discharge below 50% SOC creates irreversible damages and reduces the battery life. Above 80% SOC the charge current is drastically reduced and getting to 100% SOC takes a very long time (practically never reached in real life situation)

Usable capacity: between 30% to 40% of nominal capacity.

3 – Less weight and volume
Lithium

A 100Ah battery in 12 volts (Four 100Ah cells) weighs about 14kg for a volume of 8 dm3.

With 75% of it’s capacity being usable, the “energy density” is 5.3Ah/kg and 9.3Ah/dm3

Lead acid

A 100Ah battery weighs about 25kg for a volume of 13dm3.

With 35% of it’s nominal being usable, the “energy density” is 1.4Ah/kg and 2.7Ah/dm3.

To have 450Ah of usable capacity you need:

Lithium

600 Ah

84 kg

48 dm3

Parameter

Nominal capacity

Weight

Volume

Lead-acid

1200 Ah

300 kg

156 dm3

4 – Faster to charge
Lithium

Charge current can safely be up to 0.5C* (50A for a 100Ah battery).  The current will remain constant up to 95% SOC.

Lead acid

Charge current must be limited to 0.2C (20A for a 100Ah battery).  When the charge reached 80% SOC the battery will only accept a small current… and full charge can take many more hours.

5 – Less voltage drop
Lithium

Very small voltage variation between 20% and 80% SOC.

Limited voltage drop when a consumer draws a high current.

Lead acid

Voltage varies significantly with the battery state of charge.

Voltage drops drastically when a consumer dras a high current.

6 – Other benefits
Lithium

Efficiency above 98%

No Peukhert effect (the capacity is not reduced when drawing high current)

No need for voltage compensation when the temperature varies

Low self discharge

Lead acid

Efficiency between 70% and 90%

The capacity is reduced when discharging with high current

Need to set voltage/temperature compensation for the chargers

High self discharge

Constraints of lithium batteries

1 – Do not over-charge

Charging a battery once it is full (even in float) speeds up the aging process

Applying a charging voltage that is too high damages the battery to the point where it can be destroyed (warning: battery supplier specifications are often for electric vehicle applications where speed of charge is critical – these voltages could be too high for an energy storage application where lifespan is more important)

2 – Do not discharge completely

Below a certain level of discharge the battery voltage drops sharply to a point where polarity is reversed and the battery is destroyed

3 – Do not charge below 0°C

Lithium batteries can be used below 0°C but should never be recharged when it is freezing (if needed it can be done with a very small charge current)

4 – Sensitive to high temperature

Same as for a lead-acid battery, a high temperature speeds up considerably the aging process

5 – Cells must stay in balance

A battery is made of cells in series to obtain the desired voltage.  Cells are “balanced” when they are exactly at the same level of charge.  All cells do not have exactly the same characteristics (like internal resistance) and over a large number of cycles an unbalance can be observed.  In that case one cell is full (or empty) before the other cells, and ongoing charge (or discharge) will take that cell into a danger zone where is ages prematurely or even can be destroyed.  It is recommended to regularly balance the cells in a controlled environment.

A Battery Management System (BMS) is necessary to prevent those situations and protect the battery

Are lithium battery dangerous?

We are talking here about Lithium Iron Phosphate batteries (LiFePo4) used in Electric Vehicle and off-grid installations.  Numerous tests have been conducted on those batteries without explosion: short-cuts, high temperature, flame…  Some say they are safer than lead-acid batteries.

A short-cut between the positive and negative posts of a lithium battery will generate a very high current capable of melting metal like a spanner.  Some basic precautions are necessary to avoid short-cuts and to limit their consequences.

A battery is a concentrate of energy and is  potentially dangerous

LiFePo4 lithium battery is one of the least dangerous battery types

Are lithium batteries expensive?

A lithium battery is more expansive that a lead-acid battery of the same nominal capacity (Ah). BUT compared to a lead-acid battery:

  • it has twice the available capacity and a fraction of the weight and volume
  • the lithium battery will last at least three to five times more cycles

The cost of energy stored in a lithium battery is much lower
than when using lead-acid batteries