Lithium-ion batteries are standard for electric vehicles and portable equipment, where charge and discharge currents are high compared with battery capacity. In off-grid applications, is the extra cost justified?  What are the advantages and drawbacks when current is much lower and long-term performance is more important?

 

Advantages of Lithium-Ion Batteries over Lead-Acid/AGM/Gel Batteries

 

1 – More Cycles (*)

Lithium-Ion	Lead -Acid
Manufacturers claim 4,000 cycles or more when discharging the battery down to 20% SOC*. Real-life tests over many years show that after 750 cycles, loss of capacity is negligible. Thus, it seems realistic to expect at least 2,000 cycles	The best in class claim 400 to 500 cycles. In real situations of use we have not yet encountered a battery (where it is seldom recharged to 100% SOC) that can survive so many cycles before its capacity falls dramatically. 200 to 300 cycles seems to be a realistic expectation.

 

2 – More Usable Capacity

Lithium-Ion	Lead-Acid
Can be safely discharged down to 20% SOC with voltage steadily above 12 volts.   Charge rate is constant up to 90% SOC, allowing for a complete charge in every cycle.   Usable capacity: 70–80% of nominal capacity.	Discharging below 50% SOC creates irreversible damage and shortens battery life.   Above 80% SOC, the charge current falls dramatically and getting to 100% SOC takes a very long time (practically never reached in real-life situations). Usable capacity: 30–40% of nominal capacity.

 

3 – Lower Weight and Volume

Lithium-Ion	Lead-Acid
A 12V 100Ah battery in (four 100Ah cells) weighs about 14kg for a volume of 8 dm3.   With 75% of its capacity being usable, “energy density” is 5.3Ah/kg and 9.3Ah/dm3.	A 12V 100Ah battery weighs about 25kg for a volume of 13 dm3.   With only 35% of its nominal being usable, “energy density” is 1.4Ah/kg and 2.7Ah/dm3.

 

 

To reach 450Ah of usable capacity you need:
Lithium-Ion	Parameter	Lead-Acid
600 Ah	Nominal capacity	1.200 Ah
84 kg	Weight	300 kg
48 dm3	Volume	156 dm3

 

4 – Faster to Charge

Lithium-Ion	Lead-Acid
Charge current can safely be up to 0.5C* (50A for a 100Ah battery). The current will remain constant up to 95% SOC.	Charge current must be limited to 0.2C (20A for a 100Ah battery). When 80% SOC is reached, the battery will only accept a small current, and full charge can take many more hours.

 

5 – Less Voltage Drop

Lithium-Ion	Lead-Acid
Very little voltage variation between 20% and 80% SOC.   Limited voltage drop when drawing high current	Voltage varies significantly with the battery state of charge.   Voltage drops dramatically drawing high current

 

6 – Other Advantages

Lithium-Ion	Lead-Acid
Efficiency above 98%   No Peukhert effect (capacity is not reduced when drawing high current)   No need for voltage compensation when the temperature varies   Low self-discharge	Efficiency between 70% and 90%   Capacity is reduced when discharging with high current   Need to set voltage/temperature compensation for the chargers   High self-discharge

 

Drawbacks of Lithium-Ion Batteries

1 – They must not be overcharge

 

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

Applying an excessively high charging voltage will damage the battery to the point where it can be destroyed. (Caution: Battery supplier specifications often refer to EV applications where charging speed is critical. Voltage indications may be too high for an energy storage application where lifespan is more important.)

 

2 – They must not be vompletely discharged

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

 

3 – They must not be charged temperatures below 0°C

Lithium-ion batteries can be used below 0°C but should never be recharged when it is freezing. If there is no alternative, use a very low charge current.

 

4 – They are sensitive to high temperatures

As with lead-acid batteries, high temperatures considerably speed up the aging process.

 

5 – Cells must remain in balance

A battery is made up of cells connected in series to obtain the desired voltage. Cells are “balanced” when they are exactly at the same level of charge. Not all cells have precisely the same characteristics (such as internal resistance), and over a large number of cycles an imbalance can arise. One cell can be full (or empty) before the other cells, and continued charging (or discharging) will push that cell into a danger zone where it ages prematurely or may even be destroyed. Users are advised to balance cells regularly in a controlled environment.

 

An Energy Management System (EMS) is needed to prevent such situations and protect the battery

 

Are lithium-ion batteries dangerous?

This discussion concerns lithium iron phosphate batteries (LiFePO4) used in electric vehicles and off-grid installations. These batteries have been subjected to numerous test conditions, such as short-circuits, high temperature, or flames, that have not resulted in explosion. Some say they are safer than lead-acid batteries.

A short-circuit between the positive and negative posts of a lithium battery will generate a very high current capable of melting metal objects like wrenches. Some basic precautions are needed to avoid short-circuits and to limit their impact.

 

A battery is a concentration of energy and is potentially dangerous.

 

The LiFePO4 lithium-ion battery is one of the least dangerous battery types

 

Are lithium-ion batteries expensive?

Lithium-ion batteries are more expansive than lead-acid batteries with the same nominal capacity (Ah). However, compared with lead-acid batteries :

• they have twice the available capacity at a fraction of the weight and volume
• lithium-ion batteries will withstand at least three to five times more cycles

 

The cost of storing energy in lithium-ion batteries is much lower
than in lead-acid batteries