Measuring
the life expectancy of a lithium battery is difficult to pinpoint. So we thought that we would like to expound
on the subject to help an anticipated lithium user understand the issue and
conclude how to get the maximum life from their battery pack.
Understanding Charge Cycle Life
– Lithium vs. Lead Acid
Typically
the life of any battery is measured in the number of charges the battery has
before it deteriorates to a point where it can only hold 80% of its capacity
when it was new. This number is called
the batteries ‘Charge Cycle Life’.
As
a comparison I would like to start with a lead acid battery. Lead acid batteries have a charge cycle life
of between 350 charges all the way up past 600 charges. With the lower charge cycle life usually
found in car starter batteries and the higher charge cycle life found in
laboratory or solar storage applications.
But here is where things can become a little misleading. The honest way of measuring a battery’s life
can be manipulated. Because of the Peukert
effect which exists on all lead acid batteries you cannot discharge more
than about 55% of the batteries new 20 hour capacity rating. A 100 Amp Hour ‘Ah lead acid battery will
only yield about 55 Ah before it is considered fully discharged.
Lithium
batteries are not affected by Peukert’s Law to the extent that lead acid
batteries are so you can discharge a lithium battery down to 20% of its full
State of Charge ‘SOC’ before the battery is considered discharged. With many lithium batteries if the discharge
is more that 80% DOD it will not hurt the batteries life but is generally
accepted that the lithium battery is fully discharged at 80% DOD. To run honest tests to determine the exact
life of a lithium battery the battery must be fully charged and then fully
discharged to 80% DOD.
Charging
and discharging thousands of times takes a lot of time therefore most companies
will do it for a number of complete cycles and then extrapolate the remaining
life expectancy based upon some typical known curves for the chemistry of the
battery.
Lithium Titanium Cells and
Polymer Battery Drawbacks
Lithium
Titanium cells have the longest life.
But they are 3 to 5 times more expensive than the common Lithium Iron
Phosphate ‘LiFePO4’ battery. Lithium
Nickel Cobalt Magnesium (also called a polymer battery) or NCM batteries
exhibit about half the life of the LiFePO4 cells. NCM batteries are lighter in weight and lower
in cost and are often seen in electric vehicle applications where cost and
weight are paramount.
How Companies Cheat on Charge
Cycles
Some
companies cheat and they say that their LiFePO4 batteries will last over some
ridiculous number - like 8000 charge cycles.
Those numbers are not realistic if they fully discharge the battery down
to 80% DOD, which they do not. However,
there are several other factors that play into the battery life scenario. One of them is the case of the battery. When batteries charge or discharge they
create heat. Heat that is trapped inside
the battery will cause the battery life to go down. A plastic encased battery of a given Ah
rating will have a shorter life than a metal encased cell. That is because the plastic is a poor thermal
conductor.
The
smaller the Ah rating of the battery the easier it is for the battery stack to
dissipate the heat away from the core of the stack and therefore will have a
longer life. Honest manufactures of
LiFePO4 plastic encased batteries which I call a prismatic cell will normally
claim that their cells will have more than 2000 charge cycles. A prismatic plastic encased battery is
illustrated in the graph that is in this dissertation.
Why Aluminum Encased Lithium
Batteries Perform Better
A
typical aluminum encased battery that has better heat dissipation will start
out higher. Dependent upon the size of the cell the expectant charger cycle life of a smaller LifePO4 cell
can be as much as 4000 charge cycles and follow the same trajectory of the
plastic prismatic cell shown below. The
simple fact is that if you have an application where you want a long life, like
for solar storage, and you do not discharge the cells below their nominal 3.2
voltage you will have a cell that will probably outlive you. Depth of discharge has a very profound effect
on a lithium battery’s life.
-
Factor 1: Depth of
Discharge…Percentage of Capacity Used Per Cycle.
-
DOD, short for the Depth of
Discharge, is used to describe how deeply the battery is discharged. If we say
a battery is 100% fully charged, it means the DOD of this battery is 0%, If we
say the battery have delivered 30% of its energy, here are 70% energy reserved,
we say the DOD of this battery is 30%.
-
Factor 2: Discharge Rate, The
AMPS Divided by the Capacity
-
To Read the Chart Above, Here
are Some Examples
A.
All Numbers are at a 1C
Discharge Rate, Which is Aggressive…
B.
At a 90% Depth of Discharge,
Our Batteries Will Provide 2400 Cycle to 80% State-of-Original-Capacity (SOIC)
C.
At a 10% Depth of Discharge (Say,
Engine Starting) Our Batteries Will Provide 35,000 Cycles Until 80% SOIC.
Beyond 35,000 Cycles the Battery is Still Functioning But Gradually Losing
Capacity, but not Performance
D.
Conclusion: By recharging a
lithium ion battery more frequently, thus reducing the DOD (depth of
discharge), the battery cycle life is Increased. Solar is very often a beneficial
method to reduce DOD and increase life!
