This post will constantly be updated with latest studies which I do regarding LiFePo4 batteries and how they perform under various conditions. So stay tuned.
EVs have become quite popular with the usage of NMC battery chemistry. NMC batteries became popular due to the lower amount of battery complexity, which led to a much easier BMS design for those companies. But the prime reason EVs didn't become mainstream was because of the battery longevity. We all know NMC batteries need to be operated between 20 to 80% SoC to maintain its battery health for a long time. While it's not the same case with LFP batteries. Now in this article, we will be discussing in depth about functioning of LFP batteries.
Introduction
LFP batteries are also one kind of Lithium-ion
batteries. Its full form is referred to as Lithium Iron Phosphate. It was
mainly originated from EVs manufactured in China. It gained popularity due to
its capacity to withstand extreme climatic conditions and also high cycle
life which it tends to offer.
LFP: Basics
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| Source: Powmr |
The above graph gives a basic idea about the voltage change
in the battery vs the battery percentage left. Battery will only
experience voltage jumps at very high SOC and very low SOC. The middle part of the
battery percentage curve is very flat and hence it is very difficult for BMS
(Battery Management System) to know the actual battery percentage.
To solve this issue battery manufacturers have
developed a method to know the actual battery percentage. It is referred to as the coulomb count method. This method will calculate the amount of current
entering and leaving the battery. Thereby it will display the actual
battery SoC. But even this method is not accurate as there will be current
losses and minor miscalculations. So over time these calculations can become
inaccurate. As a result, one needs some reference point from where it can reset
its calculations. These reset points are either 0% battery or 100% battery.
Hence, one is required to either fully charge the battery or completely discharge
the battery to reset this calculation.
Now the next question that arises is when to know
that battery is exactly at 100% SoC. This is a little bit complex in the case of LFP
as compared to NMC battery chemistry. To understand this one needs to
understand the concept of Absorption Time.
Absorption Time
We know LifePo4 batteries have constant voltage or
flat voltage curves up to 96% SoC. So, it becomes very difficult to determine the SoC
of individual cells. Hence, it won't be possible for cells to balance themselves
or by use of a balancer during charging unless there is a substantial difference
in voltage. This particular situation is developed only when battery SoC goes
above 95%.
Absorption time is referred to as the time that
battery takes for both the battery and balancer to settle down and also perform
balancing as the balancer is most active at higher SoCs.
What happens during Absorption Time?
To understand this let's take one example. Consider 3
cells of LifePo4 connected together to a BMS and with a Passive cell Balancer.
These cells have reached the absorption voltage stage.
Case 1 - 98% SoC.
Cell 1 - 3.58V
Cell 2 - 3.51V
Cell 3 - 3.6V
Here cell 3 has reached the end limit of voltage for
LFP batteries. Hence, BMS will start discharging Cell 3 until a certain limit.
After that, it will again charge all 3 cells together. But here cell 1 and cell
2 have greater voltage differences. So,
Case 2 - 99% SoC
Cell 1 - 3.60V
Cell 2 - 3.53V
Cell 3 - 3.56V
Here again, Cell 1 hits the limit voltage and hence
the same process will be repeated until all cels are within the accepted limits
of the voltage difference.
There is one more method of balancing at Absorption
time. In this method, the battery is let to sit at 100% SoC for a certain fixed
interval of time (though your display shows 98% internally it's 100% SoC). This
time is decided based on a number of incomplete charges done, the time interval
between consecutive 100% charges, and the amount of fast charging done. As this
absorption time is in the steep area of the curve, one can observe more voltage
difference between cells even for 1% cell imbalance. This will help the balancer to
easily balance all the cells. During this time charger is just letting the battery
sit at 100% voltage and a constant amount of current is supplied to maintain
that voltage. As soon as the voltage goes above a certain threshold it displays 100%
and stops charging.
Now all the batteries in Ev's have more than 1 LFP
cell and hence it is important that all the cells are always properly balanced
for consistent battery performance. If cells are not properly balanced than a
situation like the one described below can occur.
Consider a battery consisting of 5 LFP cells.
Cell 1 - 3.18V (38%)
Cell 2 - 3.12V (22%)
Cell 3 - 3.06V (14%)
Cell 4 - 3.15V (27%)
Cell 5 - 3.10V (19%)
Here, in this case, all the five cells are highly
imbalanced. So when they are discharging when one of the cells hits 0% (2.5V,
i.e. Cell 3) whole battery pack will shut down to avoid excessive discharge of
that cell. This phenomenon is very dangerous for EV users as it might stop the
car which makes cell balancing very important.
How to keep the battery pack
balanced?
There are 2 types of balancers available for LFP
battery packs. They are Active Cell Balancer and Passive Cell Balancer. In the
section the Absorption time we have discussed considering our EV has a Passive Cell Balancer. Passive Cell Balancer only works at higher SoC when
there is a higher voltage difference visible between cells of different SoC's.
Now let's discuss Active Cell Balancer. This balancer
will be constantly balancing cells even at different SoCs. Even a minute
voltage difference between the cells will trigger an active cell balancer to
balance the pack. This kind of balancer doesn't require an outer power source to
perform its task. It will be utilizing the power of the battery itself while
performing the task of balancing.
Best balancer for EV's
Most of the low-cost Evs come with only a passive
cell balancer even in LFP batteries. Where high-end EVs come with active
cell balancers. One of the YouTubers did an experiment on LFP batteries by using
both kinds of balancers to check their effectiveness. He used 3 different types
of Passive Cell Balancers from different companies and charged them to 100% to
measure the voltage difference which they show even after being charged to 100%
using a passive balancer. As per his test using 3 different types of balancer,
still, his BMS was still showing a voltage difference of 200mV in all of the balancers
at 100% SoC.
After this test, he connected the Active Cell Balancer
with all 3 battery packs, and within a fraction of second, the voltage difference
between min and max cells started to reduce. This shows that to properly
balance cells even at 100% SoC an active cell balancer is required in LFP
cells.
Is it required to always
charge LFP batteries to 100%?
The graph shown above depicts that when you are doing
incomplete charging in LFP cells for a certain number of cycles it will start to
reduce its capacity. So let's say someone is constantly charging from 20 to 60%,
here 60% will have lesser error as it is calculated based on the current input to
the battery. But as we are charging based on voltage during balancing at 100%,
one might notice that 3.28V which might previously be 60% SoC might also get the same for 55% SoC after a certain number of incomplete cycles (where end SoC is
not equal to 100%)
To avoid this, one should charge their batteries to
100% whenever it is convenient to them. This will ensure that their BMS is
completely calibrated and the memory effect of LFP is also taken care of which
will revert back the original capacity of LFP batteries.
Imbalanced Battery: Can it fail BMS?
LFP batteries have trait of lasting very long if they are properly balanced for a longer time. All BMS are designed to ensure that LFP batteries always stays balanced. Below is one of the video explaining what happens when an LFP battery gets too much imbalanced.
LFP degrades at 100%?
Some people are skeptical that charging their LFP
batteries to 100% might actually degrade their batteries more rapidly. In
reality, this isn't the case, LFP batteries have more resistance to degradation
at 100% as compared to NMC batteries. So it is completely fine to charge LFP
batteries to 100% once a week. Most company manufacturers recommend
charging their LFP battery-powered cars all the way up to 100% at least once a
week or once every 15 days for accurate SOC calibration, cell balancing, and battery
equilibrium.
Ideal way of using LFP
batteries
Most people have a habit of plugging in their
battery for charging every now and then. They plug in their car as soon as they
reach home to keep their battery at 100% always ready to go out whenever
required. This is not the ideal way to charge LFP batteries. NMC batteries
love small charges like 40 to 70%, 30 to 70%, but not LFP batteries.
Let's discuss the most idealistic and best way to charge LFP batteries in your EVs
for battery longevity.
1. Always try to charge to 100% using the AC charging
method and try to minimize the use of DC charging. DC chargers should only be
used on trips or on emergency occasions.
2. Don't repeatedly plug in charging for your car.
Let it drain to lower SoC before plugging in back to charge your car. LFP
batteries love it if you use it till 20% before charging it back to 100%.
3. After 4-5 fast charging sessions, try to slow
charge (AC charge) your car till 100%. This is required for the battery for
proper calibration, balancing and battery equilibrium.
4. Avoid excessive use of acceleration and braking
for battery longevity.