We must have heard in recent times that few of the LFP BMS are failing in recent times. Main reason for that will be discussed over here based on my studies and understanding. Do keep in mind that these studies can likely have errors as I am still studying about functioning of LFP batteries. To start our understanding we will be starting with the concept of memory effect.
Memory Effect
Word of Caution: Don’t google definition of
memory effect. That definition doesn’t apply to LFP batteries. Memory effect
observed in LFP batteries is completely different.
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| Source: Indiamart |
Memory effect is influenced by the point of
starting and stopping the charging cycle. Some papers are showing that starting
point will be showing memory effect point while some are showing that even end
point will also showcase memory effect. I know this will be little confusing
for you as you are not aware about the exact functioning of memory effect in
LFP yet. So lets first understand that. Lets consider an example, where we have
started charging of an LFP battery from 20% and charged it till 85%. Here, 20%
and 85% will be our starting and ending points of our study.
Now during the next cycle when we have consumed
this charging and plug in the charger again, then we will experience a minor
voltage jump at these points of 20% and 85%. These voltage jumps are so minor
that normally they don’t affect the charging cycles. But when incomplete
charging happens for a number of consecutive cycles that’s where the problem
kicks in.
Problem No. 1
After lots of cycles, in which few of the
cycles had resulted in incomplete charging till 90 to 95%. Here, it will show a
voltage jump between 90 and 95% which will be substantially higher. Now we know
that BMS judges 100% of the cells from their cell voltages. Any kind of voltage
jump nearby 100% can confuse BMS about the actual 100% SoC of the battery. This
can cause BMS to judge 93% as 100% of the battery after certain number of incomplete
cycles nearby 93%. When this happens, one can experience their LFP batteries to
pre-maturely fail.
Solution
For LFP batteries to stay healthy, they should
always be charged to 100%. This will avoid building up of memory effect. Also
when LFP is properly charged to 100%, some of the previously built memory
effect is also released, which will actually make your battery healthy. So some
people may question that whether they can always charge from 80 to 100%. Answer
to that is “YES” and “NO” both. Main reason for that is LFP batteries likes to
stay at low SoC. So when you are always cycling LFP batteries at higher SoC
then it will degrade much faster. So ideally its better to discharge it till 20
to 30% before charging it back to 100% for better battery life.
Problem No. 2
When LFP batteries are getting incompletely charged for quite a long period of time, it can likely happen that the cells of the complete battery pack might get imbalanced. This can even happen if the balancer or BMS is not functioning well. Due to that with time, cells of the complete battery pack will keep on getting more and more imbalanced with time. After a certain period of time or a certain number of cycles, this imbalance will increase to such a large extent that BMS will no longer be able to handle the imbalance and balance it further. This would also result in failing of the LFP BMS. Always remember that in most cases, its BMS that is failing in case of LFP batteries, not the battery cells.
Solution
Once your battery is highly imbalanced in a way that can't be handled by BMS anymore than it needs to be disassembled and balanced manually or using some external balancer which is more effective as compared to the standard Balancer that comes with the battery. Normally connecting an active balancer and then charging till 100% with active balancer connected will solve the problem.
Charging
techniques for best battery life
1.
Charge
from different SoC every time like from 20, 30, and 40% to 100%.
2.
On
fast charging limit the end charging percentage to 85% or below 90%.
3.
On
slow charging too avoid stopping the charge once it crosses 90% SoC.
4.
It
is totally not recommended to disconnect the charger once the balancing process is
started.