Understanding Absorption Time in LFP batteries

By Electrifying India -
Source: Off Grid Garrage

Understanding LFP charging is not as simple as other battery chemistries. This is because of their weird behavior during the charging phase. There are several misconceptions floating around about charging behavior of LFP batteries. All of them are not true, while some of them are true but not for the reason they are mentioning, while some reasons are actually true too.

Misconceptions

1. Charging 10 to 100% does better cell balancing than 50 to 100%: This concept holds 100% true, if your battery is NMC battery. If its LFP battery then its not true. As per experiments conducted by several people this is not proving to be true. But doing 10% to 100% is actually good for LFP battery but not for cell balancing but for battery longevity. To understand this one guy did experiment on group of lfp cells connected together. these were his findings.

  • If you want all cells to be balanced to 3.4V, then keep the balancing start voltage to 3.4V only.
  • Keeping balancing start voltage to 3.35V or 3.45V does no good to battery but rather increases balancing time.
2. You can overcharge LFP cells only when its 3.65V: This is a very wrong misconception. It has actually came from NMC batteries which doesnt apply to LFP batteries. You can actually overcharge your cell at 3.45V too if you keep on absorption time for your LFP cell at constant voltage for a very long time.

Source: Off Grid Garrage

As you can see, absorption time plays a very crucial role in case of LFP cells. Hence, it will be important to understand what actually is absorption time.
Absorption Time: When we are charging LFP batteries to 100%, we can actually do it with 3.65V or 3.55V too. It is also possible with 3.45V but will take very long absorption time to reach the 100%. Suppose we are using 3.65V for doing 100%. So when cell reaches 3.65V, BMS will let it charge with decreasing current while keeping voltage constant. For that current will be constantly dropping unless it reaches minimum threshold current. When it reaches this point, charging stops.

This period of constant voltage charging is termed as absorption time. This absorption time will be very less at 3.65V and we can actually achieve 100% SOC. But when we use 3.5 or 3.55V then this absorption time will be 45 minutes where as for 3.65V it will hardly be 5 to 10 minutes. Against this if you want your battery to absorb till 100% using 3.4V then it will take time around 4 hours.

When we do 100% (with absorption - SOC reached) with cutoff charge at 0.1Amps
at 3.65V - 99.9% SOC
at 3.5V - 99.7% SOC
at 3.4V - 98.2% SOC

Without absorption
3.6V - 99.5% SOC
3.5V - 97.6% SOC
3.4V - 89% SOC

So its quite clear that if you want to maintain battery health by doing 100% at 3.5V instead of 3.6 or 3.65V, then absorption time is crucial to reach very close to 100% SOC.

Source: Off Grid Garrage

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