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Charge Rate Question for LiFePO4 Batteries

Aislinn Fox

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Feb 11, 2020
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A beginner type of question related to LiFePO4 batteries. I have noticed that manufactures use C ratings for battery charge and discharge rates. For example, a 1.0 C rate is identified as the maximum rate you should use and 0.2 C rate is used to have a more extended life.

There is also information from the manufacture on loss of capacity of the battery at 3000 or 3500 cycles with typical capacities at 80% after that many cycles.

My question is when looking at the maximum discharge rate of 1.0 C, is that rate based on the new fresh battery capacity or does it change as the battery gets older? For example, if a 100 Amp battery is new then I would expect 1 C is 100 Amp/hr. However, if it is 10 years old and degraded to 80% of its original max capacity is the 1 C rating still 100 Amp/hr or is it now 80 Amp/hr?

Thank you for your help.
 
0.5C is a more common rate for maximum rated charge.

A battery's spec is a minimum performance standard. If you charge slower, discharge slower and use less % per cycle, you will improve cycle life.

Good catch. It technically does decrease, but it does it so slowly, it doesn't matter for awhile and most don't consider it. More often than not, the BMS may be the current limiting device rather than the cell - particularly with the higher capacity cells.

This decrease is true of all chemistries. It's why lead-acid can seem to nose-dive after working well for so long. One doesn't notice that what used to be a 50% discharge is now a 60, 70 or even 80% discharge, and that accelerates the deterioration.

Most of the conservative design approaches result in less than the targeted depth of discharge, so this doesn't typically come into play.
 
I'll chime in to clarify a bit.
There are different "classes" of LiFePO4. By classes, I mean for Usage Levels, such as Energy Storage (our usage) or EV Grade (which have Higher C-Rates). Typically ESS class cells have a max discharge rate of 1C and a charge rate of 0.5C. EV Class cells can have up to 5C Discharge and up to 3C Charge Rate and with some even higher ~NB These get very pricey BTW and too much for an ESS.

As Snoobler says, the decrease is not really that significant and it does take time. Because ESS usually does not operate at 1C Discharge rate or charge at even 0.5C there really is not much stress on the battery assembly, UNLESS it is seriously undersized and being overdriven. What is not quickly obvious, is that multiple cells in the pack are an aggregate across all cells, if you have batteries in parallel then the load & charge are divided between battery packs which also reduces the "hit" for charge & discharge which in turn is less stress on the battery & cells within. So there is a bit of a dance in all of this.

Typically, manufacturers state that 30% Capacity Loss is considered a "dead cell", so once a cell can only reach 70% of it's rated capacity, it's done.
 
100AH @ 0.5C=50A, @ 0.2C=20A Charging @ 20A for an empty battery (2.500 per cell) you'll be counting the Grey Hairs popping out while you wait. Maybe not to much with 100AH cells but 200AH and above and if you have more than One battery is your setup...

There is a Practical Reality that has to be met for people... Especially for those of us who do not have 24 hour daytimes.
 
0.5C is a more common rate for maximum rated charge.

A battery's spec is a minimum performance standard. If you charge slower, discharge slower and use less % per cycle, you will improve cycle life.

Good catch. It technically does decrease, but it does it so slowly, it doesn't matter for awhile and most don't consider it. More often than not, the BMS may be the current limiting device rather than the cell - particularly with the higher capacity cells.

This decrease is true of all chemistries. It's why lead-acid can seem to nose-dive after working well for so long. One doesn't notice that what used to be a 50% discharge is now a 60, 70 or even 80% discharge, and that accelerates the deterioration.

Most of the conservative design approaches result in less than the targeted depth of discharge, so this doesn't typically come into play.
Thank you for the quick reply and explanation snoobler. The caution on how depth of discharge will increase as a battery ages is something I was not taking into account in the design. I will need to consider that even if my daily power usage remains the same over time it will draw down a battery more when it is on cycle 1000 vs cycle 1… and this will cause the battery to lose capacity quicker over time.
 
When you size a battery for a full 24 hour period without charge, and a depth of discharge of 80%, you rarely get to 80% DoD if you generally get your daily solar. That renders is a non-issue in most cases. If you have really inconsistent solar, it can be an issue.
 
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