The various hobbyist/enthusiast communities constantly debate cell ratings and what they mean when choosing a good cell for our use. But there’s a lot of misinformation out there.
What do the ratings for a cell actually tell us? How can we use them to choose a good cell? This series of posts are my personal opinions on all of these ratings and how to best use them.
The ratings I’ll discuss are Voltage, Capacity, Wh (Energy), Charge Current, Discharge Current, Temperature, and Cycle Life.
Voltage
How can this be something that’s debated? The basic ratings aren’t but when to stop charging and discharging has been the subject of many an argument. First, the basics.
The “standard” Li-Ion cells we use are rated for use at 2.5V to 4.2V. There are a couple rated down to 2.0V and a couple that stop at 2.75V but these are rarely used. If you are unsure of the voltage ratings for your cells check for a datasheet online.
There are also a few “HV” (high voltage) LiPo cells that can be charged to 4.35V. But be careful, some LiPo HV cells might just be 4.2V-rated cells that the seller or LiPo pack assembler decided to call HV cells to increase profits. Charging these mis-labeled 4.2V cells at 4.35V will damage them and can eventually make them dangerous to use.
Standard Li-Ion cells can be brought down to 2.5V every time you use them without extra damage. This is their rating and how long the manufacturer says they will last, their “cycle life”, is based on bringing them this low.
LiPo” cells are another type of Li-Ion cell but are typically not used at below 3.0V and often not below αround 3.5V. Good LiPo’s are very high performance cells but they are delicate, both electrically and physically.
LiFePO4 (or LFP) cells are another type of Li-Ion cell but with lower voltage ratings due to their different “chemistry”. They are extremely durable with a longer cycle life and use a safer chemistry that standard Li-Ion or LiPo cells. It can vary a little but LiFePO4 cells are typically rated from 2.0V to 3.6V. Never charge one to 4.2V! It might not burst but it will be damaged.
There are other Li-Ion chemistries with different voltage ranges but they are rarely used by the various hobbyist/enthusiast communities.
So what gets debated? What voltage to charge and discharge to.
What voltage you charge to affects the overall life of the cell, its “cycle life”. This is how many times you can cycle the cell from 4.2V to 2.5V and back to 4.2V again. If you only removed 1/2 of the capacity and then recharged that would count as 1/2 of a cycle.
Do not make the same mistake so many others do! For a full cycle the discharge/recharge must be equal to the capacity of the pack. You cannot say that five uses down to 3.6V and back up to 4.2V are five cycles. That would be the equivalent of only about 2.5 cycles.
The lower the charging voltage is the longer the cell’s overall life (if not otherwise abused). A cell charged to 4.1V each time will run for more cycles than a cell charged to 4.2V every time. A cell charged to 4.0V will last even longer (overall) than one charged to 4.1V, and so on.
But charging to a lower voltage means there is less charge in the cell so it won’t run for as long. You might lose about 10% capacity charging to 4.1V instead of 4.2V but that can extend the overall life of the cell a lot. Charging to only 4.0V can significantly extend the life of a cell.
Some of the debates are about whether the loss of capacity (run time) is worth the gain in cycle life when charging to lower voltages. It’s not really worth debating IMO since it’s just a personal decision. If cycle life is the priority you might want to consider charging to a lower voltage. If run time is the priority then you will want to charge to 4.2V at a slow rate to make sure you cram in as much charge as possible. Do not charge any Li-Ion cell above 4.20V though unless you are absolutely sure it’s a true high-voltage 4.35V cell.
There are a LOT of debates on how far down you should discharge a cell. Some would argue that you should never go below 3.6V. Others say you can go down to 2.5V with standard Li-Ion cells and 3.0V with LiPo’s every time.
As always with Li-Ion cells, what to do depends on your priorities. There is no one “correct” voltage. Is the longest overall (cycle) life important or is maximum run time? Perhaps you just want a good balance between those two?
In my opinion you can increase the overall life of standard Li-Ion cells by staying at or above 3.0V or so. The exact voltage doesn’t really matter. Staying above the point where the cell’s voltage really starts to dive down during a discharge (around 2.8V-3.2V) is what’s important. This is where the cell’s internal resistance goes up and the cell gets hotter, which ages the cell faster. There’s very little charge left in the cell below this point anyway.
You can stop at 3.6V but the additional increase in cycle life you get probably won’t be worth the loss of up to 1/2 of your capacity (run time). Your choice though. You can certainly stop here if you have an application where max cycle life is critical. Electrical vehicles (EV’s) do this since their battery packs are so expensive.
For LiPo’s I recommend always staying above 3.0V as they are easily damaged from overdischarging. I haven’t seen enough good data to say how much better it is to stay above the 3.6V point many LiPo users say to stop at. It certainly won’t hurt your LiPo’s to do this and it will help to extend their life. I just don’t know if it’s worth the huge loss of run time.
For LiFePO4 cells I recommend staying above 2.5V for high power use and up to 3.0V for low power use. Below these voltages there’s just not a lot of capacity left.
Voltage ratings, and the others I’ll discuss in upcoming posts, may not seem exciting but these ratings determine how our cells will perform. A little time spent learning about them can go a long way towards getting the most out of our cells.
What do the ratings for a cell actually tell us? How can we use them to choose a good cell? This series of posts are my personal opinions on all of these ratings and how to best use them.
The ratings I’ll discuss are Voltage, Capacity, Wh (Energy), Charge Current, Discharge Current, Temperature, and Cycle Life.
Voltage
How can this be something that’s debated? The basic ratings aren’t but when to stop charging and discharging has been the subject of many an argument. First, the basics.
The “standard” Li-Ion cells we use are rated for use at 2.5V to 4.2V. There are a couple rated down to 2.0V and a couple that stop at 2.75V but these are rarely used. If you are unsure of the voltage ratings for your cells check for a datasheet online.
There are also a few “HV” (high voltage) LiPo cells that can be charged to 4.35V. But be careful, some LiPo HV cells might just be 4.2V-rated cells that the seller or LiPo pack assembler decided to call HV cells to increase profits. Charging these mis-labeled 4.2V cells at 4.35V will damage them and can eventually make them dangerous to use.
Standard Li-Ion cells can be brought down to 2.5V every time you use them without extra damage. This is their rating and how long the manufacturer says they will last, their “cycle life”, is based on bringing them this low.
LiPo” cells are another type of Li-Ion cell but are typically not used at below 3.0V and often not below αround 3.5V. Good LiPo’s are very high performance cells but they are delicate, both electrically and physically.
LiFePO4 (or LFP) cells are another type of Li-Ion cell but with lower voltage ratings due to their different “chemistry”. They are extremely durable with a longer cycle life and use a safer chemistry that standard Li-Ion or LiPo cells. It can vary a little but LiFePO4 cells are typically rated from 2.0V to 3.6V. Never charge one to 4.2V! It might not burst but it will be damaged.
There are other Li-Ion chemistries with different voltage ranges but they are rarely used by the various hobbyist/enthusiast communities.
So what gets debated? What voltage to charge and discharge to.
What voltage you charge to affects the overall life of the cell, its “cycle life”. This is how many times you can cycle the cell from 4.2V to 2.5V and back to 4.2V again. If you only removed 1/2 of the capacity and then recharged that would count as 1/2 of a cycle.
Do not make the same mistake so many others do! For a full cycle the discharge/recharge must be equal to the capacity of the pack. You cannot say that five uses down to 3.6V and back up to 4.2V are five cycles. That would be the equivalent of only about 2.5 cycles.
The lower the charging voltage is the longer the cell’s overall life (if not otherwise abused). A cell charged to 4.1V each time will run for more cycles than a cell charged to 4.2V every time. A cell charged to 4.0V will last even longer (overall) than one charged to 4.1V, and so on.
But charging to a lower voltage means there is less charge in the cell so it won’t run for as long. You might lose about 10% capacity charging to 4.1V instead of 4.2V but that can extend the overall life of the cell a lot. Charging to only 4.0V can significantly extend the life of a cell.
Some of the debates are about whether the loss of capacity (run time) is worth the gain in cycle life when charging to lower voltages. It’s not really worth debating IMO since it’s just a personal decision. If cycle life is the priority you might want to consider charging to a lower voltage. If run time is the priority then you will want to charge to 4.2V at a slow rate to make sure you cram in as much charge as possible. Do not charge any Li-Ion cell above 4.20V though unless you are absolutely sure it’s a true high-voltage 4.35V cell.
There are a LOT of debates on how far down you should discharge a cell. Some would argue that you should never go below 3.6V. Others say you can go down to 2.5V with standard Li-Ion cells and 3.0V with LiPo’s every time.
As always with Li-Ion cells, what to do depends on your priorities. There is no one “correct” voltage. Is the longest overall (cycle) life important or is maximum run time? Perhaps you just want a good balance between those two?
In my opinion you can increase the overall life of standard Li-Ion cells by staying at or above 3.0V or so. The exact voltage doesn’t really matter. Staying above the point where the cell’s voltage really starts to dive down during a discharge (around 2.8V-3.2V) is what’s important. This is where the cell’s internal resistance goes up and the cell gets hotter, which ages the cell faster. There’s very little charge left in the cell below this point anyway.
You can stop at 3.6V but the additional increase in cycle life you get probably won’t be worth the loss of up to 1/2 of your capacity (run time). Your choice though. You can certainly stop here if you have an application where max cycle life is critical. Electrical vehicles (EV’s) do this since their battery packs are so expensive.
For LiPo’s I recommend always staying above 3.0V as they are easily damaged from overdischarging. I haven’t seen enough good data to say how much better it is to stay above the 3.6V point many LiPo users say to stop at. It certainly won’t hurt your LiPo’s to do this and it will help to extend their life. I just don’t know if it’s worth the huge loss of run time.
For LiFePO4 cells I recommend staying above 2.5V for high power use and up to 3.0V for low power use. Below these voltages there’s just not a lot of capacity left.
Voltage ratings, and the others I’ll discuss in upcoming posts, may not seem exciting but these ratings determine how our cells will perform. A little time spent learning about them can go a long way towards getting the most out of our cells.
