So often I see that this is the only rating looked at when deciding which cell to buy. I can understand why because it seems that the larger this number is the less often we have to charge…a good thing. But like so many things involving Li-ion cells, it’s not that simple.
Capacity is a rating that essentially tells us the size of a cell’s “gas tank”. It’s how much charge the cell stores and it is expressed in milliampere-hours (mAh) or ampere-hours (Ah). One thousand mAh is the same as one Ah. The rating tells us theoretically how many milliamps we can draw from the cell and for how long.
For example, a cell with a 3000mAh capacity rating can be discharged at 3000mA for 1 hour. When you multiply 3000mA by 1 hour you get 3000mAh. But that also means if we discharge the cell at 1500mA the cell should run for 2 hours since 1500mA x 2h = 3000mAh.
You can also go the other way. If we discharge the cell at 6000mA (6A) then it will theoretically run for 1/2h since 6000mA x 0.5h = 3000mAh. But there are things that affect how accurate this is when discharging at above 1A or so. I will address that later on in this post.
There are two main capacity ratings for the big manufacturers, Minimum and Nominal (or Typical). Minimum is just what it sounds like, the lowest capacity that any cell for that model number should have.
The Nominal or Typical rating is what most of the cells for that model number will typically test out to if they are not old and were not stored badly. This number is always higher than the Minimum rating. For example, most 3000mAh minimum rated 18650’s from the big manufacturers test out near to 3100mAh.
The cell wrapping/rewrapping companies like Vapcell, Efest, etc., usually only have one capacity rating and they often use the Typical rating on their wraps. For example, a 3000mAh-rated cell from Samsung would be rewrapped with 3100mAh or 3200mAh on the wrap. A 3000mAh-rated cell from one of the China manufacturers would probably be given a 3000mAh rating on the wrap since the cell often only has a Typical rating.
Some wrapping/rewrapping companies are better than others with this and use accurate ratings. Some inflate the capacity rating a bit (rounding up to 3200mAh for 3000mAh minimum-rated 18650’s, for example) to try to get more sales.
There can be a problem with these ratings though…they are all measured using a very slow discharge, typically at or under 1A. The standard is to use a discharge level equal to 1/5th of the capacity rating number. For example, a 3000mAh cell would have its capacity measured using a discharge rate of 3000 / 5 = 600mA. A cell with a 5000mAh capacity rating would have been tested at 5000 / 5 = 1000mA = 1A.
At lower power levels the capacity rating can be used to get a decent estimate of how the run times of two cells might compare. But as the power levels increase the capacity rating becomes less and less useful. Here’s why…
The internal resistance of a cell causes its voltage to drop down, “voltage sag”, when it’s used. Higher capacity cells typically have a higher internal resistance which means their voltage will sag down more.
This is different from the slow voltage drop we see as a cell is discharged down. Voltage sag is instantaneous and temporary, it only happens during the discharge. When the current stops flowing the cell’s voltage bounces back up to its true “resting voltage”, determined by how much charge is left in the cell.
As the power level increases, this voltage sag gets worse and worse. The higher capacity cells will sag more and at lower power levels than most lower capacity, high performance cells. This means that at higher power levels a good 2500mAh-2600mAh cell like the Molicel P26A or Murata VTC5A can actually run for longer than high capacity 3000mAh cells like the Samsung 30Q or Murata VTC6 when above 15A or so.
You can use the true continuous current rating to judge which cells might be fairly close in performance so you can directly compare their capacity. You can compare the capacity of two 25A cells but comparing the capacities of a 5A and 35A cell at anything over 3A-5A or so would be pointless.
If the current ratings of two cells you are comparing are very different then do not compare them at anything much over about 75% of the lower current rated cell. For example, the 35A-rated, 3000mAh Samsung 30T will run for much, MUCH longer than any 3500mAh, 5A rated cell if you are discharging them at much over 3A-5A or so.
But if you are using them at 1A or 2A (flashlight, toy, fan, powerbank, etc.) then there will be very little voltage sag in the 3500mAh cell and you can use all its extra capacity. It will run for longer than the high performance, but lower capacity, 3000mAh cell in the example above. The extra performance of the Samsung 30T is just not used, or needed, in this example.
So while we we can use capacity ratings to get a rough idea of how two cells might compare we need to be careful not to trust those ratings to be helpful at higher power levels. Choose cells based on their current ratings first and only then compare their capacities.
Capacity is a rating that essentially tells us the size of a cell’s “gas tank”. It’s how much charge the cell stores and it is expressed in milliampere-hours (mAh) or ampere-hours (Ah). One thousand mAh is the same as one Ah. The rating tells us theoretically how many milliamps we can draw from the cell and for how long.
For example, a cell with a 3000mAh capacity rating can be discharged at 3000mA for 1 hour. When you multiply 3000mA by 1 hour you get 3000mAh. But that also means if we discharge the cell at 1500mA the cell should run for 2 hours since 1500mA x 2h = 3000mAh.
You can also go the other way. If we discharge the cell at 6000mA (6A) then it will theoretically run for 1/2h since 6000mA x 0.5h = 3000mAh. But there are things that affect how accurate this is when discharging at above 1A or so. I will address that later on in this post.
There are two main capacity ratings for the big manufacturers, Minimum and Nominal (or Typical). Minimum is just what it sounds like, the lowest capacity that any cell for that model number should have.
The Nominal or Typical rating is what most of the cells for that model number will typically test out to if they are not old and were not stored badly. This number is always higher than the Minimum rating. For example, most 3000mAh minimum rated 18650’s from the big manufacturers test out near to 3100mAh.
The cell wrapping/rewrapping companies like Vapcell, Efest, etc., usually only have one capacity rating and they often use the Typical rating on their wraps. For example, a 3000mAh-rated cell from Samsung would be rewrapped with 3100mAh or 3200mAh on the wrap. A 3000mAh-rated cell from one of the China manufacturers would probably be given a 3000mAh rating on the wrap since the cell often only has a Typical rating.
Some wrapping/rewrapping companies are better than others with this and use accurate ratings. Some inflate the capacity rating a bit (rounding up to 3200mAh for 3000mAh minimum-rated 18650’s, for example) to try to get more sales.
There can be a problem with these ratings though…they are all measured using a very slow discharge, typically at or under 1A. The standard is to use a discharge level equal to 1/5th of the capacity rating number. For example, a 3000mAh cell would have its capacity measured using a discharge rate of 3000 / 5 = 600mA. A cell with a 5000mAh capacity rating would have been tested at 5000 / 5 = 1000mA = 1A.
At lower power levels the capacity rating can be used to get a decent estimate of how the run times of two cells might compare. But as the power levels increase the capacity rating becomes less and less useful. Here’s why…
The internal resistance of a cell causes its voltage to drop down, “voltage sag”, when it’s used. Higher capacity cells typically have a higher internal resistance which means their voltage will sag down more.
This is different from the slow voltage drop we see as a cell is discharged down. Voltage sag is instantaneous and temporary, it only happens during the discharge. When the current stops flowing the cell’s voltage bounces back up to its true “resting voltage”, determined by how much charge is left in the cell.
As the power level increases, this voltage sag gets worse and worse. The higher capacity cells will sag more and at lower power levels than most lower capacity, high performance cells. This means that at higher power levels a good 2500mAh-2600mAh cell like the Molicel P26A or Murata VTC5A can actually run for longer than high capacity 3000mAh cells like the Samsung 30Q or Murata VTC6 when above 15A or so.
You can use the true continuous current rating to judge which cells might be fairly close in performance so you can directly compare their capacity. You can compare the capacity of two 25A cells but comparing the capacities of a 5A and 35A cell at anything over 3A-5A or so would be pointless.
If the current ratings of two cells you are comparing are very different then do not compare them at anything much over about 75% of the lower current rated cell. For example, the 35A-rated, 3000mAh Samsung 30T will run for much, MUCH longer than any 3500mAh, 5A rated cell if you are discharging them at much over 3A-5A or so.
But if you are using them at 1A or 2A (flashlight, toy, fan, powerbank, etc.) then there will be very little voltage sag in the 3500mAh cell and you can use all its extra capacity. It will run for longer than the high performance, but lower capacity, 3000mAh cell in the example above. The extra performance of the Samsung 30T is just not used, or needed, in this example.
So while we we can use capacity ratings to get a rough idea of how two cells might compare we need to be careful not to trust those ratings to be helpful at higher power levels. Choose cells based on their current ratings first and only then compare their capacities.
