A comparison of four eneloop cells from new.
I have recently obtained a ZKETECH EBC-A05+ electronic load and charger:-
I marked the cells with a Sharpie and these cells in this study are identified as cell 1 - cell 4. These four cells were due to have similar uses after I return them toTess so I would follow similar test patterns, to keep them reasonably balanced. Two would be discharged from new to 1.0v at 1A current (constant current), the other two would be given a 1W discharge for comparison (constant power). This first discharge run was to see what percentage charge was on the cells from new as these are sold as "pre-charged":-
The lines in blue and red are voltage(blue) and current(red) for the pair of cells discharged at 1A (Constant Current) and purple (voltage) and pink (current) for the cells discharged at 1W (Constant Power). The difference is that both although both finished at 1 Ampere load the constant power discharge starts at lower current whilst the cell voltage is higher, hence the run time is longer!
If instead of plotting the charts against time we plot against delivered mAh we get :-
showing the discharge profiles as very similar for the four cells.
To summarize the capacities were :-
If instead of plotting the charts against time we plot against delivered mAh we get :-
To summarize the capacities were :-
|
Cell number
|
mAh
|
discharge type
|
% stated Capacity
|
|
1
|
1400
|
CC 1 A
|
73.7
|
|
2
|
1396
|
CC 1 A
|
73.5
|
|
3
|
1380
|
CP 1 W
|
72.6
|
|
4
|
1390
|
CP 1 W
|
73.1
|
The cells were then recharged:-
The cells were the all discharged again at a current of 1 Ampere:-
Unfortunately a problem has crept into the test arrangement and a banana plug has decided to become high resistance. This has little effect on these runs and makes the current curves messy, later in the tests this was located and the offending plug was replaced!
Next a series of automatic tests were setup as follows:-
The Waits are for temperature recovery and to see how the cell voltage recovers.
An example graphics output from one of these cycles for cell 4 was:-
As can be seen the notes at the bottom only show the last test values, however at the end of the run all the charge values can be written down before closing the program:-
enloop cycle results
Cell - 1 2 3 4
D 1.9A 1.806 1.767 1.755 1.777
C 1.9A 1.918 1.880 1.871 1.915
D 1.9A 1.841 1.824 1.802 1.844
C 1A 1.942 1.948 1.912 1.922
D 300mA 1.894 Interrupted 1.886 1.893
Cell 2 had it's run broken by WINDOWS 10 deciding it was idle and it needed to apply patches.
The values here are in Ampere Hours.
So the best capacity can as expected be obtained at lower currents and with a 300 mA discharge rate the cells very nearly attain their rated capacities. Self discharge although low is obviously present and the capacity differences between the first 1.9A discharge and the second is due to the first discharge being several hours after charge and the second discharge is 5 mins after the charge.
The cells were then all recharged to a nominal state for their return.
As expected no real surprises the cells are all very similar and match out of the box reasonably well. You can't rely on the "pre-charged state" but it is useful in an emergency. These cells tolerate recharging up to 1C with no issues. Like all NiMh cells detection of end of charge is difficult for a simple (cheap) charger as the voltage needs to be monitored for that slight drop that occurs when the cell is fully charged. Trickle or other very low charge rates are an issue as it becomes impossible to detect that drop or the cells increase in temperature which is the other valid way of detecting fully charged.
The best chargers use typically three methods to detect fully charged with delta v/delta t as primary and temperature as a second method, the third is an emergency shutdown after a fixed period of time.
NiMh eneloops are best charged at between 700ma to 2000ma, it is better to shut off charge early with a high charge current than to overcharge at a low current!
For comparison with other AA cells I include some discharge curves for Alkaline cells and a 2400ma Amazon Basics cell:-
Note the Alkaline cells were only running either 300mA or 500mA the NiMh's are cell 1 from the eneloop set above and an Amazonbasics 2400mAh cell. Note the NiMh's are flat averaged at about 1.2v, the Akaline cells rapidly drop below 1.2v quickly and over most of their usable capacity run at lower voltage than a NiMh cell does. This is why in most uses a NiMh cell works well in devices designed for alkaline cells. The NiMh discharges were stopped at 1v to protect the cells. The Alkalines were stopped at 0.5v and thrown away!
I hope this is of interest.
Peter G4DCP
|
Cell number
|
mAh
|
charge type
|
% stated Capacity
|
|
1
|
2068
|
NiMh 1 A
|
108.8
|
|
2
|
2082
|
NiMh 1 A
|
109.6
|
|
3
|
1992
|
NiMh 1 A
|
104.8
|
|
4
|
1996
|
NiMh 1 A
|
105.1
|
Unfortunately a problem has crept into the test arrangement and a banana plug has decided to become high resistance. This has little effect on these runs and makes the current curves messy, later in the tests this was located and the offending plug was replaced!
|
Cell number
|
mAh
|
discharge type
|
% stated Capacity
|
|
1
|
1819
|
1A CC
|
95.7
|
|
2
|
1804
|
1A CC
|
94.9
|
|
3
|
1779
|
1A CC
|
93.6
|
|
4
|
1801
|
1A CC
|
94.8
|
Next a series of automatic tests were setup as follows:-
|
Step
|
Type
|
Param 1
|
Param 2
|
Param 3
|
|
1
|
Discharge CC
|
1.9A
|
1V end voltage
|
|
|
2
|
Wait
|
|
|
5 mins
|
|
3
|
NiMh Charge
|
1.9A
|
1 cell
|
|
|
4
|
Wait
|
|
|
5 mins
|
|
5
|
Discharge CC
|
1.9A
|
1V end voltage
|
|
|
6
|
Wait
|
|
|
5 mins
|
|
7
|
NiMh Charge
|
1.9A
|
1 cell
|
|
|
8
|
Wait
|
|
|
5 mins
|
|
9
|
Discharge CC
|
0.3A
|
1V end voltage
|
|
|
10
|
Wait
|
|
|
5 mins
|
The Waits are for temperature recovery and to see how the cell voltage recovers.
An example graphics output from one of these cycles for cell 4 was:-
enloop cycle results
Cell - 1 2 3 4
D 1.9A 1.806 1.767 1.755 1.777
C 1.9A 1.918 1.880 1.871 1.915
D 1.9A 1.841 1.824 1.802 1.844
C 1A 1.942 1.948 1.912 1.922
D 300mA 1.894 Interrupted 1.886 1.893
Cell 2 had it's run broken by WINDOWS 10 deciding it was idle and it needed to apply patches.
The values here are in Ampere Hours.
So the best capacity can as expected be obtained at lower currents and with a 300 mA discharge rate the cells very nearly attain their rated capacities. Self discharge although low is obviously present and the capacity differences between the first 1.9A discharge and the second is due to the first discharge being several hours after charge and the second discharge is 5 mins after the charge.
The cells were then all recharged to a nominal state for their return.
As expected no real surprises the cells are all very similar and match out of the box reasonably well. You can't rely on the "pre-charged state" but it is useful in an emergency. These cells tolerate recharging up to 1C with no issues. Like all NiMh cells detection of end of charge is difficult for a simple (cheap) charger as the voltage needs to be monitored for that slight drop that occurs when the cell is fully charged. Trickle or other very low charge rates are an issue as it becomes impossible to detect that drop or the cells increase in temperature which is the other valid way of detecting fully charged.
The best chargers use typically three methods to detect fully charged with delta v/delta t as primary and temperature as a second method, the third is an emergency shutdown after a fixed period of time.
NiMh eneloops are best charged at between 700ma to 2000ma, it is better to shut off charge early with a high charge current than to overcharge at a low current!
For comparison with other AA cells I include some discharge curves for Alkaline cells and a 2400ma Amazon Basics cell:-
Note the Alkaline cells were only running either 300mA or 500mA the NiMh's are cell 1 from the eneloop set above and an Amazonbasics 2400mAh cell. Note the NiMh's are flat averaged at about 1.2v, the Akaline cells rapidly drop below 1.2v quickly and over most of their usable capacity run at lower voltage than a NiMh cell does. This is why in most uses a NiMh cell works well in devices designed for alkaline cells. The NiMh discharges were stopped at 1v to protect the cells. The Alkalines were stopped at 0.5v and thrown away!
I hope this is of interest.
Peter G4DCP
