Kentli AA 2800mWh (Blue)

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Official specifications:
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This battery is not a NiMH battery, but a LiIon battery with electronic to make it look like a 1.5V AA battery. Due to this it will not be a normal battery review, I could not use my normal test scripts, but had to do a lot of the test manually. I have looked quickly at the charger and looked inside the battery.
The first thing I notice when I see the specifications is that capacity is specified in mWh not the usual mAh, this tells me that they try to make the battery look better than it is, as can be seen above the real capacity is about 1600mAh.

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When I got the battery there was some trouble with shipping loose LiIon cells, this means I got the cells in a flashlight.

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The top of the cell has a ring around the button top, this is used for charging the cell.

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The cells has very stable voltage, until they are empty. This high voltage means they can be used in equipment that will not work with NiMH cells. The very stable voltage also means that any battery indicator will show full, until the battery is empty, there will be no indication of 25% left or anything like that.
The cells have a small difference in capacity.

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I did try a low current discharge at 10mA, as can be see above the battery lasted a bit above 6 days and the capacity was around 1600mAh. This means that the extra loses due to the buck converter inside the battery is not really significant for a 6 day period.

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I did not really hit the 2800mWh in my test, only 2500mWh.

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The batteries requires a special charger or rather a LiIon charger, due to the voltage difference this charger do not need to charge 1600mAh, but only about half that.



Charger

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To use the batteries you need the special charger for them or you can use a normal LiIon charger if you can make an adapter.

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The trick is the connection to the battery, it must connect to the ring around the plus pole and not the button top on the battery.

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The charger can only be used for Kentli batteries and can handle up to four at a time.

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A fairly normal LiIon charge curve with 300mA charge current and 50mA termination.

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The 4 channels do not completely matches in voltage, that might be the reason for the capacity differences in the batteries.

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With 4 batteries the charger must be powered from a usb power that can deliver a bit above 1A.



Inside the battery

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I had to see what’s inside this battery.

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Some of the electronic is on a flex circuit board

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The metal can is just a metal can, inside there is a lithium polymer pack and all connections are done with the flex circuit board.

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This electronic handles two function. One function is the buck converter that reduces the LiIon voltage to 1.5 volt, the other function is shutting down when the battery is empty (To avoid over discharge).
The buck converter is the 2R2 inductor and the chip besides it, this chip is a 2.5A buck converter and does also contain a 2.45 volt under voltage lockout.
The other chip is in series with the battery and must also be some sort of protection IC, maybe to prevent overcurrent if the ring is shorted to the can.
Click on the above pictures for larger versions.

When I did have the battery open, I did some more measurements:



Conclusion

This battery is an interesting idea, what you get is:
With the above I believe the batteries are most useful when replacing lithium cells in equipment that uses them fairly fast, but batteries will have to be replaced/charged more often.
They are not good to replace alkaline in low power equipment that last a year or more on a single cell, due to the self discharge.
If equipment can run on NiMH, they are preferable compared to this cell, due to higher capacity and lower self discharge (for LSD cells).



Notes and links

How is the test done and how to read the charts
Compare to other AA/AAA batteries: Alkaline/NiMH/Lithium