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Bonai AAA Rechargeable batteries compared with Eneloop

I have been using rechargeable batteries for many years. The old NiCd cells left a lot to be desired -- fast self-discharge, low capacity, memory effect and short lifetime. Many people who now refuse to use rechargeable batteries probably developed their aversion from using these early NiCd cells. Then in 1989 NiMH cells were introduced which virtually eliminated the memory effect and offered increased capacity and life, but they still self-discharged fairly quickly. A fully charged NiMH might be unusable just a few months later due to self-discharge, and for this reason were sold uncharged because within the time of manufacture, packaging and display on store shelves most of the charge would have already dissipated. Because of fast self-discharge, they were generally unsuited for any use requiring long-term power such as remotes and emergency flashlights.

Then Sanyo totally changed the game in 2005 when they introduced the famous Eneloop cell which conquered the problem of fast self-discharge, retaining up to 90% of its charge after one year and up to 70% after 5 years. This still doesn't match non-rechargeable alkaline cells which can retain almost 100% of original power after 10 years. Alkaline should still be the choice for emergency devices which will go unused for many years but need to work when needed (but always check periodically to make sure the alkaline cells are still working and are not leaking). Sanyo's Eneloop technological advance dramatically reduced self-discharge by improving the electrode separator. Sanyo and its Eneloop technology was subsequently acquired by Panasonic.

Now, many manufacturers are making low self-discharge NiMH batteries. The technology requires precise and high-quality manufacturing so the rate of self-discharge can vary, as well as the total mAh capacity and maximum recharge cycles. Recharge cycles rating is usually unimportant to most users because all can be recharged many hundreds or thousands of times, which equates to years of heavy use.

Note that the nominal voltage of NiMH cells is 1.2v. A freshly charged cell is usually 1.35-1.45v, but this voltage quickly drops to 1.2v in use. The NiMH discharge pattern is a nearly flat line 1.2v level over almost the entire discharge period, then the voltage drops rapidly as it reaches full discharge. In contrast, the discharge pattern of an alkaline cell is a slow steady drop from about 1.6-1.5v down to the point its power is insufficient to power the device. This is why a flashlight powered by a NiMH cell will maintain steady brightness over a long period, then suddenly go dim with little warning as the battery dies. A flashlight powered by an alkaline cell will start out bright and then slowly and steadily decline as the battery discharges.

I have been using Eneloop batteries since they first came out and still have some a decade old in daily use. I'm a heavy user of batteries since I'm a heavy user of electronic devices which use AAA and AA cells... such as flashlights, headlamps, audio devices, blood pressure machines, remotes, etc. As I need to acquire more and more NiMH rechargeables to power more devices, I am considering less expensive brands of low self-discharge NiMH rechargeables. I saw these and they were a good price so took a chance and ordered them. I am impressed.

At first glance, these appear to be high-quality and attractive batteries. They are obviously low self-discharge as advertised since they arrived fully charged. I immediately put them through some tests to see how well they stacked up to Eneloop. I wanted to know their storage capacity, internal resistance, and physical measurements. These are the results:

Physical size: 44.4 x 10.4 mm (Eneloop is 44.2 x 10.4 mm)
Some batteries are a little larger than standard to achieve higher capacity, but this can prevent them from fitting into some battery compartments. These Bonai cells are almost exactly the size of Eneloop, which is very good.

Internal resistance varies between about 55 to 65 milliohn, which is very good. Low internal resistance allows the batteries to operate in high discharge applications.

Storage capacity (all 16 tested):
Range from 721 to 855 mAh
Median: 815 mAh (this is center point where there is as many higher and lower)
Mean: 810 mAh (the average of all cells)

So my bottom line: These Bonai batteries appear to be very good. I especially appreciate the fact that they are labeled as 800 mAh capacity, instead of being labeled with an overly optimistic rating as is the practice of too battery manufacturers. Eneloop batteries are also rated at 800 mAh and test out at about the same capacity as these Bonai cells. The self-discharge rate of these Bonai remains to be seen, but appears to be at least adequate. I'm happy so far.

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