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(More customer reviews)The product description of PowerGenix Nickel-Zinc Rechargeable AA cell promises many advantages over other rechargeable batteries: higher voltage, higher energy density, lighter weight, longer cycle life, and so on. But how does it stack up against the current gold-standard of low-self-discharge NiMH cell, the Sanyo Eneloop? Let's examine the following aspects:
[Operating Voltage]
The NiZn cell has a nominal voltage of 1.65V, which is 30% higher than that of a NiMH cell (1.25V nominal). When freshly charged, its terminal voltage is even higher at 1.85V!
[Power Density]
Since 'Power' is defined as 'Voltage * Current', most people may assume that "30% higher Voltage" translates to "30% higher Power". But it depends on the application:
- Most electronic gadgets (digital camera, MP3 player, etc) contain internal DC/DC regulators which operate in constant-power mode. If the battery voltage is higher, then input current will drop to maintain the same output power. That's why higher voltage alone does not make your digital camera shoot any faster.
(One exception is photo flash unit, which operates in constant-current mode. So in this case, 30% higher voltage does translate to 30% higher power and shorter cycle time)
- For an unregulated appliance such as a flashlight or power tool, its current increases with voltage (although not linearly). A typical 2-AA flashlight bulb is rated for 2.3V * 0.5A = 1.15W. When you apply 3.6V to it, the current may increase to 0.65A, so the power consumption is now 3.6V * 0.65A = 2.3W. Twice the power means twice the heat. That's why the bulb may burn out in seconds.
[Energy Density]
The PowerGenix AA cells are marketed as "2500 milli-WATT-hour" (energy capacity). Most people may confuse this with "2500 milli-Ampere-hour" (charge capacity). But the data sheet for NiZn AA cell shows that its current capacity is only 1500mAh (this is verified by my own testing). It turns out that an 1500mAh NiZn cell actually contains the same amount of energy as a 2000mAh SANYO eneloop AA cell:
- Energy in eneloop AA cell: 1.25V * 2000mAh = 2500mWh
- Energy in PowerGenix AA cell: 1.65V * 1500mAh = 2475mWh
[Weight]
The weight of each NiZn AA cell (25g) is essentially the same as that for eneloop AA cell (27g). For certain applications, it is possible to use three NiZn cells (3*1.65=5V) to replace four NiMH cells (4*1.25V=5V). Doing so reduces battery weight by 25%, but it also shortens run time by 25%.
[Cycle Life]
Technical data found on PowerGenix web site says NiZn cells are rated for 200 cycles (at 100% deep-discharge). This is much shorter than the 1000 cycles cited for eneloop cells.
I have tested a set of 4 PowerGenix AA cells. After subjecting them through 12-16 deep discharge cycles, the average charge capacity already dropped 5% from original value. In comparison, I have a set of 4 eneloop AAA cells that have went through about 100 cycles in the past 2.5 years, and they still exhibit over 95% of original capacity.
[Self-Discharge Rate]
In my long-term storage test after two months, a set of four NiZn cells retained 74% original charge capacity on average. This translates to about 13% loss per month, which is much better than that of ordinary high-capacity NiMH batteries (with capacity > 2500mAh). But it cannot compare to low-self-discharge NiMH cells.
Previously, I have conducted storage test for several brands of LSD cells, including Sanyo eneloop, Rayovac Hybrid, and Kodak Pre-Charged. All those cells can retain 85-90% of original capacity after FIVE months in storage.
[Recharging Time]
The PowerGenix NiZn cells must be recharged using a specially designed PowerGenix NiZn Charger. The charger is called '1-hour', but it actually takes 2.5 hours to fully recharge a set of four 1500mAh cells.
Eneloop (and all other brands of LSD cells) can be recharged using any good quality 'smart' NiMH charger. Depending on which charger you use, the charge time can be as short as 15 minutes, or as long as 8 hours.
[BOTTOM LINE]
The only verifiable benefit for PowerGenix NiZn cell is its higher operation voltage. However, even this may turn out to be a curse rather than a blessing, as higher voltage can cause unregulated appliances to burn out.
If you have a digital camera that works well with eneloop, stay with it. Switching to NiZn will not offer more power, nor longer run time. Only if your camera does not work well with eneloop, then it is worthwhile to try NiZn cells as a last resort. But you better contact PowerGenix, and ask whether they will assume warranty liability for the use in your camera. Otherwise, try it at your own risk![Update on Nov 21, 2010]
My first set of four NiZn cells was used for capacity testing and long-term self-discharge testing. They have gone though maybe 20-30 deep discharge/recharge cycles (discharged down to 0.9V only, not 0V) over the past year. As of right now, two out of four cells have already failed, suffering from voltage depression and rapid self-discharge problems. The other two suffered from reduced capacity (~1200mAh, down from the original 1500mAh). The PowerGenix '1-hour' charger needs to detect 1.9V during charging, before it can change from constant-current mode to constant-voltage mode. When voltage-depression hits a cell, its voltage cannot reach 1.9V during charging, so the NiZn charger will simply fry the cell!
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