Review: Nickel-Zinc (Ni-Zn) Rechargeable Batteries and Charger

The Ni-MH rechargeable battery is the most common AA/AAA rechargeable battery chemistry. It took over as the superior technology both environmentally and technically compared to Ni-Cd. These cells provide a nominal 1.2v, but have improved over time to offer larger capacities and lower self discharge. In fact, when it comes to AA/AAA’s, my favourite has been the Sanyo Eneloops because of their extremely reliable and consistent performance (soon to be called Panasonic Eneloop following the acquisition of Sanyo’s battery division by Panasonic). The price is also reasonable, and their low self discharge rates are amazing. Having suffered through Energizer 2500mAh cells which were useless after about seven days rest, the Eneloop really turned the tables on the Ni-MH technology and the majority of the Ni-MH cells on the shelves tend to be low-self discharge versions in various names (e.g. Varta Ready2Use).

But aside from Ni-MH and Ni-Cd, there was another battery chemistry which was struggling for some attention – the Ni-Zn battery. The main claim of the Ni-Zn battery was a higher output voltage, often stated on batteries as 1.6v or 1.65v, which better matched the 1.5v of alkaline cells and would prolong runtime in voltage-sensitive electronics.

It is the same marketing strategy as used by Rechargeable Alkaline Manganese (RAM, Grandcell) batteries which boasted an “exact” 1.5v, although those suffered from very short cycle life (and I was convinced that some of them were merely repackaged disposable alkalines). Over time, they also grew higher in internal resistance making them unusuable in heavy draw devices, such as early digital cameras. Several of mine had leaked after about 15 or so cycles, by which point they were useless. None of these batteries even bothered advertising their capacity. Despite winning several technological accolades at the time, they fell out of favour quickly.

The Ni-Zn, by being much more similar to the Ni-Cd in nature, was supposed to provide the benefits of both worlds. As it turns out, I decided to give them a go almost a year ago, and here are my findings.

The Batteries

For my Ni-Zn adventure, I had to turn to eBay. As it turns out, no local retailers stock Ni-Zn, but there was a plentiful supply on eBay. For the AA’s, I opted for the better looking “branded” cells. These were the Powergenix 1.6v 2500mWh cells, Made in China.

Despite Powergenix introducing it to the market in 2009, they exited the Ni-Zn consumer battery market merely a couple of years afterward. They continue to target their technology at hybrid vehicles, but I haven’t heard any interest at all. Regardless, apparently there was a lot of excess stock, or someone else decided to manufacture under the same name, which is how I ended up with these.

For the AAA’s, I went with the “unbranded” option, which was almost certainly a Chinese generic manufacturer. It’s interesting to see them actually bothering to manufacture these types of cells given many have not heard of it. This was rated at 1150mWh at 1.6v.

The battery manufacturers plaster the cell capacity ratings in mWh as they claim that it’s unfair to label it in mAh. The reason is due to the higher cell nominal voltage, and mWh represents the cell’s energy content, rather than the cell’s current capacity (which needs to be multiplied by the nominal voltage to get the energy content).

However, it is trivial to convert it. Taking the voltage and energy given, we can do the conversions to compare them with the “regular” eneloop cells of today:

             mAh     mWh
AA Ni-Zn    1563    2500
AA Ni-MH    2000    2400
AAA Ni-Zn    719    1150
AAA Ni-MH    800     960

Keeping in mind that we’re not comparing with the largest capacity Ni-MH cells, the Ni-Zn batteries seem to be competitive on energy content – in both cases being rated superior.

The Charger

The charger supplied was a bundled charger. It is quite cheaply constructed, and features a single bi-colour LED for charge level indication.

Unfortunately, I had opened it up before, so some of the data is missing on the rear label. The model number is HG-1206W, and it seems like there are multiple versions of this charger suitable for different chemistries.

Internally, the charger is constructed on a single piece of PCB, single sided. It seems there is a little attention paid to isolation as you can see from the milled groove in the PCB separating the high voltage from the low voltage. Many positions on the PCB are not populated – it seems that these are options which are used for different chemistries or DC power input charging.

Interestingly, there seems to be a current limiting polyfuse or similar at the bottom, and an LED which remains inside the case. I have a hunch that this LED provides the voltage reference for the charger, as the Ni-Zn love to be charged up to about 1.9v, and that’s coincidentally quite similar to the voltage drop across a red LED. How about that!

Nothing flashy about the components they used either, but the silkscreening seems to be rather non-conventional, and some holes aren’t even drilled!

The underside of the PCB has solder resist and a smattering of surface mount components which seem rather unusual for a low-cost charger.

A few more pictures of the PCB from different angles.

Lets just say, this charger gets quite hot in operation. The transformer on it is probably quite poorly wound or set as it buzzes quite audibly during charging. The circuit itself has really only two modes – charging or stopped. There’s a little hysteresis between the two, and so as the battery approaches full charge, the charger “alternates” between charging and stopped causing an irritating “bzzt-…-bzzt-…-bzzt-…” sort of noise.

Performance

I think the clearest indication of now useful Ni-Zn cells are is the fact that, after just under a year, all of my cells have hit the bin for one reason or another. Some of the blue AAA Ni-Zn decided to swell slightly on the negative pole, but all of them developed extremely high internal resistance and extremely high rates of self discharge. They would become pretty uselessly flat after only five days of rest, and wouldn’t handle any significant draw. This was only after 3-5 cycles.

Their internal resistance got so bad that charging took excessively long – and the phase where the charger alternates between charge-and-stopped continued for many hours. Some of the cells never lit the charger green, which implies they may have developed dendrites and internal shorts (which was, incidentally, a shortcoming of the Ni-Cd days).

The AA’s fared slightly better, but not much so. Barely 15 cycles in, all of them had such high internal resistance that the runtimes got quite low. Much lower than the equivalent Ni-MHs.

Did I see any increased run-times? Generally, I’d have to go with no. Even fresh out of the wrapping, most of my devices using AA’s and AAA’s are relatively newer devices and will work quite happily all the way down to 1V/cell (the “end point” for Ni-MH – alkaline cells are often rated down to 0.9V/cell). The argument that you need more voltage seems a bit of a fluff to me – the reason is that internal resistance comes into play as well.

The reason why many high draw digital cameras worked better on Ni-MH compared to Alkaline is the same reason why Ni-Zn just isn’t that useful. In high draw devices, while the Ni-MH cells provide a lower initial voltage, their lower internal resistance means that at heavy loads, the cell voltage drops less! They can maintain their ~1.2v even under heavy multi-ampere loads. Alkaline cells cannot do this, and neither does it seem Ni-Zn can. This means that their cell voltages may fall below 1v under heavy load even before they are empty.

For any appliance to be economical with batteries, they (as a rule) must be designed to work down to about 0.9-1V/cell. Anything higher and you will not see full utilization of even 1.5v dry cells (which they would be designed for, at the least, with most modern appliances designed for Ni-MH use as well).

Another reason for shorter run times comes from basic dumb appliances such as unregulated old flashlights and heating element based devices. These will see the higher voltage and consume more energy as a result – and also shorten their lifetimes.

The voltage of the Ni-Zn battery is misleading – most of them have 1.6v written on them, but they need about 1.9v to charge completely, and about 1.85v off the charger. Their higher voltage can and does cause problems for appliances especially when you have three or more in series. I managed to get smoke coming out of a few appliances and permanent damage with four in series – the excess voltage was too much. Use only two to three in series for best results.

The other thing is the voltage discharge curve drops sharply when the battery approaches about 1.5v. This means that devices with low-battery indication or reliance on cell voltage indications to fail miserably. This can mean that they fail to shut down gracefully or signal impending battery doom.

The best application I found for my Ni-Zn batteries (until they failed) was in my LED flashlights. Both regulated and non-regulated ones managed to stay in the bright mode for a little longer. The regulated ones I use might be a bit aggressive in their voltage-sensing regulators, and switch into a dim mode before completely discharging the Ni-MH cells. The unregulated ones dissipate more heat and energy due to the higher voltage, giving a better brightness but possibly at the cost of LED lifetime.

Aside from that, I saw no other great uses for them. My digital camera hated them – their internal resistance screws with the flash recharge and the power-downs prior to low-battery-warning left the lens in need of juice to retract itself. A battery operated glue gun and a flash unit which used four cells both managed to smoke to death.

Conclusion

While by no means a scientific and methodical exploration into the performance of Ni-Zn, I think it’s clear why they failed to gain favour. Their performance degraded quickly, and they suffer from the same bugbears as early Ni-MH cells – self discharge. Further to this, their energy density doesn’t match modern Ni-MH cells either, meaning less run time for devices which don’t care too much about voltage. You have to watch out for devices which aren’t aware of the voltage curve of the Ni-Zn cells, as they won’t have enough time to gracefully shut-down on low-battery conditions.

Worse still, if you’re planning to use them in devices with several cells in series, the added excess voltage may cause problems – overheating, to appliance damage. I’d keep it limited to two or three cells. The voltage rating is also a bit of a lie – they probably “chose” 1.6v and 1.65v to look closer to 1.5v, although the batteries often sit near 1.85v open circuit after charging and need about 1.9v to charge properly. The electrochemical potential is 1.73v.

The very much touted increased run-time proved to be very much a fallacy in my experience, and the eneloops win out virtually every time.

All in all, it wasn’t an expensive exercise, but it was one which was very much a disappointment. Try Ni-Zn at your own risk.

For more battery related goodness, there is an excellent comparison of chemistries at Michael Bluejay’s Battery Guide. There is also careful methodical testing of rechargeable batteries by Michael “The Doc” Hains.