Best charger for NiZn?

For general discussion of the UltraSmartCharger
B-2Admirer
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Best charger for NiZn?

Post by B-2Admirer » Fri Aug 14, 2015 8:51 am

Hello.

I really, perhaps desperately, need a reliable NiZn charger, the sooner the better (because I already have AAA NiZn cells and I already have an urgent use for them, but I haven't got anything worthy to charge them with). Unfortunately UltraSmartCharger seems to be little known, so I wasted a small fortune (by my standards) on several supposedly advanced chargers I thought I could try to adapt to charging NiZn before I was lucky enough to find a link to yours on candlepowerforums.

So my question is which of the current/upcoming chargers is the best one for my task and why? Do any of them charge/discharge with constant (not power-width modulated) current?

Thanks!

Mark
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Re: Best charger for NiZn?

Post by Mark » Fri Aug 14, 2015 8:59 am

There's only one UltraSmartCharger version at the moment - there have been earlier versions, but there is only one design that Paul is making at the moment. The current and last few versions all support charging NiZn equally well.

I believe that Paul still has a backlog of Kick Starter backers chargers to send out, so I'm not sure when he will be able to send out any new orders - it's probably best to check with him directly.

Just keep in mind that NiZn in general are nowhere near as robust at NiMH cells, so they do tend to fail relatively quickly even with a good charger.

All UltraSmartChargers use PWM (pulse width modulation) for both charging and discharging. Is there are reason why you're wanting something that doesn't use PWM?

B-2Admirer
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Re: Best charger for NiZn?

Post by B-2Admirer » Fri Aug 14, 2015 9:06 am

What's with the LCD model and the four-cell model, are they discontinued?

The recommended charging profile for NiZn is constant current / constant voltage, so I don't like the idea of PWM. What is the current model's peak value for charge/discharge and was it the same for the earlier models (I'd rather have one with a lower peak value)?

pfeerick
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Re: Best charger for NiZn?

Post by pfeerick » Fri Aug 14, 2015 10:13 am

I believe that the LCD model is shelved for now due to too many issues such as faulty LCDs, intermittent failures after extended use, etc. The 4 battery model is still planned AFAIK, but I suspect won't be happening for a while until the Kickstarter backlog for the 2 battery units is dealt with.

PWM shouldn't be an issue for these batteries. It is used to generate the constant current / constant voltage charge curve that they need. Most, if not nearly all chargers use some form of PWM, with capacitors to smooth it out so that the battery doesn't know any different!

Just remember as Mark said that NiZN is a temperamental chemistry. It is good while they work, but they go from usable to dead at the drop of a hat (think all green bars on your camera one picture, and then none, red bar... er... scratch that... camera powering off! the next), and will pack up after a while even when looked after.

B-2Admirer
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Re: Best charger for NiZn?

Post by B-2Admirer » Fri Aug 14, 2015 10:38 am

Would you mind explaining how you suppose PWM could be used to "generate constant voltage"? Because that's what you said.

In any case, I was originally asking developers.

Mark
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Re: Best charger for NiZn?

Post by Mark » Fri Aug 14, 2015 11:34 am

B-2Admirer wrote:What's with the LCD model and the four-cell model, are they discontinued?
As already answered by pfeerick, there were a number of issues with the LCD model and the four cell version is still some way off - there is still a lot of work before we even get to a prototype of the 4 cell version, so I wouldn't go holding your breath while you wait for it.
The recommended charging profile for NiZn is constant current / constant voltage, so I don't like the idea of PWM. What is the current model's peak value for charge/discharge and was it the same for the earlier models (I'd rather have one with a lower peak value)?
The PWM used by the UltraSmartCharger is at a fairly high frequency - I don't believe that it causes a problem for NiZn cells - despite many people reporting getting only around 10 cycles on their NiZn cells, I've done cycle testing on NiZn cells with this charger and obtained around 300 cycles out of them:
viewtopic.php?f=5&t=85

With NiZn, I think the key for longevity is to not over discharge them.

B-2Admirer
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Re: Best charger for NiZn?

Post by B-2Admirer » Fri Aug 14, 2015 11:37 am

What is the PWM frequency and the peak value?

Mark
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Re: Best charger for NiZn?

Post by Mark » Fri Aug 14, 2015 11:40 am

On the current charger, the frequency is about 31 kHz.

You would need an oscilloscope to measure the voltages at that frequency and unfortunately, I don't own one myself.

pfeerick
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Re: Best charger for NiZn?

Post by pfeerick » Fri Aug 14, 2015 11:42 am

Depends on how much you know about electronics. In a nutshell, varying the duty cycle of a PWM signal varies the output voltage. i.e. a if you have a mosfet controlling the output from a 5v power source, and you run the PWM control signal at 50%, you get approximately 2.5v at the output once you smooth it out with capacitors. If you run it at 75%, you get 3.75v. 25% gets you 1.25v. 0% gets you 0v (off). PWM signals are usually converted to analog voltages using RC low-pass filters. When you have some form of current monitoring circuitry (using hall effect current sensors, shunts / current sense resistors, etc), you can then start altering the duty cycle to provide a constant current.

With constant current -> constant voltage charging (which is the case for most of the common battery chemistries, lithum, nicad, nimh, nizn, even lead acid), you charge at the constant current, and then once you've reached the peak charge voltage, start ramping the current down, to hold the voltage stable, until the current being pushed into the battery drops to whatever level has been determined as the minimum. Nimh batteries have the added complexity of delta-peak detection, but that is another issue entirely.

The info round for nizn batteries is quite contradictory and confusing. I purchased a PowerGenix charger and batteries, and some 3rd party cells also. two of the PowerGenix batteries have packed up after only a few cycles, and so have some of the third party cells. However, a couple are still going strong, and continue to run my cameras without any issues. Interestingly, PowerGenix advertise the chemistry as being low self discharge, but in their data sheet say to charge the cells every 30 days regardless of if used or not! and also their charger has been tested and proven to not charge the batteries in the manner which they say the batteries should be charged, so make what you will of that.

Mark
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Re: Best charger for NiZn?

Post by Mark » Fri Aug 14, 2015 11:49 am

Just to add on to what pfeerick has said, the charging circuit on the USC makes use of a buck converter which means that the current going into the cell won't look like a typical square wave from a PWM output - instead it will be fairly close to the average with a triangular ripple on it.

Other than actually measuring the current/voltage with an oscilloscope, it should be possible to calculate the ripple by plugging the known parameters into a formula - if I recall correctly, there are web pages that do just that.

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