Best charger for NiZn?

For general discussion of the UltraSmartCharger
B-2Admirer
Posts: 9
Joined: Fri Aug 14, 2015 5:27 am

Re: Best charger for NiZn?

Post by B-2Admirer » Fri Aug 14, 2015 11:58 am

pfeerick wrote: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.
Oh yes, I know enough about electronics to understand that, but how do you know that UltraSmartCharger generates constant voltage with PWM? And where did you get the information about NiMH and NiCd being charged according to CC/CV profile?
Mark wrote: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.
Can you tell just how close to the average (percentage of deviation)?

Also, how's constant voltage for the CV stage is supplied and more importantly is it really constant?

Mark
Site Admin
Posts: 415
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Re: Best charger for NiZn?

Post by Mark » Fri Aug 14, 2015 12:36 pm

Without running the numbers, I'd only be guessing as to the amount of ripple there is. Unfortunately, I'm getting really tired and don't feel like working it out before I head off to bed.

The constant voltage is actually handled by the controller continuously adjusting the current to maintain the correct voltage during the CV phase. From a quick look back on the most recent NiZn charge, I'd say that the charger was able to keep the voltage to within less than 1mV of the target 1.900V

In the end, I don't think that the way that we're charging NiZn cells with this charger is an issue - otherwise I wouldn't have got anywhere near 300 cycles during the previous test that I did. Also, keep in mind that the test that I ran was on an early prototype with a much older firmware version which ran the PWM at only about 8 kHz. Although that charger used an inductor with a larger value, it still would have had more ripple due to the lower PWM frequency.

B-2Admirer
Posts: 9
Joined: Fri Aug 14, 2015 5:27 am

Re: Best charger for NiZn?

Post by B-2Admirer » Fri Aug 14, 2015 12:44 pm

Well, you can give me the "ripple" answer tomorrow, I don't mind that.

Is charging termination current user-adjustable or is 75 mA (what I see on that graph) the only possible value for NiZn with UltraSmartCharger? Can CV supplied to the cell during the second stage be adjusted, too? I suppose a value slightly lower than 1.9V would be safer.

Mark
Site Admin
Posts: 415
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Re: Best charger for NiZn?

Post by Mark » Fri Aug 14, 2015 10:42 pm

Pretty much all settings are user adjustable with the UltraSmartCharger - you can adjust the termination current up or down as well as reducing the voltage during the constant voltage phase.

Reducing the end voltage and/or increasing the termination current is likely to be gentler on the cells at the expense of less available capacity. It wouldn't be hard to find out what difference this makes to the capacity, but working out what difference (if any) it makes to the cycle life would take a lot longer to test.

B-2Admirer
Posts: 9
Joined: Fri Aug 14, 2015 5:27 am

Re: Best charger for NiZn?

Post by B-2Admirer » Sat Aug 15, 2015 2:34 am

Have you seen this? Someone lays out theoretical basis to conclude that damage to a NiZn cell from overdischarge occurs at around 0.42 V, consequently anything above that is not harmful. No wonder you got their life shortened by setting discharge termination voltage to 0.3 V, but instead of that you should have probably tried 0.9 V or 0.5 V, any of these values makes much more sense if the goal is to determine the safe end-of-discharge voltage. As far as I understand, the 1.3 V value that you used is taken from the "low current" graph in the PowerGenix specs sheet for their AA cell, but the sheet doesn't state that discharging below that value is harmful. Furthermore, the specs sheet for their Sub C cell has just one graph plotted down to 1.1 V regardless of the discharge current, which suggests that discharge down to 1.1 V is generally safe for NiZn cells.

pfeerick
Posts: 12
Joined: Thu Feb 19, 2015 6:52 am

Re: Best charger for NiZn?

Post by pfeerick » Tue Aug 18, 2015 6:09 am

I agree with the assessment on that forum. Since the general consensus seems to be that NiMH is considered 'flat' at 0.9v, I don't see any reason that using the same rule of thumb on NiZN would overly harm them. Having said that, this datasheet (https://www1.elfa.se/data1/wwwroot/asse ... ng_tds.pdf) for AA sized cells from ANSMANN support the idea that NiZN is considered flat at 1.1v, as was indicated in the powergenix sub c datasheet you mentioned before. But, unlike that powergenix datasheet, this datasheet does indicate that the end voltage should change with the discharge current. Anyway, while 0.9v won't be ideal, it shouldn't take the batteries into that 'damaging' range (0.4-0.5v)

I've been trying to find the old powergenix datasheets, as they used to have PDF version of the manual for their 4xAA white charger, and also a datasheet for the AA cells, as well as a decent support page, but they seem to have deleted them, probably as a result of them no longer supporting them. I'm sure that charger (F100011-14 is the model number for mine) kills the batteries itself without any assistance from me or the devices used in - the batteries are quite hot the touch at the end of charge.

I have about around a dozen or more cells that when you pull them out to use after six or so months - are all at the 0.4-0.7v mark without being used - after being fully charged before being put away. For about a month earlier in the year, I checked two batches of four batteries that had had little or no use since purchasing every day or so, and charted their self-discharge rates, and out of that set, 3 were fine, 3 had dropped to 1.2 volts within a week but were stable after that, and 2 dropped to unusable within 2-3 weeks.

The reference to CC/CV charging for NiMH / nicd I made before may not have been quite correct for general purpose chargers. I was going off programmable hobby chargers that I use, which show the current and voltage parameters whilst charging, and they tend to drop the current once the battery gets to around 1.5v, in order keep current flowing without overcharging the batteries, still allow the -delta-V detection to work. The Opus BT-C3100 v2.1 lithium/nimh charger certainly does this - once the battery voltage gets to 1.48 volts, it backs the current off until the -delta-V detection triggers.

I just came across this page which compares some of the different battery chemistries (nimh, nizn, lithium) - you might find the comments on NiZn (starts about 1/4 down the page) interesting. Particularly the lack of information from Powergenix as to discharge end voltage, and some inconsistency as to discharge cycles - 500 as opposed to 200. In the end, it really depends on the application, as with all chemistries. I think NiZN is great for high drain use, such as camera flashes, etc. But for anything low drain, or where reliability is an issue - not a chance! I fear that poor quality control, and a lack of proper documentation / testing has doomed this very promising technology.

B-2Admirer
Posts: 9
Joined: Fri Aug 14, 2015 5:27 am

Re: Best charger for NiZn?

Post by B-2Admirer » Tue Aug 18, 2015 3:50 pm

Thanks for the link to the ANSMANN PDF, I couldn't find it on their website when I was looking for the information about their NiZn cells, the life expectance, self discharge and some other data make it very informative, more so than the PowerGenix datasheets.

PowerGenix datasheets are no longer available on their website, but the wayback machine still retains them: AA and Sub C. Likewise, it still retain the manual for their charger, but as far as I know it never was available as PDF, only as HTML.

You're correct about the PowerGenix 1-hour charger, it indeed has a tendency to kill cells, so it's definitely the one to avoid. As reported by several reviewers (and now you), it sometimes fails to get a cell to 1.9V to switch to the second stage and thus always charges with high current, killing the cell in the process. The NiZn charger I own is UltraCell Plus+ 4P80T-NiZn and it's not something particularly usable either. The four channels are not really independent, so for some reason the cell with the highest capacity receives much more charge than the rest of them, which remain undercharged (I was able to measure that by discharging in the OPUS BT-C3400), therefore to receive full (?) charge they must be charged individually. Even then the charge received by a cell depends on the slot it's charged in, for some reason slot 2 gives more charge than the others. Also the charge procedure it uses looks very much different from the recommended ones for NiZn, based on my measurements with an oscilloscope and an ampermeter, I can say it charges with gradually reducing pulse current (I don't mean PWM). But at least it does not heat them up, so hopefully doesn't damage them either.

Unfortunately there was no link to the page "which compares some of the different battery chemistries (nimh, nizn, lithium)" in your post. I'm not sure if the technology is doomed but I would agree that lack of documentation severely damaged the spread of it.

As for NiMH, I haven't noticed the behavior you described (charging with CV) on either of the advanced chargers I own, which are OPUS BT-C2000 V2.1, OPUS BT-C3400 V2.2 and SkyRC NC2500. The indicated current stayed constant until the end.

dino
Posts: 24
Joined: Tue Nov 04, 2014 12:54 pm

Re: Best charger for NiZn?

Post by dino » Tue Sep 08, 2015 8:36 pm

Does the USC distinguish between NiCd battery and NiZn battery automatically, or does the user have to tell the charger that the inserted battery is a NiZn chemistry?

Mark
Site Admin
Posts: 415
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Re: Best charger for NiZn?

Post by Mark » Tue Sep 08, 2015 10:14 pm

Yes, the charger will automatically detect NiZn cells when it's charging them. There are some thresholds that can be adjusted by the user to change the point at which they're detected.

dino
Posts: 24
Joined: Tue Nov 04, 2014 12:54 pm

Re: Best charger for NiZn?

Post by dino » Tue Sep 08, 2015 10:34 pm

Mark wrote:when it's charging them.
wow that sounds interesting. i wouldn't know how to implement that *gg*
looks like i need to read more literature on battery charging and chemistries :oops:

Post Reply