Mixing chemistries?

[core]

My direct drive device works just so-so with 3 NiMH AA's hot off the charger, and is pretty much unacceptable after only 30% of the battery capacity is used up. I end up recharging my cells religiously every day (by choice, not because they are drained much at all) and even then I'm not happy with the output. What I need is just a wee bit more voltage.

Thanks to this forum I have learned of these NiZn cells and have several of those on the way. I'm fairly certain that using 3 of these NiZn cells will blow my filament, even if I only charge them up halfway. So I'm thinking about swapping just one of the NiMH out for a NiZn. And errr... not with my Eneloops. I have some Energizer 2500's just begging for this kind of application. :devil:

I realize this is a "bad idea", but can someone objectively tell me why? The two remaining NiMH will not go into cell reversal here because they won't get drained halfway -- not even close. I'll continue to top them off daily. And the switch is momentary so no chance of it being left on. I suppose there is always the chance of a switch failing in the closed position and then I am at the very least left with destroyed cells.

Or maybe worse? And that's kind of what I'm asking: What kind of fireworks can I expect in the unlikely event of a failure?

 

[spencer]

There was a thread awhile ago about someone using Li-Ion and a NiMH. I believe it should be fairly safe as long as none of the cells are depleted all the way. Look into it though.

 

[recDNA]

There was a thread awhile ago about someone using Li-Ion and a NiMH. I believe it should be fairly safe as long as none of the cells are depleted all the way. Look into it though.

WOW. I thought that was like .... I don't know...nuclear fission X 5. LOL

Kidding aside I thought mixing chemistries guaranteed

I'll have to find that thread.

 

[core]

Yes, I did locate a few threads, including one from February of this year. Mainly just a slew of "don't do it" responses, which I completely understand. And a post or two saying maybe it would be ok. It did "seem" that things would be ok as long as no cells got discharged.

Well, barring no "you're a fool for even trying" posts, my first test will start tomorrow so I guess I'll follow-up with some sort of update after a week or two of use.

 

[KiwiMark]

Kidding aside I thought mixing chemistries guaranteed

I would say that is not true at all. But I would also say that caution is essential - best to take great care to avoid any chance of cell reversal. What happens when you turn on a light with different chemistry cells depends on the combined voltage and combined resistance of all cells in use - generally nothing bad would happen but cell reversal will quickly damage cells, so you need to avoid that happening.

The biggest danger is with primary lithium cells (like CR123a cells) where cell reversal can definitely lead to 'vent with flame' situations. I would not want to mix and match at all with those cells.

 

[csa]

Yeah... be careful. Mostly we advise not to mix cells because it makes it much easier to make a mistake, no matter how careful we normally are.

 

[Linger]

my first test will start tomorrow

Be safe, I hope you're starting on the presumption that the cells will react violently and are thus prepared for it. If you expect an explosion and it doesn't come, it's nice.
And you're also aware that cell degradation is (largely) not visible, nor will faults necessarily manifest themselves immediately. So the internal faults (crystalline formations, gas pockets) may only reveal themselves after the tests, like next month when you loan a headlamp to your 3 year old nephew. (just random example)

 

[core]

So the internal faults (crystalline formations, gas pockets) may only reveal themselves after the tests

I'm assuming/hoping that if anything bad happens I'll at least see it as reduced capacity when I run the cells through the analyzer later. I think I'll be safe as long as I continue to monitor the states of each cell extremely closely. No problems so far, not that I expected it to blow up on the first use.

You know though, all this time I was under the impression that the NiMH cells would be the ones to get reversed if anything. But today I came across a post from the infamous 'NLee the Engineer' saying that the higher voltage (NiZn) cells are going to be the ones taking the hit, not the NiMH. Hrmmm.

Unfortunately NiZn is new enough to me that I probably wouldn't know a good cell from a bad one at this point.

 

[TakeTheActive]

[Mixing Two Chemistries in One Device] 2 NiMh + 1 NiZn = ???

My direct drive device works just so-so with 3 NiMH AA's hot off the charger, and is pretty much unacceptable after only 30% of the battery capacity is used up... ...What I need is just a wee bit more voltage.

Thanks to this forum I have learned of these NiZn cells and have several of those on the way. I'm fairly certain that using 3 of these NiZn cells will blow my filament, even if I only charge them up halfway. So I'm thinking about swapping just one of the NiMH out for a NiZn...

..What kind of fireworks can I expect in the unlikely event of a failure?

I find it 'interesting' that none of the CPF Gurus have yet replied to your query. :thinking:
Possibly a better / More 'Catchy' Title: [Mixing Two Chemistries in One Device] 2 NiMh + 1 NiZn = ??? :candle:

(IMHO, *SO* MANY folks put *SO* LITTLE thought into their thread titles...)

Personally, with *NO* NiZn experience (or interest at this time ), I'd say 'Experiment'. You know the individual CAPACITY of both the NiMh and NiZn cells.

1. What's the:
   • Current draw of your 'Direct Drive Device'?
   • CAPACITY of your NiMh cells?
   • CAPACITY of your NiZn cells?
2. Run the LOWEST CAPACITY cell(s) down 25% (timed)
   • Check *ALL* the cells
   • Any near FULL DEPLETION?
     - No?
3. Run the LOWEST CAPACITY cell(s) down 50% (timed)
   • Check *ALL* the cells
   • Any near FULL DEPLETION?
     - No?
4. Run the LOWEST CAPACITY cell(s) down 75% (timed)
   • Check *ALL* the cells
   • Any near FULL DEPLETION?
     - No?
5. Run the LOWEST CAPACITY cell(s) down 90% (timed)
   • Check *ALL* the cells
   • Any near FULL DEPLETION?
     - No?
6. Yada, Yada, Yada...

From my 'non-experienced' / 'non-technical' point-of-view, if you don't run *ANY* cell past it's point of FULL DEPLETION, how will it know that the 'other cell(s)' aren't of the same chemisty?

 

[VidPro]

ya slam them together then take a look at how they discharge like TTA is saying.

my question would be WHat kind of DD device? a incan bulb will certannly wreck things, but if you calculate the cut-off on a DD white, it might not hurt a batt from reverse charge, because of an automagic cutoff.

if you toss a 2000, with a Zinc , and a 2500 (worst case)
you get something like this for capacity

Capacity
-{1950]-{1300]-{2300]-
-{0700]-{0000]-{1000]- approx end of use capacities.
With the zinc being the first victom, assuming the 2500 didnt self discharge before the zinc

Voltages for cutoff if they ended the same which they wont.
-{.09]-{1.2]-{.9]- 3.0v White led still running ok. If you had a way to do a added cut-off it would be around there.

Likely voltages at White LED way way slow down
-{1.2]-{.0]-{1.2]- 2.4v <--- see here white led be going really slow, gates barely triggering, Zinc below specs already.

now put in 2 zinc and only 1 ni-mhy
-{1.4]-{.0]-{1.2]- 2.6v <-- led still going some.

sooo, basically the LED will just barely be pulling anything by then. so with 3X and DD of White LED, the poor zink wont be torn up by reverse, even though it will be dropped lower than it wants to be.

will everything go to heck? No it wont be terrible As Long As it is DD and a white led that doesnt do much of anything at the voltages where the batteries are way low.
you certannly wouldnt SELL anything like that, leave it running when its run down, or hand it to someone who doesnt know how crasy it is. there is some assuption that you already know how wrong it is, and would stop using it when it goes way down, both today and years from now, when you forget
Differance being: no Hard pushing driver, or motor , or Bulb or something that wont cool out or slow down or stop.
If you DD is NOT a White LED, or UV LED , or HAS some driver or is an incan or something, then it would be a bloody mess.

It still "Just aint Right", i am just pointing out it does have a slowdown/cutoff this time.