2 NiMH AAs, 1 lithium primary, together?

[Paul_in_Maryland]

I know, I know, we're not supposed to mix cells of different chemistries. The thing is, I would like to standardize on 3AA bodies (Javelin, FiveMega), and the 3.6V delivered by three 1.2V NiMH Eneloops is just a bit low to keep my modules regulated. If I could replace one cell with an Energizer L91 lithium (1.7V), the module would stay regulated till the two Eneloops were depleted. And it would cost me only one-third as much to run as three lithiums.

What's the downside? What's the risk? What precautions would I have to take?

 

[ltiu]

The L91 will drop down to 1.5v (under load) after a few seconds of use. It won't stay at 1.7v.

 

[Niconical]

I really think that is a bad idea.
I don't know enough about the physics or the chemistry to prove why, I just know mixing battery types is never good, ever.

It seems you're looking for a long term solution, so how about the ni-zn batteries?
They are rechargeable but with a higher voltage than nimh.

"They have a capacity of about 1400 mAh and can maintain a discharge voltage declining from 1.7 V to 1.5 V for most of the discharge at a 1C rate."

You'd need new batteries and a new charger, but whatever that costs is surely better than mixing battery types.

Click here for more info.

 

[Paul_in_Maryland]

Thanks; I've read up extensively on nickel-zinc AA cells. Their voltage under load is attractively high. But I'd rather wait till the next generation of charger appears. With today's NiZn charger, you're advised can't leave the cells in place overnight. Also, they lose a charge faster than LSD NiMH cells.

Unfortunately, telling me "it's a bad idea" adds nothing to what I had already heard. Specifics, please. Surely some combinations--say, NiMH and NiCd--are less dangerous than others--say, 1.2V NiMH and 3.7V Lithium-ion.

 

[rmteo]

Go for it. Let us know how it works out. :whistle:

 

[Niconical]

Well, based on my limited knowledge, this is what I would see as the most obvious problem. I'd assume that the 2 x nimh would run out of power before the lithium. If that were the case I don't suppose it would be a big deal, you'd easily see that the output of the light was way down, and you would then (presumably) replace the lithium and charge the 2 x nimh. However, and this where I think it might get ugly. What if you make a mistake, like for example putting back the same partially used lithium with 2 new nimh, or if you just have a lithium that you think is new and fresh but has self discharged for whatever reason? You could get to a point where the 2 x nimh start to charge the lithium, or at least attempt to.

If that did happen you still might be fine as by then the output would be low enough to be obvious (based on it working on just 2 x nimh at that point), but what if you don't notice, what if you leave it on, what if.... whatever. Like I said, I'm no expert but I just can't see how mixing battery chemistries can be anything other than "unadvisable" at the very least. If Silverfox ever writes down his years of knowledge and gets published (I'd buy that book! :paypal:), I'm sure that chapter one, page one will be "don't mix battery chemistry". What would be on page two, his explanation, well, maybe he'll see this and chime in :)

 

[Paul_in_Maryland]

Thanks, Niconical. That's what I was hoping [not] to hear. :candle:

 

[Bullzeyebill]

Paul, just curious, but what is the lowest voltage you need to run your modules, and which modules are they.

Bill

 

[Paul_in_Maryland]

Re: voltages of my two drop-ins

Off the top of my head:

• 1 to 5.5V for the Malkoff M30WF. Which makes me fear that I've damaged one or more of my Eneloops, because recently I walked to the bus as the Malkoff became as dim as a Mag Solitaire. Is it possible that two cells had gone totally depleted, leaving one cell to struggle along at 1V? I thought the Malkoffs were regulated; where does it say so on the linked page?

• I don't know the bottom limit for my direct-drive NailBender MC-E warm. Anyone?

 

[Elliot]

I think it might make a difference where in the 3 cell + LED series you place the Energizer. Last might be the best place. Anyway, I sure would try it out in your flashlight.:poke:

 

[MorePower]

What if you make a mistake, like for example putting back the same partially used lithium with 2 new nimh, or if you just have a lithium that you think is new and fresh but has self discharged for whatever reason? You could get to a point where the 2 x nimh start to charge the lithium, or at least attempt to.

In this case the lithium cell wouldn't get charged. It could be driven into an overdischarge condition, but that is not as dangerous as charging a primary lithium cell.

 

[Lynx_Arc]

I know, I know, we're not supposed to mix cells of different chemistries. The thing is, I would like to standardize on 3AA bodies (Javelin, FiveMega), and the 3.6V delivered by three 1.2V NiMH Eneloops is just a bit low to keep my modules regulated. If I could replace one cell with an Energizer L91 lithium (1.7V), the module would stay regulated till the two Eneloops were depleted. And it would cost me only one-third as much to run as three lithiums.

What's the downside? What's the risk? What precautions would I have to take?

how did you come up with 1.2v on nimh? everyone keeps saying that because they look at the labels and don't measure them while in fact most of the operating range is above that voltage typically most of the capacity of a nimh is used up before it hits 1.2v.

 

[ltiu]

how did you come up with 1.2v on nimh? everyone keeps saying that because they look at the labels and don't measure them while in fact most of the operating range is above that voltage typically most of the capacity of a nimh is used up before it hits 1.2v.

Then why the fxck are they labelled 1.2v ???

 

[Paul_in_Maryland]

Go for it. Let us know how it works out. :whistle:

I think it might make a difference where in the 3 cell + LED series you place the Energizer. Last might be the best place. Anyway, I sure would try it out in your flashlight.:poke:

You guys are too funny! :crackup:

 

[Bullzeyebill]

1.2 volts nominal voltage, just like Li Ion can be 3.6, or 3.7 volts, nominal voltage, and obviously can be charged higher.

Malkoff M30 is regulated, but it is voltage regulated, and can really draw voltage down and still put out good light. Need to really watch it. It also does quite well with two NiMh AA's, putting out about 130 lumens, or so. Use a lightmeter with bounce to check output and runtime, comparing output with some of your other lights.

Bill

 

[Lynx_Arc]

Then why the fxck are they labelled 1.2v ???

most likely because there was a time nicads and nimh batteries were used mainly for heavy loads and under those heavy loads the voltage was closer to 1.2v, but under loads that common electronics and led lights use the voltage doesn't drop that low. A load that would sink a decent nimh AA to 1.2v would also sink an alkaline AA to even lower levels that had the same capacity left in it.