Can someone explain this?

[apagogeas]

Ok, I got a UV light insect killer powered by 4xD cells. It does the job, however the D alkalines last for 2 days only, so I naturally want to look into using rechargeables. I don't want to invest on D NiMH (I don't have a D cell charger either) so I got some cheap 2xAA -> D holders to do the job. Each holder accepts 2 AA cells in parallel and last night I put this thing to work. It did deliver for 4-5 hours, then the electronics of the device turned off due to lower voltage (it turns off around 1.24V or so), so I really don't get the maximum capacity out of AAs but this may also be a good thing, avoiding cell reversal.

What troubles me here is the actual drain occured in each holder. I didn't wanted to buy new cells of the same brand/capacity, so I popped in some spare cells I had, 4xEnergizer 2450mAh (non LSD which currently deliver around 2Ah) and 4xTronic LSD 2100mAh (which actually deliver around 1900mAh). Each holder has one Energizer and one Tronic. All cells were fully charged yesterday, put the device in operation, after 5 hours it turned off due to lower voltage and today I put them in charge and I observe something strange. The energizers accepted around 1200-1500mAh and the Tronic ones around 800-1000mAh. None of these has been fully drained obviously by the device but I can't explain that huge difference in actual drain between the brands within each holder. Any ideas?

 

[45/70]

Hi apagogeas. Interesting results. From your "testing" (although that's not really what you intended to do) I'd say your Energizer cells have a lower internal resistance than the Tronics. By paralleling the cells, you ensured that the voltage level of the two different cells would remain the same, but this will not keep the cell with a lower IR from supplying more current than the cell with a higher IR, and that's just what happened.

This is a problem I see with paralleling cells of different make, model, age etc. While it helps to ensure that the cells will discharge to the same voltage level, it does not ensure that the cells will be evenly discharged. I've never tried using mismatched cells in parallel for this reason, although I guess there's really nothing wrong with it. The cells simply discharge unevenly (capacity wise) and one works harder than the other. Nonetheless, it will work.

Dave

 

[apagogeas]

Probably you are right on this, it is similar to placing in parallel two resistances, a smaller and a bigger one therefore most current flows though the smaller resistance. I have another question regarding this setup. If I put a fully charged cell and an empty cell in the holder, will the charged one put charge in the empty one till both equalize their voltage? What I mostly worry about here is the case of accidentally putting a charged and an empty cell, I might have a burst of current flowing from one to another. Will this happen or do NiMH actually needs quite higher voltage applied for charging to take place?

 

[45/70]

......I have another question regarding this setup. If I put a fully charged cell and an empty cell in the holder, will the charged one put charge in the empty one till both equalize their voltage?

Yes, the charged cell would charge the discharged cell, but I think you'd be OK. I wouldn't recommend doing so, but if by accident this were to happen, I think you're correct, the voltage difference would not be enough to cause an inappropriate amount of current to flow from the charged cell to the discharged cell. The only downside would be an unnecessary discharge, or actually about a half discharge, to the charged cell, adding to it's cycle count.

Also again, keep in mind that if this were to happen, the voltages would equalize, but not necessarily the capacities.

Dave

 

[apagogeas]

Thanks for the remarks 45/70. Some further observations, the device after two nights of usage can deliver for more than 7+ hours, possibly more but I turned it off in the morning. The gap between the brands almost disappeared and now batteries accept around 1500-1600mAh at the moment I switched it off. In two holders both batteries (energizer & Tronic) accepted almost the same charge, which means that higher impedance of Tronic has been restored and equaled Energizer. Indeed Tronic was sporadically used whilst Energizer was in regular use. This indicates that batteries do suffer by sitting idle, even if they are LSD and the higher impedance occurred can be restored by use (at least in this case). Furthermore, I was able to get about 3Ah out of each holder - probably more than that - surely these 2xAA -> D holders do better than just using a single AA -> D holder. The reason is that now basically both brands participate equally. Therefore, instead of one battery being drained at 400mA in a AA->D holder, they now are drained at 200mA each, which helps in a lower voltage sag, which results in higher overall voltage per holder which is especially important in fuzzy devices like this one. I'm sure if I used just a single AA->D holder I wouldn't be able to get 7+ hours of usage here.
All the above really suggest that your best bet for performance in D cell applications (especially if they are fuzzy on voltage) is to use 2xAA -> D holders and use the same brand/capacity cells to be equally drained.

 

[45/70]

It sounds like your cells of different origin are beginning to even out. If it were me, I'd still try to better match the cells. While only using cells of the same brand, capacity, age, etc. applies more to the use of cells in series, it still applies somewhat when using cells in parallel. As I mentioned, it is likely that one, or the other of your mismatched cells is still supplying the bulk of the current requirement at different stages of the discharge. It will work, but personally, I prefer better matched cells.

I too have had success with DX's 2AA to D adapters in various projects. Really though, I don't have many devices that use D cells, even fewer that use C size. One thing you have to watch out for is draining the paralleled cells within the adapter, down too far when using the adapters in series. If you go too far, you may run into cell reversal, or "reverse charging". In this case you will be damaging not one cell, but both cells that are paralleled in each adapter that is reversed. This, in effect, causes twice the damage. For this reason, whenever I use these D adapters, I try to stop the discharge well before the adapters in the series string are depleted, and replace/recharge the cells more often.

Dave

 

[apagogeas]

True, I am aware of cell reversal situations. Based on the current consumed by this device I know I enter a dangerous state for the cells beyond 8 hours of run based on the charge I put in the cells after 7+ hours of run, which is around 1600mAh drain per cell in 7 hours, or 220mA drain (440mA per holder). This is why I started with 2xAA->D adapters, to sort of double the overall capacity so not to get drained whilst I'm still asleep. I just didn't expect this initial observation of uneven discharge but everything looks good now. I'll get proper cells for this task anyway, just wanted to test this setup before spending money for new cells. The uneven cells problem is not the parallel placement in each holder. It comes from the holders being in series in the device, and I have two cells in each holder to worry about now. So I still have this cell reversal problem and better matched cells will help the most here knowing the holders will drain almost equally. Any 2Ah+ cells would do the job and still have some time margin before entering cell reversal. I may not enter at all however given the device turns off on low voltage, I just don't want to test this scenario. On the other hand, if I wake up and see the device turned off... I'll know I've overslept hehe :)