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NiMH & NiCad batteries

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mabauti

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What precautions must I take when connecting (2 1.2VNiMH 1600mAH batteries in series) and (2 1.2VNiCad 600mAH batteries in series) in parallel in order to have 2.4V? :confused:
 
Your question is too vague. Under most circumstances 2 3 or 4 cells in series will require no additional precautions as long as the cells are from the same production line at the same time.

With multiple cells in series it's common to be a little understated in it's charging requirements compared to a full cell and to do a low current equalizing charge periodically.

12 volt car batteries are after all in fact 6 2 volt (nominal) Lead acid cells in series.
 
I think he wants to charge the old small-capacity Nicads from the newer higher capacity Ni-MH battery. That wastes power.

Instead of using 1600mAh plus 600mah= 2200mAh but it will never be reached, buy new 2500mAh AA Ni-MH calls. They just keep on going and going and going and going and going and going and going and going and going and going and going and going and going and going and going and going and going and going and ....
 
Even if you had 2x IDENTICAL 2.4V 1.6AH NiMH packs, you can't put them in parallel. Neither NiCd nor NiMH can be put in parallel, even though basic electronic theory predicts they'd be fine.

At odds with this practice is the charging voltage profile on these batts. When charging, typically one will take more current than the other leading to one fully charged batt and one partially charged batt. Continuing to charge may only result in further overcharging the already full batt.

So can we chare them separately and put them together to get one big storage batt? Unfortunately, no. When put in parallel, one batt will typically discharge into the other one- leading to one mostly dead batt and one which has been overcharged (which permanently decreases the batt's capacity).

Technically, you could charge separately and put each series string in series with a diode so it cannot take charging current from the other batt. Problem there is that's a 0.7v diode drop (Schottky may not be a great choice because of its own reverse leakage current). In a 2.4V batt losing 0.7v is probably not going to be acceptable.
 
I built a little stereo amp for the beach. It used two series strings of 6 AA Nicads with the strings connected in parallel. It worked fine and lasted for many years.

After about 10 years it didn't work and I discovered that all 12 cells were shorted.
 
I feel , Mabauti wants to parallel two sets of 2.4V batteries. one is a set wirh ni-mh and another with ni-cd with different capacities.
 
I discovered the hard way that ni-cads' propensity for self discharge is magnified 198728912738912738912 times (OK, that's an exaggeration, but not by much) if they are in parallel. I built an external power pack for a very old laptop that ran for a dissappointing 20 minutes from some small ni-cad cells. I figured I'd get 40 minutes if I used another pack wired in parallel. I did, but if I let the thing sit for long after charging it would be stone dead. Eventually all the cells crapped out after a really short time.

Don't do it.
 
Was the server down yesterday? I could not connect it >_>

anyway, I forgot to poste a figure ,sorry :

**broken link removed**
 
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But straight series connections are fine, yes?

I also doubt ballast resistors to balance current output from each would be of any use here?
 
You can connect identical battery cells in series but never allow them to completely discharge in series. Stop discharging when each cell still has 0.9V across it.

Like any battery, its charging current must be limited and the charging must be stopped when the cells are fully charged.
 
Nigel Goodwin said:
You must use identical batteries!, same make, same type, same amount of charge - never mix different types or makes, or old and new batteries - in ANY combination.

That's not the end of it.

Even if they were the same tech, you can't put two NiMH series strings in parallel. You can't put two NiCd strings in parallel either. Basically you cannot ever put NiMH/NiCd cells in parallel without an inordinately complicated isolator to prevent one cell or series string of cells from dumping their charge into the other one in parallel. When it dumps it drains one batt and overcharges the other resulting in damage to the overcharged cell or string of cells.

2 lead acid batts of similar type can be put in parallel ok in most cases. But there are technical reasons why this will not work on NiMH/NiCd.

Your diagram is clear, it's something you just can't do. There's no reasonable way to make it work, either. I mean there's a low dropout isolator scheme I can think of but it's insanely complicated and expensive compared to getting a bigger NiMH batt or going with a lithium ion.
 
Even in Ni-Cd or NI-Mh cells in series, the eventuality of some cells discharging faster, and thereafter the load current (though reducedto some extent), chrging these specific cells IN REVERSE, is not uncommon.

Such being the case Paralleling two sets of batteries, whether same capacity or make or otherwise, will have ODD consequences.The OP can, if he still wants, enjoy this experience at some cost. But such cases it is better to learn from scientific logic and at the worsrt from others experiences, rather than spoiling few more cells . these cells will be spoiled, we won't realise the reason and helplessly throw them.
 
mabauti said:
so, what extra connections/elements I must use?
Just don't do it, mixing battery chemestries is asking for trouble.
 
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