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.

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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.

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Uncle $crooge

 

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.

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I thought what I'd do was I'd pretend I was one of those deaf-mutes.

 

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.

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Regards,
Sarma.

 

Just don't do it, mixing battery chemestries is asking for trouble.

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Lead acid (wet and SLA) don't seem to have problems when put in parallel and series-parallel configurations, but other types will just discharge into each other and severely shorten the life of the cells.

Like I mentioned before, I learned this the hard way...

 

Ok understood. I wont' mix them.

Thanks to you all for your comments!

 

whenever I've tried to parallel voltage sources, I've gotten sparks and blown fuses.

 

Couldn't you simply use a diode on each positive lead? As long as the packs have similar performance I would think that would work. The current load could still be a little uneven, but the diodes would prevent the stronger pack from charging the other, wouldn't they?

 

The diodes would make an OR gate. One battery OR the other battery would power the load. The battery with the highest voltage. They won't be in parallel so their currents are not added.

When the battery with the highest voltage discharges down to the voltage of the other battery then they will be in parallel only for a moment.

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Uncle $crooge

 

Perhaps one has to use schottky diodes to reduce power wastage. then the supply to the load will be reduced by the voltage drop of the diode .
thus there will be reduction in the effective working time of the battery.

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Regards,
Sarma.