Why no rechargable "C" or "D"?

ceptimus said:
Putting NiCad (or NiMH) cells in parallel is NOT recommended.

These cells have a peculiar voltage vs charge curve. As they come up to full charge the curve flattens, and the voltage then falls slightly as more charge is added (partly due to the cell heating during charge).

If you connect freshly charged cells in parallel, then the cells will try to equalise voltage with each other, but because of the negative slope on part of the curve, this can result in a runaway situation. In exceptional circumstances this may even result in fire, but will more likely just partially discharge some of the parallel cells while damaging others by overcharging/overheating.

The one time when it's okay to connect Nickle cells in parallel is when you're about to discharge them fast. If you intend to put sufficient load on the cells to flatten them in, say, ten minutes, then the internal resistance of the cells means that they all only discharge, rather than attempt to transfer charge to each other.
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Can't you get around this by using diodes to prevent the backwash?
 
Zombified said:
You're gonna drop 0.6V across each diode...
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Just thinking quickly without trying to do it on paper, wouldn't you also need a second set of diodes if the batteries are to be charged in parallel as well as discharged? One set to isolate them from each other in the circuit that powers the load, but also one set in the charging circuit?
 
bruto said:
Just thinking quickly without trying to do it on paper, wouldn't you also need a second set of diodes if the batteries are to be charged in parallel as well as discharged? One set to isolate them from each other in the circuit that powers the load, but also one set in the charging circuit?
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The temperature coefficient of the diodes will mess with the "full charge" detection in the charger, virtually guaranteeing that it won't work right. Add in that you are trying to detect whether multiple cells are charged through just the one charge connection, and you've got a guarantee that some will be over charged and some under charged.

The discharge side isn't much better. The diodes will have to be hefty enough to dissipate .6V*Amperage watts. That heat is pure loss as far as your capacity goes. On top of all of that, you also won't get full use of what is in the cels anyway. That .6V may be the difference between go and stop on an electric motor, or between on and off in an electronic device.
 
Zombified said:
LED lighting is a lot more efficient (and more reliable) than incandescent, provided you don't have any complaints about the color or spread pattern.
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For those who to want to pierce the blackness and see farther, the less intense central part of the spread pattern of LEDs is still a limitation.

Nothing is best at everything. The LED's great strength is reliable, efficient, medium-range instant-on illumination.
 
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