voltage value??

[RCinFLA]

To original question. It depends on design of power supply.

Most power supplies have a series pass regulator, meaning there is a transistor in series to the output. The output voltage is sensed and used to control how hard the series connected transistor turns on. If something external pulls the voltage higher then the set output voltage for that power supply the series transistor just shuts down.

This is likely the design for the two supplies you are using. Which ever of two power supplies has the higher voltage setting wins. All supplied current to any load is from the higher voltage supply.

You can still damage some things doing this however.

 

[tariq7868]

I mended many old shorted Ni-Cad cells with the ZAP from a 4700uF capacitor charged to 20V. But the mended cells developed the same problem over and over.

I think the problem with shorted cells was due to over-charging them in a product that keeps charging them.

can you plz tell me ,, how old-shorted ni-cad cells are mended ? and what is Zapping ??

 

[xtrmi]

Woohoo go Texas!

 

[audioguru]

can you plz tell me ,, how old-shorted ni-cad cells are mended ? and what is Zapping ??

Read a tutorial or document from a batterty manufacturer like Energizer about old Ni-Cad cells. Energizer does not make Ni-Cad cells anymore (they switched to Ni-MH cells) but their tutorial about Ni-Cads is still on their website.

Ni-Cads that are continuously over-charged develop spikes of crystals inside that cause the cell to be short-circuited. Applying a high current for a short duration will burn out the spike of crystal then the battery works again. In English a high current for a short duration that burns something is called a ZAP.

But many spikes of crystals form and you burn out only the longest one that caused the short-circuit. Other spikes of crystals will soon be long enough to cause another short-circuit.

I don't think spikes of crystals form in Ni-MH cells so they do not develop short-circuits.

 

[tariq7868]

thnx for explaining.. going to try on some of my Ni-Cad cells (drains very quickly and while charging ,the charger gets very hot)..

 

[marcbarker]

Really good explanation AG

I seem to notice that the shorted nicad phenonomen occurs with nicads that have been left flat for period of time. Maybe when the nicad holds a charge, the voltage is enough to zap any spike as it occurs. If not, I'll guess if it doesn't the cell has self-discharge, and ends up flat anyway

 

[audioguru]

thnx for explaining.. going to try on some of my Ni-Cad cells (drains very quickly and while charging ,the charger gets very hot)..

Ni-MH cells have 4 times the capacity of old Ni-Cad cells. They can be charged fast. They do not have the "memory" problem and cost the same.

Ni-Cads were used in 1975. Ni-MH and lithium cells are used today.
Maybe your Ni-Cad cells are old and need to be replaced by the newer chemistry.

 

[Hero999]

Nicads have one advantage: they can produce higher currents than MiMH cells for short periods of time.