I need some advice about charging. Is it true that input voltage must be higher than output?

E.g. I need to charge 12 cells NiMH (12 * 1.2v = 14.4v), so I need Power Supply that can produce at least 14.4v into the charger?

Thanks for the enlightenment.

Its really a matter of wattage, the charger handles the required voltage. The charger has to be at a higher voltage to 'drive' any current into the battery. For NiMh, this means not only being higher than the total pac voltage, but also overcoming the packs internal resistance (IR) that is relatively high and causes some of the heat you'll notice when these are charging. A good NiMh sub c cell will get to +/-1.5v/cell too so a 12 cell pac will see 18+v right off the charger.

As fas as a power supply is concerned, it needs about 20% more watts than the charger is outputting to enable charger to take care of business. So for the 12 cell batt, charger will put out say 20v at maybe 5amps or 100watts of power (this is an example, but won't be that far off for a real 12 cell pac charging at 5amps). You'd need a power supply capable of delivering at least 120watts, so if its a true 12v supply, it'd need 10amp minimum output. You'll find a lot of RC oriented power supplie slisted as 12v really deliver 13.5-13.8v.

Power supply is an area where more better. For the above, I would look for a 15a supply (180watts). Power supplies last longer and run cooler when they are not running at 80-90% of their rated capacity.

Generally with most modern chargers you won’t need a higher voltage supply. It depends on the charger, what type of design it is. What charger do you have?

If it’s a modern switch mode based charger then the old voltage limitation you’re asking about doesn’t apply. Usually just need to consider wattage as mentioned. The charger will draw more amps than what you are charging at to get the needed power.

The older linear based chargers did have this limitation, the needed perhaps a couple of volts higher input than what you needed for charging. The higher the difference the hotter the charger ran. They draw the same amps they are charging at.

Ah so it is wattage. Now it all makes sense. Thanks for clearing my doubt. I truly appreciate your advice. Thanks heaps!

Whether or not the charger will charge 12 cells may not depend on wattage or input voltage at all. It depends on the specific charger like Dave pointed out.

If the wattage is low then it will simply slow the charge down on some chargers. If the voltage is low then it will simply not charge on some chargers and others will not care.

It is best to see your specific chargers max number of cells, if it happens to be 12-15 then you're good to go even with a 11-12 volt input. You still have to have the same or more power on input than on the output like 360 said.