Basic question

How many volts and ampere a battery need to be able ti drive 4 cree q5 leds at maximum capacity??
How can I preview how burn time I 'll have?
I am unable to understand how volt influence the luminosity

is a 12 volts 4000 mAh battery better than a 3,7 volts battery 4000 mAh??
If I remember correctly V= I x R so se if R stay the same I assume that the first one (12 V) could give more current? ( more burning time?)
Is it right?
Can someone explain me how it works or point mein the right direction?

Thank you
Andrea

you need about 12Watts per hour of operation. for your 4X3Watt leds, more for any curcuits losses.

you could calculate the runtime based on the total wattage of your batteries Volts X Amps =Watts . You would want to find the ACTUAL wattage the battery could output at the actual drive current it will run at, vrses some manufactures specs that mean little. then add in any losses involved in the rest of the curcuit.

12V of battery at 4000ma is much more total power than 3.7v at 4000ma, just depends on how you waste what you got

for whatever your battery, you need to have the current to the led, or at least the voltage to the led, low enough so you do not burn it out. Keep it within specs.
generally this is done with a driver that pushes the right ammount of current to the led or led set.

Thank you for your replay.
So if I'll ending use a battery pack of 8 AA NiMh cell rated 2500 mAh
I am expecting about 2hr run time

1,35*8*2,5= 27W
Cos I'll need only 1hour and 30 minutes run time I suppose this setting will be fine with reasonable margin of safety is it right?

Thank you for let me learn , this forum is great
Andrea

NiMH AAs put out around 1.2v.
So with 8 you have 9.6v
You LEDs need 3.7v x 4LEDs= 14.8v
You want to drive them at max power 1A (at the LEDs).
So you will be drawing from the batteries 1A x 14.8/9.6 =1.5A =1500mA

They are rated 2500mAh ... 2500mAh / 1500 mA = 1.7hours.

But you need a driver circuit between the LEDs and batteries(to turn 1.5A @ 9.6v into 1A @ 14.8v) which will drop 10-20%, and batteries are never as good as they claim, so drop another 10-20% ... you're down to around 75minutes.

Running the LEDs at 700mA instead of 1A will give you the required runtimes.