Number of alkalines to power 6 Cree P4s?

[Rando]

I'm going to be using 6 Cree P4s (these) in a fixed lighting application with heat sinks. I'll be powering them from an alkaline D cell battery pack, most likely direct drive with resistors. Would 4 D cells be sufficient to handle the drain from 6 P4s? I'd be running them at around 800 ma using 2.1 Watt 3.3 ohm resistor each, per this calculator.

I'm willing to add more cells and change the resistor values if necessary.

Suggestions?

 

[Black Rose]

I think you are using the wrong calculator, as the one you linked is for a single LED. You should be using the serial/parallel calculator.

How are you wiring them, all serial or a serial/parallel combination?

4 D cells seems to work the best, although according to the serial/parallel calculator, a lot of energy will be wasted by the resistors.

You'll be drawing 4800 mA from the D cells, so you'll start to see dimming after about 3 hours.

 

[Rando]

Well... I'm open to suggestions on how to wire them. Although I had linked to the single LED calculator, I had planned on wiring them as shown in the series/parallel calculator (it shows the same resistor values, just six of them). How could I wire them more efficiently? I'd prefer not to use a driver board due to budget concerns.

In this application, the lights won't be on continuously, probably no more than a couple minutes at a time for a couple times a day max. I'll be mounting the stars to 23x23x10mm passive heat sinks.

 

[Rando]

After playing with that calculator, it looks like bumping the voltage up will reduce the number of resistors required, increasing the efficiency of the circuit:

4 Cells = 6 volts, 6 resistors, 4.8 amps for the entire circuit.
6 Cells = 9 volts, 3 resistors, 2.4 amps for the entire circuit.
8 Cells = 12 volts, 2 resistors, 1.6 amps for the entire circuit.

Since space isn't going to be a problem, I'll probably add another 4 cells, dropping the drain to 1/3 and increasing the capacity by 2x.

Should result in 6x longer runtime, or am I oversimplifying something?

 

[Yoda4561]

If you're running a 12v circuit, why not just use a 12v SLA battery ? Better still if you have some AC power or a window nearby, you can run a float charger for it.

 

[VidPro]

just so you know 4D alky cells pushing ~3+ amps, isnt going to be ~6V when under that kind of load, it would depend on the freshness of the cell (in shelf time) and how long it has run for, but your probably looking at more like ~4.8-5v under that load. tiny resistance to each led unit will probably be sufficient. this also assumes that ALL are ON at the time, you wouldnt want to suddenly only have 1 connected.

if you doubble up parelell, then the battery could cope with the ~1.5+A probably just fine (the voltage would be higher under that kind of load), or 6V lantern batteries dont they have a bit more than D in them? then you wouldn't be "current limited" by the battery so much.

note: i am NOT saying anything bad, or wrong with it, i am just saying with 4x alkaline D cells you might get close to a Direct drive (slight resistance) because the batteries themselves will only put out so much, which is sorta good, easy, cheap. With 4X ni-mh or if anyone should Decide LATER to go rechargable it would be quite different.

with that setup it might be good to use them AMC1735 instead, because they would be sorta made for that kinda setup, and your resistance would reduce as it was not needed, but its easy still to add just slight resistance, and go with it.

all/any added resistance letting off power in heat, is waste, raising the voltage then blowing it out in resistance is not efficent, unless your resistance is putting out Light but changing from series to parellel, will reduce the amps and increase the voltage, which would completly vary the current and resistance ammounts.


sooo
if your going to use the calculator , which is a good thing, then put in all your parmeters, and see what your variances are. put in your max high voltages, along with your voltages under a hard load, and even your discharged low voltages, then the calculator can give you more real results.

 

[Rando]

I could use a rechargable 12v battery, though it isn't in a place that could be conveniently recharged. I see a 12v 4.5 amp hour battery here.

Looking at the font of infallable wisdom (Wikipedia), they're showing 12000 mAh typical for a D cell. I realize that this is at 1.5 volts, but that's 12 amp hours (right?). How does this translate when you line eight of them up for 12 volts? Surely that doesn't equal 96 amp hours at 12 volts...?

I understand that the alkalines will sag more under load than lead acid will.

 

[Black Rose]

How does this translate when you line eight of them up for 12 volts? Surely that doesn't equal 96 amp hours at 12 volts...?

Voltage increases when connected in serial, but the capacity remains that of as a single cell.

One the opposite end, capacity increases when connected in parallel but the voltage remains that of a single cell.

12 volts and 12,000 mAh in serial.
1.5 volts and 96,000 maH in parallel.

 

[Rando]

Thanks for that explanation!