Battery voltage for Maxflex

[piesoup]

Evening all

I have a quick question that I cant find the answer too..
I want to run 4x XRE R2s at 1000mA with the MaFlex. What would my optimum input voltage be? I know I have to have the Vin less than Vout, but I dont know how much less.
Thank you in advance!
Andrew

 

[steve6690]

What sort of a battery or battery pack did you have in mind i.e battery chemistry ?

 

[gillestugan]

Look at the table on the taskled homepage: http://www.taskled.com/techmaxflex.html
Efficiency is higher the closer to output voltage you get. (because of lower currents in the circuit)
Choose a battery setup with a maximum (fully charged) voltage just below Vout.
You have to measure total Vf for your leds to get the correct Vout.

 

[Offroad'Bent]

It looks like you'd get 4% more efficiency (91% vs 87%) if you use 14V vs 7V from the table. Is that right?

 

[gillestugan]

the table contains numbers for both 6 led in series and 4 leds in series at different voltages and output currents. See the 3rd and 5th columns.

These are the numbers for 4 leds at 1A
5.89V 83%
7.07V 88%
9.52V 91%

 

[piesoup]

Thats perfect, thanks! I remember seeing that table now, forgot where it was!
I've found a 13.2v 4500mAh NiMH on batteryspace which I think will do.
Is there a decent battery supplier in the UK that you guys and girls use??
Thanks again for the help :twothumbs

 

[trout]

Thats perfect, thanks! I remember seeing that table now, forgot where it was!
I've found a 13.2v 4500mAh NiMH on batteryspace which I think will do.
Is there a decent battery supplier in the UK that you guys and girls use??
Thanks again for the help :twothumbs

wildwood on here
and a 11.2 volt li ion would be a good option

 

[steve6690]

wildwood on here
and a 11.2 volt li ion would be a good option

Agreed. The reason I asked about battery chemistry was that you have to consider what the pack voltage will be fresh from the charger, and also when fully discharged. A 13.2V Ni-Mh pack consisting of 11 batteries in series could be around 15.8 volts freshly charged and this might be more than the total Vf of your led's. This could be a very bad thing..
A 12V Ni-Mh would probably be ok though. Ideally you should use a bench power supply to run the string at 1A and measure the total Vf of the string. Then you can choose your battery pack. If you don't plan on doing that a 3S 11.1V Li-Ion pack would be fine for your application considering it will be c. 12.6V freshly charged, and around 8V fully discharged.
I always calculate what the input current to the driver would be when the pack is near fully discharged - as the battery voltage drops, the driver will draw more current as it needs to boost more. If you get your sums wrong (or don't do them at all) you could find that as the pack gets near to being fully discharged the input current rises above the level that the driver/heatsinking can handle and you end up with a fried driver....
I'm certainly not as knowledgeable as some on here, but my recommendation would be for the Li-ion pack, unless you can measure the Vf of the string in which case there might be a slightly better option.

 

[piesoup]

Steve and Trout, thanks for your replies!
Electronics is kinda new to me, all these wires and wiggly amps can get confusing at times! I knew about the various calculations but I didnt realise about using the lowest voltage. It all seems rather obvious now! Definitly dont want to fry my driver, been waiting ages for it to arrive.
Trout, your lights are amazing, I cant wait to get mine finished now.
Cheers again for the replies