Which driver maxflex or simular

[Goldigger]

Im thinking of stuffing 4 cree R2's into a Lumicycle bike light, with a fat heat sink of course. Using this board + R2's

Here's my puzzle the leds need 14.4volts @ 1amp. If i were to use 4 x 18650 batterys i would have 14.8volts, freshly charged 16.8 max.. i would think i could use a nflex here? i think the nflex needs 1.5volts itself.

Or would i be better of with using 3 x 18650's = 11.1 volts freshly charged 12.6v with a maxflex??

I've given up on the P7 leds, they sounded great when the came out 900 useless lumens..

Thanks in advance

 

[dnlwthrn]

Why not run your 18650's in 2S2P orientation and use the Maxflex. That gives you ~7.4V and twice the life (i.e. your 18650s are 2400mAh, now your pack is 4800mAh)

That's what I've been running using 3 Q5s and am very happy with runtime.

 

[Goldigger]

Why not run your 18650's in 2S2P orientation and use the Maxflex. That gives you ~7.4V and twice the life (i.e. your 18650s are 2400mAh, now your pack is 4800mAh)

That's what I've been running using 3 Q5s and am very happy with runtime.

Hi dnlwthrn,
I was under the impression that the more i boost the voltage the hotter the maxflex will be?
How hot is yours getting?

On the other hand i wouldn't be getting twice the life as im boosting the voltage to nearlry double if im not mistaken?

Cheers

 

[georges80]

4 Cree (3.5V @ 1A) in series from 7.4V nominal li-ion pack is well within the capability of maxflex4. Yes, it will get hot, but that's what heatsinks are for

Read the thermal white paper in the maxflex tech section of the taskled website to see what you will need to deal with. In this case (LEDs 4s and battery 2s2p) you would have likely around 1.8W of heat loss in the driver. Of course this is only while you are running the full 1A to the LEDs and I bet most of the time you will have plenty of light with around 0.5A going to them.

4 Cree and 4 li-ion in series is not a viable configuration for either a buck or a boost converter, so you need to change the LED or Battery voltage. Running the battery 2s2p is your most obvious choice in this case.

cheers,
george.

 

[Goldigger]

4 Cree (3.5V @ 1A) in series from 7.4V nominal li-ion pack is well within the capability of maxflex4. Yes, it will get hot, but that's what heatsinks are for

Read the thermal white paper in the maxflex tech section of the taskled website to see what you will need to deal with. In this case (LEDs 4s and battery 2s2p) you would have likely around 1.8W of heat loss in the driver. Of course this is only while you are running the full 1A to the LEDs and I bet most of the time you will have plenty of light with around 0.5A going to them.

4 Cree and 4 li-ion in series is not a viable configuration for either a buck or a boost converter, so you need to change the LED or Battery voltage. Running the battery 2s2p is your most obvious choice in this case.

cheers,
george.

Hi George,
Why is the 4 cree and 4 li-on in series not an option?
If I'm running the crees @ 1amp it's my understanding that each led would need 3.7 volts?
4 x 3.7 = 14.8v

How about 8 AA NiMh 9.6 volts with a maxflex and 4 cree's?

I suppose the ideal question is if i wanted to use all the features of the UIB2 as this is a bike light after all, what is my ideal battery config for 4 crees and a bflex?

Cheers

 

[georges80]

Hi George,
Why is the 4 cree and 4 li-on in series not an option?
If I'm running the crees @ 1amp it's my understanding that each led would need 3.7 volts?
4 x 3.7 = 14.8v

How about 8 AA NiMh 9.6 volts with a maxflex and 4 cree's?

I suppose the ideal question is if i wanted to use all the features of the UIB2 as this is a bike light after all, what is my ideal battery config for 4 crees and a bflex?

Cheers

It's not an option since the li-ion fullly charged is 4.2 and fairly discharged is 3.3V. So your 4s pack of 4 can range from 16.8V down to 13.2V.

Your LEDs at 1A can be up to 14.8 (or higher if your Vf's are higher) and dimmed down will be 2.8 x 4 = 11.2V so the LEDs range from 14.8V to 11.2V.

So, you can't boost (you'll be in HEAVY direct drive with fully charged pack) and have no dimming at all. You can't buck since you can have battery voltage < LED voltage.

Yep, 8 nimh cells is also viable with maxflex.

cheers,
george.

 

[NovaNath]

Hi George,
Why is the 4 cree and 4 li-on in series not an option?
If I'm running the crees @ 1amp it's my understanding that each led would need 3.7 volts?
4 x 3.7 = 14.8v

How about 8 AA NiMh 9.6 volts with a maxflex and 4 cree's?

I suppose the ideal question is if i wanted to use all the features of the UIB2 as this is a bike light after all, what is my ideal battery config for 4 crees and a bflex?

Cheers

Pasted from the TaskLED website:
"(bflex)
Can drive up to 24V (V2B shipping 6 Jun 2008) of series connected LEDs (from minimum input = (output voltage + 1V)).
Input operates from 4V to 25V. Being a buck converter, input voltage must be greater than output voltage - see the bFlex technical section (manual) for specific information. "

Basically, being a buck converter, it steps the voltage down. So the batteries need a high enough voltage so that they can step down and supply the LED's as well as the driver board with power.
Batteries vary their voltage output as they discharge, so fresh will be 4.2V, going to 3.3V when empty (cut off here on a protected cell).

The confuguration of the battery pack you use needs to suit the type of driver and the number of LED's, for both the fully charged condition as well as empty.


So, 4 LED's at 3.7Vf will need 14.8 from the battery.
Say the driver needs 2V (conservative)

Buck driven, the supply needs to be 16.8V min. (the danger is suplying too little)
With the buck driver, you need to consider the batteries at the empty voltage, so you would need 16.8/3.3= 5.1 = 6 cells in series (6s)

Boost driven, the supply needs to be less then 14.8V (how much less i don't know how to calculate) (the danger is in suplying too much)
With the boost converter you need to consider the batteries at the fully charged voltage, so you would need 14.8/4.2 = 3.5 = 3 cells in series (3s).

Note, to increase run time of the battery pack, you need to increase the current capacity (Ah). To do this you marry the cells up in paralle.
So with the boost example above, you could parallel conect: 2 sets of [3 cells in series]. Anotated as 3 series, 2 parallel (3s2p).


Hope that's not too confusing

 

[Goldigger]

Hi Novanath,
Thanks that makes perfect sense, all i need to know now is how to calculate the optimun battery voltage to use for a boost..

Cheers

 

[NovaNath]

Try this page
http://www.taskled.com/techfatman.html

There's some useful info and calculations at the bottom of the page