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How do they do it?

725 Views 10 Replies 5 Participants Last post by  TheBigYin
All the pro builders seem to use 7.2v batteries when the forward voltage is more like 14+v.
Do they use a seperate driver per Led?
Please enlighten me.
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Sorry,do you mean one driver each Led? off one battery.
One of these will run multiple XR-E's hapily from a 7.2v battery.

And in best TV Presenter Stylee - Here's one I Made Earlier...

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TOMMO said:
Sorry,do you mean one driver each Led? off one battery.
It all depends on the boost driver that you are using. You can run multiple LEDS off the one driver using a lower battery voltage than the combined Vf of the LED's. The number of LED's you can run is determined by the max output voltage of the driver.

You need a battery voltage at least 1V lower than the Vf of the LED's. If you use a battery voltage higher, then the driver goes into direct drive and the LED's go POOF!

Here is an example of a boost driver

Edit: Nice work YIN.. good boost driver example ;)

The fatman is another example that comes to mind
sorry misread your last reply,why dont you guys do this instead of going for the bigger voltage batteries.
I am asking as i have been given a brand new 7.2 Li-ion laptop battery 7.5aph but would like to make a twin mc-e and use that battery.
A good question....

Ultimately, you need to compare apples with apples when you price out batteries. Your overall battery capacity is best defined in Watt hours. i.e mAh x V

so if you have a 14.8V 2200mah battery or a 7.2V 4400mAh battery, you are really both packing the same amount of energy. So there is no cost saving (or weight saving) to be made as you will be buying the same number of cells.

Historically, buck converters were more efficient and everyone was using them however there is little splitting buck v boost now.

There is definitely more choice with buck drivers than boost drivers at the moment which is probably the biggest reason. You can get super cheap DX drivers through to expensive inteligent drivers like the hipflex.
Cheers that helped alot.
Guess i will be purchasing a boost driver and using my freebe battery.
Cheers guys keep up the good work.
Many people claim that the boost driver efficiency is the best when the battery V is close to the Vf. So driving a 4 serial LED light from a 7.4V batt does not seem like a good idea?

Tesla uses 7.4V, but cuts it down to 3.1Vf of the SSC P7 I guess, which must be close enough for the driver they use, as the run times are great.

Dinotte 600L Vf is at least 10V, plus the voltage drop on the driver, so around 11-12Vf boosted from a 7.4V batt source. Not very close if you ask me :)

Either we are not well informed, or the drivers are ok to boost 7.4V to 12 with negligible losses?
A boost converter can and will happily double the voltage, even 4 led from a 7.2v is safe. The efficiency of a 7.2v battery running a set of leds is indeed better if the battery is closer to the forward voltage. The better boost converters out there will be above 80% even with a 7.2v nominal source and a combine vf of 12. A 11.1v nominal source could be and often are as good or better than 90% for a 12v combine vf. I made a previous post a while back with a formula for matching your battery voltage. With the new 4 die MCE and multiple multi die emitters things can get complicated quick. A very simple rule of thumb for good efficency is that if you are using a boost converter your leds should out number your lion cells by one. The opposite is true for a buck converter.
TOMMO said:
...why dont you guys do this instead of going for the bigger voltage batteries.
Well, to be honest I did do it that way for pretty much the same reasons as you state - I already had a 6.0v Electron Bottle Battery and a 7.2v RC Battery pack which I wanted to use. :winker:
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