Li Ion battery for LED flash application

[John_Matrix]

For mobile flash applications the LED is typically driven with 1 - 2A current in pulse mode.

How much battery current is typically available for an LED flash in a mobile phone application? Assuming most phones are using Li-Ion batteries these days? The reason I ask is because this limits maximum current rating of the flash LED.

Thanks for your help.

 

[AnAppleSnail]

Assume 2C. For my HTC Hero, that's around 2A once you're running the camera and stuff. Many things limit the LED power. Design envelope, heat, driver cost, power load, battery, and LED rating. I don't know what my flash actually runs at.

 

[John_Matrix]

Thanks for the reply. The reason I ask is because the current that is supplied from the battery to the LED driver is higher than the current supplied from the driver to the LED.

If anyone has any experience with system design in a mobile application maybe they can tell me how much battery current is typically available. There must be some limit since it is supplying other subcircuits (RFPA, cpu, etc) Also what typical driver efficiency might be.

(Maybe the question is better suited for an EE forum but worth a shot)

Thanks!

 

[AnAppleSnail]

Li-Ion batteries respond poorly to current draw over twice their capacity. So on a 1000mAh battery, the phone needs to live on less than 2000mA. Used continuously, this would drain the battery in half an hour. Many phone designers strive for better life even with heavy use, building their phone for lower peak power. But the battery is happy below that 2C rate.

It sounds to me like you asked the same thing. That tells me that I misunderstood your question.

 

[rmteo]

Lithium batteries used in the R/C hobby industry are rated to 40+C continuous. For example, this 3S-1800maH (11.1V) battery is rated at 40C (72A continuous) and 50C (90A burst).

 

[AnAppleSnail]

Lithium batteries used in the R/C hobby industry are rated to 40+C continuous. For example, this 3S-1800maH (11.1V) battery is rated at 40C (72A continuous) and 50C (90A burst).

So they are. But cell phone batteries tend to be LiFePO4 or other more-common chemistries that suffer at over 2C drain. Energy density is more important for most phone uses than power density. Of special note is the capacity loss in Li-Ions which varies with temperature, but sits at 10% per month in most pockets. Here's an article of interest gotten by googling "Cell phone battery current limit" and clicking on the first five articles:

Supercaps lighten load, which mentions top-end phone camera flashes taking about 2A as I predicted. Sucking this and more requires special design considerations as phones (And batteries) slim down more.

 

[rmteo]

But cell phone batteries tend to be LiFePO4 or other more-common chemistries that suffer at over 2C drain.

You sure?? All that I have seen are Lithium-Polymer (higher energy density than LiFePO4) Prismatic cells. In any event, an A123 LiFePO4 26650 http://www.batteryspace.com/A123-Sy...O4-26650-Rechargeable-Cell-3.2V-2500-mAh.aspx cell is rated at 40C 120A (impulse discharge <10sec, 70A 28C continuous).

 

[AnAppleSnail]

You sure?? All that I have seen are Lithium-Polymer (higher energy density than LiFePO4) Prismatic cells. In any event, an A123 LiFePO4 26650 http://www.batteryspace.com/A123-Sy...O4-26650-Rechargeable-Cell-3.2V-2500-mAh.aspx cell is rated at 40C 120A (impulse discharge <10sec, 70A 28C continuous).

*shrug* You're welcome to quote all the specialized-for-high-drain application batteries like the A123 or racing packs you like. But if someone asks about tractor parts you wouldn't go off on carbon-fiber landing struts, either... Just because both are used in wheeled vehicles. Broadly speaking, 3.7v Li-Ion batteries outside high-drain applications tend to be LiCo chemistry precisely because of the significant boost in overall capacity at reasonable drain rates. That A123 Systems battery you linked to has about half the capacity of a similarly-sized LiCo 26650. In most consumer devices, runtime is at such a premium that LiCo is the most common chemistry.

There are exceptions. Motorola has a new chemistry out with a +0.1V nominal voltage (2.5% more energy even disregarding capacity increase). Some products (Some Apple ones, perhaps others) use LiFePO4 cells, which can be configured for impressive discharge (As the A123 specialized cell you linked does) or... not. A battery's ability to deliver power (Quick energy) is almost directly proportional to the severity of consequences of damage to the battery. Batteries that are ready to send a few hundred amps through a short circuit are less stable after impact, severe vibration, or damage. These are all reasonably likely events to happen to pocketable devices, and a good designer has probably considered and discarded super-output batteries for this reason. The bonus runtime and less risk of a slightly larger boom are appealing.

I stand by the 2C limit being the most common 'throttle' on LED output in a phone-type device. There will be exceptions, including the unusually-large "mobile" slabs that have much larger batteries. In those cases, 2A is still the common limit. Soon it will stretch. Usually a change in battery chemistry requires exchanging some very desirable capacity or safety bonuses of sober battery design, limiting the appeal of the possibility of a thousand-lumen, ten-watt LED flash on a phone.

Edit: I have a 1500 mAh 3.8v nominal cell in my phone. It has, therefore, over half the capacity of that honkin' big 26650, but is slim and fits in my pocket.

 

[John_Matrix]

So they are. But cell phone batteries tend to be LiFePO4 or other more-common chemistries that suffer at over 2C drain. Energy density is more important for most phone uses than power density. Of special note is the capacity loss in Li-Ions which varies with temperature, but sits at 10% per month in most pockets. Here's an article of interest gotten by googling "Cell phone battery current limit" and clicking on the first five articles:

Supercaps lighten load, which mentions top-end phone camera flashes taking about 2A as I predicted. Sucking this and more requires special design considerations as phones (And batteries) slim down more.

Thanks you answered my question!