Hey guys,

I'm building a battery powered LED project.
I need a maximum of 1A @ 5v. Current draw will typically be less.
I want to use "14500" lithium cells.
I plan to use a separate regulator/board (to meet my current demands) and bypass the onboard regulator of the arduino i'll be using.
Size/weight are issues. Through-hole components preferred for ease of prototyping.

1. Is it more efficient to step up a single 3.7v cell or step down two cells in series (7.4v)?
2. Will I run into issues using 14500 cells in parallel to increase my capacity?
3. Will I run into issues using 14500 cells in series? (They have onboard protection circuits)

Thanks!



Possible equipment options:
Batteries: (upto 2C discharge rate) http://dx.com/p/trustfire-protected-14500-3-7v-900mah-rechargeable-lithium-batteries-2-pack-26124
Voltage Regulator:
Both: http://dx.com/p/mini-dc-dc-voltage-stabilizer-regulator-module-red-126106
Step up (TDFN package): http://www.maximintegrated.com/datasheet/index.mvp/id/5770
Step up (TDFN package): http://www.maximintegrated.com/datasheet/index.mvp/id/5127
Step up (SOT23 package): http://www.maximintegrated.com/datasheet/index.mvp/id/2451

 

Use a Simple Switcher e.g. LM2475 and the two cells in series. Properly charging them will be something you need to consider.

 

Stepping down will use a smaller inductor and be a bit more efficient as a result. It will also use a NFET transistor instead of PFET (probably) which will also help.
I would buy one from Ebay. Hard to build one for the same price. Check out Buck Regulator.
The series configuration will need to be balanced.

 

Using a larger cell, like and 18500 or 18650 is better than parallelling two smaller cells. I would use two in series and a buck converter.

Bob

 

Definitely buck down from a higher voltage.

The problem is the cell voltage: an Li cell is 4.2V peak, but drops to about 3V at end of cycle so that only gives about 6V from two cells. That's not enough to run a buck down to 5V.

Adding a third cell would make the design a lot easier.

 

Alright, thanks for your help guys!

 

Definitely buck down from a higher voltage.

The problem is the cell voltage: an Li cell is 4.2V peak, but drops to about 3V at end of cycle so that only gives about 6V from two cells. That's not enough to run a buck down to 5V.

Adding a third cell would make the design a lot easier.

Buck converters can be made to operate with an input all the way down to the output voltage. In this case, the switch just stays on permanently.

Bob

 

Buck converters can be made to operate with an input all the way down to the output voltage. In this case, the switch just stays on permanently.

Bob

But that's a terrible design: look at the VIN range with only two cells. The efficiency is terrible on a buck when the VIN approaches VOUT. Adding a third cell in series is the way to make it better. If only two cells can be used, he would probably be better off just using a linear reg. The power saved in a 1A design doesn't justify the added cost, size and noise of a switcher. For a linear, I would use a LP2975 controller and an external FET and it would be very cheap.

 

Not so bad.

 

Because of the great current capacity, I'd lean towards the step up. Your voltage sag will stop the buck, before your current runs out in a boost.

 

Lipos are pretty "stiff' until they go dead. With such large ones I don't think he needs to worry much about the voltage sag. His voltage should only sag a couple of millivolts at 1 amp.

 

Not so bad.

I was talking about the typical cheap stuff like simple switchers LM2575, LM2576 type. Unless you have at least 3V of headroom, not worth the trouble because the eff gain is microscopic.