Is this Li-Po too much?

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Good day freaks,

 

I've got another general electronics question. So I've got this 11.1 volt, 3-cell Lithium Polymer battery:

 

I've used it to power some of my AVR and ARM boards, some are 3.3 volts and some are 5.0 volts. Each of these are using regulators that have a maximum input voltage of +15 volts.

With one specific board (my ATSAM dev. board I built recently), I've had problems with the power regulator shorting the output pin with the input voltage. With this specific board, I've been testing a 2.8 inch full color TFT LCD. So it draws significantly more current than some of my simpler boards.

 

I've noticed if I power the board with my Li-Po, the regulator will get extremely hot.  If I power the board with a 9-volt battery, the regulator seems to be fine. Unfortunately, the specific 9-volt batteries I've used (non-rechargeable) doesn't last very long and discharges very quickly. I am using the required input and output capacitance for the voltage regulator and the CPU on my dev. board. I've got a 4.5 volt three AA battery pack that I can test with but I'll have to pick up some more AA batteries tomorrow.

 

So short question with a long explanation. But is this Li-Po too much or not stable enough for powering these low voltage, low current devices?

This topic has a solution.

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

Last Edited: Fri. Mar 4, 2016 - 05:43 AM
This reply has been marked as the solution. 
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You are going about this the wrong way.

 

Your battery has too high a voltage for 3.3volt systems. The power that has to be dissipated by your linear voltage regulator is (11.1 - 3.3)*the current (more than twice what your application needs). I suggest that you obtain some of these instead. https://www.fasttech.com/product...

 

They are inherently safer. Each battery provides 3.2 volts. I use them myself.

 

Stay safe.

 

Ross

 

Ross McKenzie ValuSoft Melbourne Australia

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At a minimum, you should give up the linear regulator and go for a buck or buck-boost regulator, a type of dc-dc switch-mode-power-supply.  Pololu has a great number of regulators, many of which are likely drop-in replacements for your linear regulator, and are in the neighbourhood of 75% - 95% efficient (versus the 25% efficiency of your 3.3V reg running from an 11.1V source):

Check out their product selector.

 

Plenty of other source for these things, too.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Maybe I wasn't clear enough. With a battery producing 3.2 volts, you don't need a regulator at all.

Ross McKenzie ValuSoft Melbourne Australia

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valusoft wrote:

You are going about this the wrong way.

 

Your battery has too high a voltage for 3.3volt systems. The power that has to be dissipated by your linear voltage regulator is (11.1 - 3.3)*the current (more than twice what your application needs). I suggest that you obtain some of these instead. https://www.fasttech.com/product...

Thanks for the fast reply and good information! After seeing some posts recently here about Li-Pos, I kind of suspected this may have been the case. It is nice to get a confirmation.

So is that current the draw of the entire device or the current potential of the battery?

 

Can you provide any information into why a 3.3 volt regulator can have a maximum input voltage of 15 volts if 11.1 volts is too much? Or is it more related to the maximum current discharge of the battery and the voltage?

I will certainly pick up a few pairs of those batteries! Thanks for the recommendation.

 

joeymorin wrote:

At a minimum, you should give up the linear regulator and go for a buck or buck-boost regulator, a type of dc-dc switch-mode-power-supply.  Pololu has a great number of regulators, many of which are likely drop-in replacements for your linear regulator, and are in the neighbourhood of 75% - 95% efficient (versus the 25% efficiency of your 3.3V reg running from an 11.1V source):

Check out their product selector.

 

Plenty of other source for these things, too.

Thanks for the recommendation. I will look into these as well.

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

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valusoft wrote:

Maybe I wasn't clear enough. With a battery producing 3.2 volts, you don't need a regulator at all.

Jamison wrote:

I've used it to power some of my AVR and ARM boards, some are 3.3 volts and some are 5.0 volts.

Jamison wrote:

With one specific board (my ATSAM dev. board I built recently), I've had problems with the power regulator shorting the output pin with the input voltage. With this specific board, I've been testing a 2.8 inch full color TFT LCD. So it draws significantly more current than some of my simpler boards.

No mention of the specific voltages required on the board in quesiton.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Can you provide any information into why a 3.3 volt regulator can have a maximum input voltage of 15 volts if 11.1 volts is too much? Or is it more related to the maximum current discharge of the battery and the voltage?

Assuming you're talking a linear regulator, these work by dumping the extra energy as heat.

 

For example, if you've got 11.1 volts going in, and 3.3 volts going out, that's a drop of 7.8 volts.  If you draw 250 mA, the regulator will dissipate 7.8V * 0.25A = 1.95W of heat.  That's a lot.  probably too much, depending upon the regulator.

 

What's the part number?  How much current are you pulling?  Do you have a heat sink?

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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joeymorin wrote:

No mention of the specific voltages required on the board in quesiton.

My apologies. The specific board in question was using a 3.3 volt regulator (3.63 maximum input voltage for the ATSAM D21, the CPU I am using on that board). The 5.0 volts were for some other tests I've done with some breadboard projects using ATmegas.

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

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joeymorin wrote:

For example, if you've got 11.1 volts going in, and 3.3 volts going out, that's a drop of 7.8 volts.  If you draw 250 mA, the regulator will dissipate 7.8V * 0.25A = 1.95W of heat.  That's a lot.  probably too much, depending upon the regulator.

Ah, that definitely makes sense!

 

joeymorin wrote:
What's the part number?  How much current are you pulling?  Do you have a heat sink?

I am using an AZ1117CH-3.3. Last I checked the draw of the LCD used by the board, it was drawing about 400 milliamps alone. I'm not entirely what the entire draw of the whole board would be but it's only got a small 30 milliamp LED and then the CPU. Without the LCD,  I can't imagine the board draws too much current. I don't really have a good way of measuring current for the whole board. In order to do that wouldn't I need a way to put my meter in series after the regulator output?

I am not using a heat sink.

 

Here is the voltage regulator for that specific board in question:

https://www.digikey.com/product-detail/en/diodes-incorporated/AZ1117CH-3.3TRG1/AZ1117CH-3.3TRG1DICT-ND/4505206

 

[Edit] Added the link to the voltage regulator

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

Last Edited: Fri. Mar 4, 2016 - 05:27 AM
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3.63 maximum input voltage for the ATSAM D21,

Take care with the 3.2 V lithium 'AA' batteries.  They have a different chemistry and use a different charge profile than does your 11.1V pack.  You'll need a charger designed for LiFePO4 chemistry.

 

This page might be helpful:

http://lygte-info.dk/review/batteries2012/Coolook%2014500%20LiFePO4%20700mAh%20%28Black%29%20UK.html

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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(11.1 - 3.3)*the current

"the current" is the current consumed/drawn by your AVR or ARM device and its externally connected peripherals. Let's say you have a board that draws 50 mA and a connected display that draws another 50mA... that makes a total of 100mA. So your linear regulator would have to tolerate/handle/dissipate (11.1-3.3)*0.1 = 780 milliwatts while your "load would only be handling/consuming 3.3*0.1 or 330 milliwatts. Of course as Joey has pointed out, you could replace the linear regulator with a switcher design and reduce that 780mW to around 80mW; a big saving indeed. But if you only need 3.3 volts, a 3.2v LiFePO4 battery can run your design without the need and expense of any regulator.

 

I agree though that if you need 5 volts you need another approach. In my case I use a boost switcher and go from 3.2 volts to 5 volts.

 

The "lesson" to be taken from this discussion is that you need to consider your actual needs and try to minimise wastage of power and money... at least that has always been my approach. YMMV.

 

Cheers,

 

Ross

 

Ross McKenzie ValuSoft Melbourne Australia

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Semiconductor devices are thermally limited. Even though you are gelow the voltage spec, that doesn't mean you are exempt from the thermal spec. At a guess a sot223 package has a temp rise of 80C/Watt. The reg was wasting 0.4 times 7.8 which is a few Watts times 80 means you've gone beyond the max of 125C. The thermal protection hopefully kicked in a saved the show.
Note: use a fuse on the LiPo. It can deliver enough current to melt pcb tracks and wires if there is a short circuit.

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joeymorin wrote:

3.63 maximum input voltage for the ATSAM D21,

Take care with the 3.2 V lithium 'AA' batteries.  They have a different chemistry and use a different charge profile than does your 11.1V pack.  You'll need a charger designed for LiFePO4 chemistry.

 

This page might be helpful:

http://lygte-info.dk/review/batteries2012/Coolook%2014500%20LiFePO4%20700mAh%20%28Black%29%20UK.html

Thanks for the heads up. I will be sure to pair up the batteries with a proper charger.

 

valusoft wrote:

(11.1 - 3.3)*the current

"the current" is the current consumed/drawn by your AVR or ARM device and its externally connected peripherals. Let's say you have a board that draws 50 mA and a connected display that draws another 50mA... that makes a total of 100mA. So your linear regulator would have to tolerate/handle/dissipate (11.1-3.3)*0.1 = 780 milliwatts while your "load would only be handling/consuming 3.3*0.1 or 330 milliwatts. Of course as Joey has pointed out, you could replace the linear regulator with a switcher design and reduce that 780mW to around 80mW; a big saving indeed. But if you only need 3.3 volts, a 3.2v LiFePO4 battery can run your design without the need and expense of any regulator.

 

I agree though that if you need 5 volts you need another approach. In my case I use a boost switcher and go from 3.2 volts to 5 volts.

 

The "lesson" to be taken from this discussion is that you need to consider your actual needs and try to minimise wastage of power and money... at least that has always been my approach. YMMV.

 

Cheers,

 

Ross

Thank you. I figured this was but merely wanted to be sure. I've certainly learned a lot here from you both and I really appreciate the input.

 

The needs for a design has been a struggle for me as I am just learning this in my free time and gaps in my knowledge like this are filled as I go.

I realize I've probably made some pretty bad mistakes with some of the things I've done but I find electronics to be very overwhelming at times.

 

But I've been learning so many things and this site has been a pivotal role in increasing my knowledge with electronics.

Every day I learn something new about design considerations and these are things that I ensure I consider in my future designs.

 

Again, thank you both for the fantastic information and explanations. I've learned some good stuff today!

 

Kartman wrote:
Semiconductor devices are thermally limited. Even though you are gelow the voltage spec, that doesn't mean you are exempt from the thermal spec. At a guess a sot223 package has a temp rise of 80C/Watt. The reg was wasting 0.4 times 7.8 which is a few Watts times 80 means you've gone beyond the max of 125C. The thermal protection hopefully kicked in a saved the show. Note: use a fuse on the LiPo. It can deliver enough current to melt pcb tracks and wires if there is a short circuit.

Makes sense. As explained above, these are definitely design considerations that I need to ensure that I am following. Also, yes, for the most part it seemed the thermal protection was kicking in. But as I was testing the last few days to figure out where the problem might be, I think eventually the regulator just had too much and shorted out the input to the output.

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

Last Edited: Fri. Mar 4, 2016 - 05:47 AM
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400 mA is a lot with a 7.8V drop.  Let's say another 50 mA for the rest of the board.  It might be more.  But let's say 450 mA total.  At 11.1V, the reg has to dump 7.8 * 0.45 = 3.51W.  Note that your 11.1V pack will be as high as 12.6V after a full charge, but we'll ignore that.

 

The datasheet has a graph which shows the maximum power dissipation vs. case temperature:

 

 

So at 3.51W you need to keep the case (regulator device package) below about 75°C.  Without a heat sink, that will be difficult.  You'd likely need a fan, and even that likely won't be enough.

 

Some other important factors are the junction to case thermal resistance (θJC), and junction to ambient thermal resistance (θJA).

 

Each depends on which package you have (SOT89, SOT23, or TO252).  θJC is 10°C/W for the TO252.  That means that for a given case temperature, the junction temperature (temperature of the silicon inside the device) is 10°C hotter for every watt.  At 3.51W, the junction will be 35.1°C hotter than the case.  If you can keep the case at 75°C, fine.  It will run below the 125°C recommended maximum.

 

A more important number is θJA.  With no heatsink that's 100°C/W.  So the junction will rise to 351°C hotter than the ambient air.  At 25°C room temperature, the junction will quickly rise above the 160°C thermal shutdown point.

 

It's better with heat sinking, but bottom line is 3.51W is >>way<< too much for this little beastie.  The other packages are even worse.

 

Note that 'heat sink' in this context, since this is an SMD part, is a minimum "100mm2(10mm*10mm) copper (top side solder mask) on 2oz.2 layers FR-4 PCB with 8*0.5mm vias."

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Fri. Mar 4, 2016 - 06:03 AM
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Note: use a fuse on the LiPo. It can deliver enough current to melt pcb tracks and wires if there is a short circuit.

You're not kidding:

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Wow, joeymorin. Fantastic information. Thanks for the good explanation about some of the details from the datasheet as well.

 

Also, as you said, with the battery fully charged, it's typically closer to 12.6 (barely ever seen it go below 12.2, even after quite a bit of usage). So with that, I'd say the board is probably closest to about 480 milliamps (400 for the LCD, 30 for the power indicator LED, and probably about 50 as you said for the CPU). So that (12.6-3.3)*0.48 would be about 4.464 watts. So almost 450 C? Yup... crazy ridiculous.

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

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The battery will give way more than 42A into a short circuit! As well, these batteries get a tad upset when mistreated. Youtube has plenty of vids.

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Kartman wrote:
The battery will give way more than 42A into a short circuit!

I once had a 11.1V LiPo short-circuit on the passenger seat of a BMW Z3 while the car was left unattended. The remarkable thing is that the car was not burnt out completely but it was full of a white powder and there was a rectangular shaped hole in the seat that went all the way down to the floor where the battery had melted its way through. Yoiks :-(

 

Anyway to OP I'd say that any decent model shop will have 1S, 2S and 3S packs so at the very least get a 2S pack to replace this 3S so you are only looking at a maximum 8.4V (nominally more like 7.2V) to drop to 5 or 3.3.

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The battery will give way more than 42A into a short circuit!

It sure can.  I was only showing what the battery is rated for.  It could turn a PCB into a cinder without breaking a sweat.

 

As well, these batteries get a tad upset when mistreated. Youtube has plenty of vids.

Most recently, an e-cigarette exploded in the someone's pocket while paying for something (e-cigarette refills?) at a gas station:

https://www.youtube.com/watch?v=k1LjSuq0rk8

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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clawson wrote:

Kartman wrote:
The battery will give way more than 42A into a short circuit!

I once had a 11.1V LiPo short-circuit on the passenger seat of a BMW Z3 while the car was left unattended. The remarkable thing is that the car was not burnt out completely but it was full of a white powder and there was a rectangular shaped hole in the seat that went all the way down to the floor where the battery had melted its way through. Yoiks :-(

 

clawson wrote:
Anyway to OP I'd say that any decent model shop will have 1S, 2S and 3S packs so at the very least get a 2S pack to replace this 3S so you are only looking at a maximum 8.4V (nominally more like 7.2V) to drop to 5 or 3.3.

It seems even at 8.4v, that may still be too much for my 3.3v regulator? As joey pointed out above, thermal shutdown is about 160 C. At 8.4v and 480 milliamps, I'm looking at (8.4-3.3)*0.48=2.448 watts. At 100C/W, it would still easily reach well over 250 C. If I continue to use the boards with the regulators, it seems my best is to use something closest to 5 volts input (4.5 volts being the minimum). So three 1.5 volt AAs seem they would work good for that but I'm not sure I can find a 1.5 rechargeable. I will continue to look around.

 

My best for the future design of this specific board is to remove the regulator and run it off of a single 3.2 volt battery as Ross pointed out.

 

What do you folks think?

 

[Edit] Meant to say AA 1.5, not 1.2

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

Last Edited: Fri. Mar 4, 2016 - 03:57 PM
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Jamison wrote:
It seems even at 8.4v, that may still be too much for my 3.3v regulator?

So get 1S for when you want to supply 3.3. Use 2S when you need 5.0V. Peak voltage at 1S is 4.2V with 3.7V nominal.

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clawson wrote:

Jamison wrote:
It seems even at 8.4v, that may still be too much for my 3.3v regulator?

So get 1S for when you want to supply 3.3. Use 2S when you need 5.0V. Peak voltage at 1S is 4.2V with 3.7V nominal.

Yup, that sounds good! I'd probably have to slice my existing regulator from my design, as the minimum voltage it needs is 4.5.

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

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To be honest you may have a job to find a regulator that can take an input as low as 3.7 to provide 3.3 on the output.

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clawson wrote:

To be honest you may have a job to find a regulator that can take an input as low as 3.7 to provide 3.3 on the output.

Yup, just a bit of searching on DigiKey proves a bit difficult.

 

So it seems I'd have a few options here...

* Power by three AA 1.5v battery pack providing 4.5v (found a couple sites that have 1.5v rechargeable). Tested this previously with regular AA 1.5v batteries with this specific regulator and had no issues.

* Knock out the regulator entirely and use a 3.2v LiFePO4 as suggested by Ross.

* Use 5 volts via usb power while testing (not viable for a completed, portable product)

 

Thank you all for the awesome advice. Fantastic as always and I've learned a lot here!

 

[Edit] Fixed a few incorrect words

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

Last Edited: Fri. Mar 4, 2016 - 06:31 PM
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As I was interested to know what the solution to this was myself I just googled "how to drop lipo voltage to 3.3v" and got some really terrific results. Seems you are not alone in this quest.

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clawson wrote:

As I was interested to know what the solution to this was myself I just googled "how to drop lipo voltage to 3.3v" and got some really terrific results. Seems you are not alone in this quest.

Yup, lots of different people wondering how to drop it down to 3.3v but a lot of the results seem to be from a 3.7v-4.2v LiPos. Honestly, I'd rather just get rid of the LiPo. It's too much for what I am using anyway.

My digital portfolio: www.jamisonjerving.com

My game company: www.polygonbyte.com

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Remember the that 3.7V rating of a LiPo is 'nominal'.  What 'nominal' means is different for every battery and manufacturer.  It is based on a specific rated discharge current.  Discharge currents are rated in units called 'C', which represent the current required to (nominally) completely discharge the LiPo.  So for an 850 mAh LiPo a discharge current of 850 mA is 1 C, and a discharge current of 1.7 A is 2 C, etc.  However, both the mAh rating and the voltage specified by a manufacturer are generally based on a much lower discharge current, often 0.1 C or 0.05 C.  In the case of the latter, an 850 mAh LiPo will be able to provide only 42.5 mA at 3.7V over the stated capacity of the cell.  If you draw more than 0.05 C from such a battery, it's voltage will drop below 3.7V long before the cell is considered 'empty'.  In effect, there is charge  still left in the battery but you can't extract it at a higher current without the battery voltage dropping.  Few manufacturers even specify under which conditions their claims of capacity are measured.

 

Ultimately, you want to look at the datasheet for a given LiPo in concert with your requirements before making your selection.  Often datasheets lack the kind of detail you want, so you can look to third-party testing.  As an example (although the nominal voltage is different at 3.2 V) take a look at the link I posted in #10.  You'll want to select a LiPo which will remain at a high enough voltage under the expected current load, so that the voltage delivered to your selected regulator is above the sum of the regulator output plus the regulator's voltage drop (at the required current load), and to do so for as long as you require before needing a recharge.  Note also that the voltage drop on a regulator, even an LDO, is dependent upon the output current, so look at the datasheet for that, too.  Higher currents spell higher regulator drop out.  The reg you mention in post #9 has a drop out of 1.1 V @ 450 mA @ 25°C, and 1.0 V @ 100°C.  There are better LDO regs, but you aren't likely to find any with a 0.4 V drop out at 450 mA.

 

I'd say that if you want to run a 3.3V device from a nominal 3.7V LiPo, you'll want an appropriately sized buck-boost regulator, such as the ones you can find at the Pololu page I linked in post #3.  This will allow you to use all of the energy in the LiPo while maintaining a solid 3.3V output, and with minimal wasted energy.

 

Using a 3.2 V battery directly may be suitable, but again look at the link in #10.  With a discharge current of 500 mA, the voltage drops below 3.0 V after about 52 minutes, leaving 25% of the battery's energy unusable.  If your device can operate below 3V, then you get at a bit more of the energy.  However if it needs even 3.1V, the run time drops to about 46 minutes, leaving 30% of the battery's energy unreachable.  A buck-boost regulator will solve this.

 

Note that most but not all LiPo and other Li-based batteries include protection circuitry to (among other things) prevent a cell from over-discharging.  Over-discharge (and over-charge, and over-current) spells death for a LiPo cell, and often fireworks, so it's critically important that you have it.  If the cell itself doesn't have it (check datasheet), make certain that the power supply (whether linear, buck, boost, or buck-boost) has it.  Indeed, there are all-in-one solutions like this:

https://www.sparkfun.com/products/11231

... but note that that particular device is only capable of 200 mA at 3.3V.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Fri. Mar 4, 2016 - 10:03 PM
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This is what I use to go from 3.2 to 5 volts. There are lots of suppliers. I have had them as cheap as 99 cents each including delivery!!!

 

http://www.ebay.com.au/itm/DC-DC...

 

 

 

 

I agree it is not perfect... no current limiting.

 

Cheers,

 

Ross

 

 

Ross McKenzie ValuSoft Melbourne Australia