What good do 1cell lipos have?

Hi everyone!

In another project I posted on this forum, someone pointed out that a power supply lower than 5V isn't that useful.
Now, a lot of electronics we use everyday use 1Cell Lipo batteries, which have a maximum voltage of 4.2V (unless you want to make fireworks, then you can charge it up to a higher voltage). Although it has a low voltage, it seems to deliver quite a low of power when you realize laptops, smartphones and RC stuff are powered by these batteries.

Now, I'm still quite a noob when it comes to general electronics, and I know that voltage isn't the only thing responsible for high power, but is there a way to step up the voltage of a 1Cell LiPo battery? The first thing I'm thinking about it a voltage regulator (LM7805), but those need the input voltages to be higher than the output voltage...

I would like to know since LiPos are awesome batteries, but a 2Cell can get quite bulky very fast, so it would be nice to be able to power a tiny project with a 1Cell 200mAh battery or something.

Thanks in advance!

You should look into boost/step-up converters, a type of switch mode power supply. It can be done fairly simply with relatively few components if you're not too picky with the output.

Sure... couple of ways:

Change fuses to run the microcontroller at 8mhz off internal oscillator w/bod disabled, run it off battery voltage. This will work fine down to 2.5v. May require modifications to the board, depending on whcih board you're using.

Use a boost converter ( http://www.ebay.com/sch/i.html?_nkw=DC-DC+step+up+converter - search for step down instead for buck converters to lower the voltage without wasting tons of energy)

Switched capacitor voltage doublers, optionally into a regulator - these have worse current handling capability, but are easy to design boards for.

It might also be worth mentioning that 3.7V, not 4.2V, is the nominal voltage of a LiPo.

syntaxterror:
It might also be worth mentioning that 3.7V, not 4.2V, is the nominal voltage of a LiPo.

Depends on the kind of LiPo you have apparently. But the one I mean are the Lithium Polymer ones, and those shouldnt be discharged under +-3.8V. Charged, they have a voltage of 4.2V.

Thanks everyone for the reactions!
Ill look into the boost converters

Michiel_:
Depends on the kind of LiPo you have apparently. But the one I mean are the Lithium Polymer ones, and those shouldnt be discharged under +-3.8V. Charged, they have a voltage of 4.2V.

LiPo is Lithium Polymer.

3.8 is way on the high side. Usually you'd consider 3V to be a flat battery. The fully charged voltage of pretty much any accumulator is always going to "spike"... and as soon as you put any load on it, it drops fairly quickly to the nominal voltage, where it should remain for most of the discharge cycle.

syntaxterror:
LiPo is Lithium Polymer.

3.8 is way on the high side. Usually you’d consider 3V to be a flat battery. The fully charged voltage of pretty much any accumulator is always going to “spike”… and as soon as you put any load on it, it drops fairly quickly to the nominal voltage, where it should remain for most of the discharge cycle.

Im only used to using LiPos from my RC hobby… When the batteries get under 3.8V, the motor shuts off. But I guess that is to protect the battery from high loads pulling the voltage down way to much…

But to be sure then: The nominal voltage of a LiPo IS 3.8V then?

The rated voltage is almost always given as 3.7V... but I think the nominal voltage can go a bit lower. I'm a bit puzzled by what you say about the battery though. RC packs should be a prime example, actually:
A 1-cell Lipo is 3.7V, 2-cells are 7.4V, 3 cells are 11.1V and so on. Those numbers should be familiar when you go shopping for battery packs, yes?
I'd consider any LiPo that measured 3.8V on the terminals to hold a decent charge?

For the discharge curve, you can have a look at this, it should give you a general idea what to expect:
http://www.rcheliresource.com/wp-content/uploads/2009/05/tp-5000mah-pro-power-40c-cell-discharge-curve-graph.jpg

syntaxterror:
The rated voltage is almost always given as 3.7V... but I think the nominal voltage can go a bit lower. I'm a bit puzzled by what you say about the battery though. RC packs should be a prime example, actually:
A 1-cell Lipo is 3.7V, 2-cells are 7.4V, 3 cells are 11.1V and so on. Those numbers should be familiar when you go shopping for battery packs, yes?
I'd consider any LiPo that measured 3.8V on the terminals to hold a decent charge

Thanks for the graph! I just realized that graph (V/A) was in one of the courses I had to study...

Of course, but in the RC world, 3.7V is pretty much considered an empty battery :-X
All the ESC's are programmed to stop working at 4.1, 3.9 or 3.7 I think, and everywhere I hear that you DEFINATELY shouldnt discharge your battery under 3.7V.
Isn't that because of the loads? A load of 50A isnt particularly high, so to avoid the battery to drop below 3V, it might cut off so early?

Huh, that is odd. Of the ESC's i've had, none have had a low-voltage cutoff that could be adjusted above 3.4V... which I would consider very safe, well before risking battery damage. And yeah, the discharge curve is of course readings under load, the resting voltage is bound to be higher.

everywhere I hear that you DEFINATELY shouldnt discharge your battery under 3.7V.

Not correct. 3.0 V is generally considered safe. Many cells have "battery protection PCBs" attached and most of those cut off at around 2.7-2.8 V. See this article for a typical discharge versus capacity curve: http://batteryuniversity.com/learn/article/lithium_based_batteries

It would depend on the load, really. Rule of thumb is, when the voltage ramp down starts to accelerate, it's time to stop.

So Ive been looking around in how to make a step-up converter, but couldnt find much... Anyone who has a tutorial or something on how these work/how to build one?

I think this might help you along:
http://www.dos4ever.com/flyback/flyback.html

Of course, it can be done in much simpler ways as well. You can hop onto your favourite electronics pusher's website, search for 5V boost converter and then just replicate whatever circuit they've drawn up under "typical applications". Here's a dedicated boost converter IC:

Be a bit wary of those though, personal experience tells me the designs have a tendency to get obsolete quicker than you can design your circuit.

I think 3.7V is considered the point at which to land the plane, ie well before you risk total
discharge. For a RC model its complicated by the voltage drops due to high current load,
you have to worry about battery self-heating due to internal resistance times load current,
so you want to worry about the terminal voltage dropping to 3.7V for a freshly charged
cell (0.5V drop inside the cell generating heat)...

Michiel_:
So Ive been looking around in how to make a step-up converter, but couldnt find much... Anyone who has a tutorial or something on how these work/how to build one?

I would suggest buying one on eBay - I couldn't buy the parts for the prices you can get assembled units for shipped for free - See the eBay link. $2-3 each for lowish current is really hard to argue with.

Indeed DrAzzy, that is a great way to go.

The various modules have several switcher chip configurations and can be tweaked to provide a "fine tuning" for your application. The advantages are low cost, designed for general use, decent specs, and compact. The main disadvantages are mounting schemes (or not) and a module often requires more space the a board integrated design, and documentation maybe non-existent.

At very least, these modules are a great way to get relatively high power from a small device for testing/proving a design. You could always copy the module design into your own board if that is your desire, to reduce space, so long as it is not a commercial product, you have no worries.

With a charge controller, and a bank of lithium batteries, you can get pretty much any power requirements met. In fact, I have a pretty nice lithium powered car I use to commute to work in. The biggest drawback...low temp/high temp operation. For most batteries, if YOU are comfortable with the temperature, they will be too! In fact, my vehicle, a Mitsubishi i-Miev, has it's own battery heating and cooling systems to keep the batteries comfortable!

Over the past few years, quality has generally increased coming out of Asia, but I still carefully inspect any assemblies I get. Keep in mind that some of these items might be items that were rejected and either reworked or not, and need to be vetted carefully if they are to be used in sensitive applications.

-fab

DrAzzy:
Use a boost converter ( http://www.ebay.com/sch/i.html?_nkw=DC-DC+step+up+converter - search for step down instead for buck converters to lower the voltage without wasting tons of energy)

How good are those cheap converters from eBay compared to those from Pololu?

And how efficient are they when there's no load, e.g. the Arduino is in sleep mode? It says: " ... no-load 18mA (5V input, 8V output, no-load is less than 18mA ...". Does that mean those converters use 400mAh per day? So after around 2-4 days the LiPo is empty - even when doing nothing.

Zealot:
How good are those cheap converters from eBay compared to those from Pololu?

And how efficient are they when there's no load, e.g. the Arduino is in sleep mode? It says: " ... no-load 18mA (5V input, 8V output, no-load is less than 18mA ...". Does that mean those converters use 400mA per day? So after around 2-4 days the LiPo is empty - even when doing nothing.

If you draw 18 mA, that's 18mA no matter how long you draw it for. What you meant, I think, that leaving it on for a day would use 400 mAh (milliamp hours ). That is an important distinction.

And yes, that's exactly what it means. A smaller converter will usually have lower quiescent current (the XL6009 is probably beefier than you need), and some are specifically designed for low quiescent current - read the specs, this is almost always listed, because everyone cares about it.

The polulu converters are almost certainly at least somewhat better than the cheap ebay ones. Pricey though.