Basic battery questions (now plural!)

[orangeterran]

EDIT: more questions added to my thread in this post:
http://www.candlepowerforums.com/vb/showpost.php?p=3074893&postcount=8

/end edit

If I take a higher volt li-po battery and use a step down regulator to lower the output voltage, does the battery have a greater (relatively proportional) mAh rating at the lower voltage?

Thanks!

 

[TorchBoy]

Re: Basic battery question

Assuming you're talking about a buck driver not a linear regulator then maybe. It would depend on the efficiency of the driver and how much you were stepping down.

 

[flatline]

Re: Basic battery question

Kinda. You'll be pulling lower current from the battery which allows the battery to deliver a higher proportion of it's capacity.

 

[orangeterran]

Re: Basic battery question

Thanks!

After looking up the specs of the AX3022 switching regulator (some Engrish to navigate, but manageable)

12V to 3.3V(2A) Efficiency: 80% (1A)

12V to 5V(2A) Efficiency: 84% (1A)

I'm not familiar with regulators in general, but I believe that is fairly efficient. I don't know if I'll make any gains from stepping down from 14.8V down to 7.2V. From what the web site shows, a larger voltage difference results in lower efficiency.

I'm trying to decide if I should just get a 14.8V battery for dual purpose. Or get a 7.2V battery now and 14.8V battery later.

 

[Mr Happy]

Re: Basic battery question

I'm trying to decide if I should just get a 14.8V battery for dual purpose. Or get a 7.2V battery now and 14.8V battery later.

It really depends on your application. The answer to such questions can never be decided in the abstract.

For instance, you might not need a step down regulator at all if you only want to go from 14.4 V to 7.2 V. A 14.4 V battery is four cells in series, wired like this:

o---|[---|[---|[---|[---o

Whereas 7.2 V can be made by putting the same cells in this arrangement:

    ----|[---|[----
o--|               |--o
    ----|[---|[----

So in principle you can just get four individual 3.6 V cells, or two 7.2 V batteries, and wire them up however you want...

 

[xenonk]

Re: Basic battery question

I'm not familiar with regulators in general, but I believe that is fairly efficient.

80-85% is decent. 90%+ would be excellent.
Some regulators actually manage to do worse than using plain old resistors :sick2: so you really need to know your application.

 

[TorchBoy]

Re: Basic battery question

If you were stepping 14.4 V down to 3.3 V (eg, for an LED or two) at 80% efficiency you'd have almost 3.5 times the capacity of the 14.4 V battery. In theory, at least.

 

[orangeterran]

Re: Basic battery question

Thanks for all the replies, I'm definitely learning!

The only thing I have ordered is the voltage regulator I mentioned earlier (which won't go to waste, I have 12V SLA batteries I'm going to step down). I'm just trying to decide on Li-Po batteries for portability (trying to keep the batteries around a pound or less in weight).

I goofed in my original post. I need 7.4 volts as someone pointed out in battery arrangements. I have two applications in mind.

The first is an external battery pack for my digital SLR camera for extended time lapse sequences which requires 7.4 volts and should not draw more than 1.5 A.

The second is a power source for a bicycle headlight I have yet to buy/build. I've done a little browsing around and from what I've seen it looks like many bike lights tend to use higher voltage.

My reasoning for sharing the same battery pack is because I will be riding the bicycle and hiking to many of the spots where I'll be doing the time lapse photography. :twothumbs

I've been looking at these battery packs:

Polymer Li-Ion Module: 7.4V 10 Ah (74 Wh) Prewired with PCB
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2333

Polymer Li-Ion Battery: 7.4V 10 Ah (74 Wh, 9A rate) with Fuel Gauge
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=4488

Li-ion 18650 Battery: 14.8V 4400mAh (65.12Wh) Battery Pack with PCB
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3600

Mr Happy suggested rearranging cells for intended application. Would it be easier/cheaper to buy single cells for that purpose instead of battery packs? Also, any suggestions for battery retailers is welcome. I have never purchased cells/packs before.

Again, thanks all! lovecpf