Series vs. Parellel Config. Runtime vs Brightness

I have a 6C batt pack here in front of me, from those old RC cars. It is labled as 7.2 Volts, 1200 mAh. (yes only 1200, I said OLD remember) /ubbthreads/images/graemlins/smile.gif

This got me thinking. I am wanting to make a regulated LED worklight. That will require very long runtimes.
When you configur a battery pack like this RC car one, (series) you get alot of Volts, for alot of power output. Lets talk in flashlight terms now... you get alot of brightness.

Well, if I make a worklight, that have a Converter board such as a BB750. I dont need a lot of volts. As little as 3V would be fine.
So,,, in this situation, shouldn't I config. a batt pack in series/parellel? So that I can get ~3V but with more mAh. ?

If my logic is correct, then why dont we see more xeries/parellel configurations?
Wouldnt a BB750 drive an 1W led just as bight off 3V, as it would 6V?

I want to make something like this mod made by georges80.


I actually had a batt just like that one, but its onld, and no good anymore. /ubbthreads/images/graemlins/frown.gif
It was 4,000mAh. I figure if I use C rechargable batts, I can get more mAh than that.

Any recommendations, comments, suggestions, ideas?

Thanks.

The BB can't take down the voltage from the battery to an LED, but the Downboy can. How many cells do you want to use?
And what are their usable voltage range under load?

Well, I am not sure yet.

Probably some rechargable C batts. Something I can find on the net relitivily cheap.
Can anyone suggest any good rechargable C batts?

As far the BB goes,,, I know it cannot take down voltage, sence it is a stepUP converter. But, that doesnt mean I cant run even 100 C batts, if wired right. /ubbthreads/images/graemlins/smile.gif

Now that I think about it,,, I probably will go with a DownBoy converter, sence it has a greater Voltage range posibility, heck, I could use a car batt if I wanted. /ubbthreads/images/graemlins/smile.gif

But back to my original question.
If two batts are wired in parellel, the voltage stays teh same, and the mAh doubles,, correct?

If that is correct,,, then what would get better runtime?
A DownBoy running off 8 C batts in series. (~9.6V, lets say 2,000mAh) OR 8 C batts connected in series/parellel (~4.8V 8,000mAh) ???
The series/parellel configuration would run longer, right? /ubbthreads/images/graemlins/thinking.gif

The lower voltage pack will run twice as long, BUT will also be half as bright.

Power (watts) is directly proportional to the voltage applied to any given circuit. If you want a 12v 100watt light to run at 100 watts, you MUST apply 12v to it.

Think of your voltage as the brightness and your Amperage as the run time.

Side note: The batteries used in the picture have a problem too. The smaller battery will go dead before the bigger one (capacity) and the bigger one will then ruin the smaller one.

Half the volts is half the brightness.... yes that makes sence, for a regular bulb.

But im talking about using a led and a DownBoy converter.

Sence it trys to supply the same voltage and current to the led,,, shouldnt the light output say the same, with both batt configurations? Until the converter falls out of regulation.?

I used to be into RC cars, if i remember rightly, the batteries were sub-c sized.

Kevin I think that there is only one battery there. The other is the electronics package.
Chris

I think as a rule of thumb the closer your Vin is to the Vout the better the efficiency of your regulator will be - so lets assume you want to run a 1W LED I would propose to use multiples of 3 cells (i.e. 2 strings of 3 cells each) - this way you might not even need to use a regulator but could tune the current to your desired brightness/runtime mix by either the right Vf of the LS or a resistor. If you want to go with a 5W LS you just use 6 cells in series which again should give you pretty much the voltage needed. I did some short trials with 6 NiMh AA cells and one U3T and that worked almost perfect without anything added - YMMV depending on cell chemistry and Vf of the LS used though.

Klaus

If the regulator circuits have equal efficiency, the runtime and brightness will be the same for either series or parallell connections.
Like Klaus said I think the less you convert the voltage the more efficient the cirquit, generally. I have been warned against using more than 4 cells in series, because of the risk of reverse charging when one goes dead.

I heard something about low dropout regulators (LDO), for when the voltages are close to each other. These I think have a good efficiency.

[ QUOTE ]
jtice said:
A DownBoy running off 8 C batts in series. (~9.6V, lets say 2,000mAh) OR 8 C batts connected in series/parellel (~4.8V 8,000mAh) ???
The series/parellel configuration would run longer, right? /ubbthreads/images/graemlins/thinking.gif

[/ QUOTE ]

I think you have your numbers wrong, it should be 4.8V 4,000mAh. That would give you 19.2 watts for each configuration. Bottom line is 19.2 watts either way. No difference except for efficiencies of converters as others have stated.

OOPS! /ubbthreads/images/graemlins/ohgeez.gif Your right 4,000mAh,,, i was thinking parellel.

OK, thanks alot for your help guys. I understand alot better now.

mAh is alittle misleading then. Just cuz you have more mAh doesnt mean it runs longer.

So basically, if I want good runtime, I need to configur the batts so that the Volts stays up at the better effeciency ranges of the converter. Which, what what you all say, is as close to Vf as possible.

[ QUOTE ]
AilSnail said:

I heard something about low dropout regulators (LDO), for when the voltages are close to each other. These I think have a good efficiency.

[/ QUOTE ]

Now y'er talkin'! Let's assume four times 1.2 Volts for our battery, and a Vf of 3.6 Volts. This means that since the .1 Volt 'headroom' requirement has been met, we can expect 3.6 Volts (Vf) over 4.8 Volts (Vbat) or 75% efficiency. If, however, we run a little lower current (lower Vf) or select for the part and drop to three cells per stirng, we can easily guarantee 90 plus percent efficiency with no danger of reverse charging.

In case those are useful features.........

Doug Owen