batteries connected in parallel

[dedalus5550]

So I get that if I connect batteries in series, they will add up in voltage (two 3 volt batteries will equal 6 volts). But what about in parallel. My understanding is that they will still equal the same voltage (assuming all batteries connected are the same) but that they will have more current or power. I wondered about the fact that I'm gonna have 20 or so LEDs connected to the same source (with varying amount of resistance, as required) to the same 9-volt battery. Will the power source last longer if I use more than one battery in parallel or is it not a big deal with 20 or so LEDs? On the same topic, if 10 or 15 of those LEDs need the same amount of resistance, can I connect them all on the same resistor (or string of resistors that equal the required amount), or does each need its own resistor. Or is this where I can connect them all directly to the battery because there are so many connected that I don't need resistance? Obviously my first foray in lighting, huh?
Thanks,
Mike Todd

[Umi_Ryuzuki]

If you find that the power is dying more quickly than you anticipated, then yes,
you can connect batteries in parallel to "make the gas tank bigger".

As with batteries in series adding their voltage together, 6v 4ahr+6v 4ahr= 12v 4ahr...
LED in series 2v+2v+2v = 6v.... so if using a six volt battery, then
three LED in series needs no resistor at all.

Conversly, if you wire the batteries in parallel, then the amperage adds up.
6v 4ahr+6v 4ahr = 6v 8ahr.
LED in parallel will also draw more amps...

While it is possible to run a single resistor in a parallel LED array, the increased
amperage needs to be accounted for. Otherwise the resistor will over heat
and burn out. So a resistor with an larger wattage may be required.

[USS Atlantis]

So I get that if I connect batteries in series, they will add up in voltage (two 3 volt batteries will equal 6 volts). But what about in parallel. My understanding is that they will still equal the same voltage (assuming all batteries connected are the same) but that they will have more current or power. I wondered about the fact that I'm gonna have 20 or so LEDs connected to the same source (with varying amount of resistance, as required) to the same 9-volt battery. Will the power source last longer if I use more than one battery in parallel or is it not a big deal with 20 or so LEDs?

Well, 20 LEDs at the standard 20ma each is 180ma.
ma ratings of some standard alkaline batteries
AA - 2000ma
D - 12000ma
9v - 500ma

So, each of these will lite those 20 LEDs for
AA - 11.1 hours
D - 66.6 hours
9v - 2.8 hours

Put another battery of the same type in parallel and you effectively add another battery worth of time

On the same topic, if 10 or 15 of those LEDs need the same amount of resistance, can I connect them all on the same resistor (or string of resistors that equal the required amount), or does each need its own resistor. Or is this where I can connect them all directly to the battery because there are so many connected that I don't need resistance?

I would still use a single resistor for each "run" - one string of LED's -> one resistor

Now, you can connect LED's in series so that the voltage equals or is a little lower than the voltage supplied by the batteries

Say you have 6 AA batteries connected in two parallel strings of 3

battery-battery-battery-
battery-battery-battery-

You're supplying 4.5 volts of power

If you're using Red LEDs, they normally need 2-2.2 volts

So you could have

LED-LED-RESISTOR

But if you were using White LED's, they normally need 3-3.4v - so you could only have one per "string"

Unless you're willing to dim the LED's - supplying less than optimal voltage to an LED causes it to dim, producing less than it's max rated light

So in the case of the White LEDs, if you put two in a string, with a minimal Resistor (1-10 ohms), you will still light them, just they will be a bit dimmer than if each was on a seperate string.

All them together? - I don't think they'd light up at all

Even if your LED string "uses" all the voltage available, I would still use a minimal resistor (1-10 ohms) on the string to protects against minor surges - which can happen at times when you take a dead set out and put in a fresh set

Obviously my first foray in lighting, huh?
Thanks,
Mike Todd

Well, there's quite a few of us who know our electronics, so ask away

[dedalus5550]

Well, there's quite a few of us who know our electronics, so ask away

The irony is that I did avionics for 4 years in the military and I was part of a group of guys that would routinely get yelled at for troubleshooting to the component level because we still had a lot of components left over in the shop from the older days when people doing our job would actually do that. (The new thing then was to just get to a circuit board and replace defective boards, but we found that boring and way too easy.) But I left the military and electronics in '92.

Anyway, until the end of the post, you kept saying "use a resistor" without mentioning an ohm measurement. In the whole post, do you mean something between 1-10 ohms?

And what's this wattage rating? The resistors I've got now have the packaging, but it only mentions the three bands for ohms and fourth gold, silver, etc. band which, though I can't remember off-hand what it means, I don't believe is wattage.

Thanks,
Mike Todd

[USS Atlantis]

Ok - the way to calculate the needed Ohms is

R=(Vs-Vl)/a

Ohms=(Source Voltage-Load Voltage)/Current Draw

Example

Source Voltage is 9 volts
LED Voltage is 3 volts
LED Current rating is 20ma (.02a)

9-3=6
6/.02=300

So to run a single LED rated at 3v off a 9v power source, you'd need a 300ohm resistor

When you run multiple LEDs in series, the voltage adds

So 2 3v LEDs in series off the same 9v power source would be

(9-(3+3))/.02 or 150 Ohms

My particular comment is based on running a full load of LEDs off a power source - I always have at least a 1-10ohm resistor, even if the rating of the LEDs in series equals the power source rating

As far as the banding

The 4th band is the % tolerance of the resistor - normally a 5th band has the wattage ratings - if it doesn't have it, I think it defaults to 1/4 watt - which for lighting projects in models should be fine

[dedalus5550]

So, regarding power surges like replacing a battery--I'm wondering how that applies to just turning the power on and off. I mean, I doubt anyone leaves the power on a model 24/7. Even when I see lighted models at shows, there is usually a switch and a note for viewers to please turn off power after they have viewed the effect so as not wear down the battery before judging.
Thanks,
Mike Todd

P.S.--can't wait to get my MF build back on track with my first lighting attempt.

[tetsujin]

So I get that if I connect batteries in series, they will add up in voltage (two 3 volt batteries will equal 6 volts). But what about in parallel. My understanding is that they will still equal the same voltage (assuming all batteries connected are the same) but that they will have more current or power.

Whenever people have questions like these my inclination is to try to teach them as much of the theory behind electronics as I can get them to learn...

Basically, a battery is typically modeled as a voltage source: a device with a consistent voltage across its terminals. (Of course that's not entirely true, voltage decreases if you draw too much power, or as the battery wears down, but it's good enough for an understanding of how to set up the circuit)

Voltage is a sort of measure of potential - how much "push" something is exerting to move charge around in a circuit - or of how much resistance a device is offering to the flow of charge through it. These "forces" have to balance out, like in physics. If there's a 6V source in a circuit, then other devices in the circuit must somehow account for all of that voltage. This is Kirchoff's Voltage Law - the sum of voltage drops across devices in a circuit (that is, a simple loop circuit) is zero.

So first thing - if you connect two batteries in parallel, the voltage has to be the same as the two individual batteries. It can't be anything else. But this also illustrates why you can't connect mismatched batteries in parallel - like a 3V cell to a 1.5V cell, or a fully charged 1.5V cell to one that's been mostly drained... If a battery is connected, in parallel, to a battery of lower voltage, then current from the higher-voltage battery will flow into the lower-voltage battery.

To understand why the two batteries in parallel last longer, I'm calling in Kirchoff's Current Law: the amount of current flowing into any point in a circuit equals the amount of current flowing out of it. (Or, the sum of currents flowing into a point is zero). So when you have two batteries connected in parallel, if the circuit to which they're connected draws 100mA, that 100mA is divided between the two batteries. Each battery will source half as much current as if that battery were feeding the circuit on its own.

If you want to hook up a bunch of LEDs in parallel and give them a single resistor, the things to remember are, first, the voltage drop across all those LEDs will be the same. Generally this means the LEDs involved must be the same type - if you had a 1.7V LED in parallel with a 3V LED, the 3V LED might not operate at all, and the 1.7V LED would likely get too much current as a result. And the current through the resistor will be the sum of the currents through the LEDs (KCL again).

As for battery life - how much power that 20 LED circuit will dissipate depends on how you lay out the circuit. The key is that resistors do nothing but convert energy to heat (wasting it, basically). We need that because it provides a helpful means of regulating circuit behavior, but to some extent we also want to minimize that waste. The power dissipated by a component is the voltage across the component multiplied by the current through it. (Likewise, the power drained from a battery is the voltage supplied times the current sourced - since we usually assume the voltage is constant, it's often sufficient to just refer to the current...)

So if you have a 9V source, and you drive 10 LEDs, each operating at 3V, 20mA, you could give each LED its own resistor (dropping 6V at 20mA) - that would be 1.2W of wasted power for 0.6W of utilized power...

If, instead, you organized the LEDs in pairs (wired in series) and gave each pair its own resistor, each resistor would drop 3V at 20mA - 0.3W wasted for 0.6W used on the LEDs.

Using a single resistor for all the parallel LEDs versus using one resistor for each parallel circuit has no effect on power waste - the only difference is that using multiple resistors gives you the option of customizing resistor values for each individual LED if necessary - and if an LED burned out this would prevent the excess current from dropping into the other LEDs.