Help me with my math here - Watt Hours/Runtime

Okay, I've got a question. I'm trying to figure out some watt hour and total runtime numbers for an LED rig I'm building and I need some help. Here's what I've got so far from my experimenting.

The "base" rig is a 5 LED assembly in a unregulated chain. IE, no resistors, bridges, or other circuitry regulating the flow of electricity. Just battery to LED and back again. It's powered by 3 alkaline cells, so it's injecting 4.5v into the LED assembly. But when I put it on a multimeter, it only shows the LED's pulling 2v. Unregulated amps is 225ma. Now, when I put a 10olm resister onto it, voltage doesn't move (at least not that I can tell) but the amps drops to roughly 100ma.

I'm generally familiar with doing watts the old fashioned way with volts x amps = watts, but given the voltage drop I'm seeing with the LED's, and then in turn the amp drop when I add the resistor, and knowing that resisters bleed off extra energy as heat, it now has me completely confused. Given the information I've listed above, with the three batteries running at 4.5v and the 10olm resistor, what's my watt usage for this rig and how many watt hours per amp hour can I expect from it? Ie, what would be my total runtime per available amp hour using a normal alkaline battery? Any help is appreciated.

Oops, I just realized that this should probably go in the LED forum. Mods, if you think it needs moving, please do. Sorry. ^_^;;

You need to supply more information here like type of LEDs, wiring diagram, type of alkaline cells etc. I'm having trouble figuring out what you are really doing.

Looks like he's got some led's being direct-driven by 3 alkalines in series and he's confused because he's not seeing 'enough' voltage.

Voltage sag. When you put a load on batteries, their voltage goes down. The greater the load, the more the voltage goes down. Under load your alkalines are NOT 1.5V. The voltage also goes down as the batteries drain. If your alkalines are not brand new, they're not 1.5V any more either.

Your batteries are dead. Get new ones. 2 volts for 3 1.5 volt alkalines is dead. At less than 1 volt each (3 volts in series) they should be replaced. A 1.0125 Watt load for three alkaline batteries (without a resistor) should not cause that much of a voltage drop (unless you are using something smaller than AAA batteries). If using a resistor, the voltage should change. That's the point of the resistor. Your base rig might have a voltage boosting circuit to regulate the voltage but that's just speculation without more info. Are you sure that the batteries are in series and the l.e.d.s are in parallel? That might be your problem. If the batteries are in parallel, they will read at 1.5 volts. If the l.e.d.s are in series, 4.5 volts might not be enough to run them. By the way, we can't tell you how long your setup will run without knowing what the power source is. All we know is voltage and amperage for operating the l.e.d.s. We have no idea for mAh on the batteries. We can only guess that you're using white l.e.d.s. Red and amber l.e.d.s might only use 2 Volts. It might help to know what l.e.d.s you're using. Please give more info if you want better responses.

Hooked on Fenix said:

Your batteries are dead. Get new ones. 2 volts for 3 1.5 volt alkalines is dead. At less than 1 volt each (3 volts in series) they should be replaced. A 1.0125 Watt load for three alkaline batteries (without a resistor) should not cause that much of a voltage drop (unless you are using something smaller than AAA batteries). If using a resistor, the voltage should change. That's the point of the resistor. Your base rig might have a voltage boosting circuit to regulate the voltage but that's just speculation without more info. Are you sure that the batteries are in series and the l.e.d.s are in parallel? That might be your problem. If the batteries are in parallel, they will read at 1.5 volts. If the l.e.d.s are in series, 4.5 volts might not be enough to run them. By the way, we can't tell you how long your setup will run without knowing what the power source is. All we know is voltage and amperage for operating the l.e.d.s. We have no idea for mAh on the batteries. We can only guess that you're using white l.e.d.s. Red and amber l.e.d.s might only use 2 Volts. It might help to know what l.e.d.s you're using. Please give more info if you want better responses.

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GregY said:

Looks like he's got some led's being direct-driven by 3 alkalines in series and he's confused because he's not seeing 'enough' voltage. Voltage sag. When you put a load on batteries, their voltage goes down. The greater the load, the more the voltage goes down. Under load your alkalines are NOT 1.5V. The voltage also goes down as the batteries drain. If your alkalines are not brand new, they're not 1.5V any more either.

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Well, that's the thing. I thought the same thing that the batteries must be dead or weak, but when I put a tester on them, and it's a load based tester, they test out at 1.5v each, so they're fresh as can be. If I put them under load I don't see any voltage drop. That's what confused me. I didn't see any change in voltage. Well, except when they're connected to the LED's.

Lynx_Arc said:

You need to supply more information here like type of LEDs, wiring diagram, type of alkaline cells etc. I'm having trouble figuring out what you are really doing.

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Well, in regard to type, I have no idea. They're basic white LED's salvaged from an LED puck light. As for the wiring diagram, they're 5 LED's in parallel, either direct drive or with the resistor in line before the LED's.

lordraiden said:

Well, that's the thing. I thought the same thing that the batteries must be dead or weak, but when I put a tester on them, and it's a load based tester, they test out at 1.5v each, so they're fresh as can be. If I put them under load I don't see any voltage drop. That's what confused me. I didn't see any change in voltage. Well, except when they're connected to the LED's. Well, in regard to type, I have no idea. They're basic white LED's salvaged from an LED puck light. As for the wiring diagram, they're 5 LED's in parallel, either direct drive or with the resistor in line before the LED's.

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You could have a bad LED in the mix. What type of batteries are you using AAAs?

Well, I've tested with AAA's, AA's, and D's. All give the same results. All are fresh batteries. Also, I've done the same test on three different identical units. All render the same results. So I'm doubting it's any of my gear, the batteries, etc, especially given the consistent results. Also, oddly enough, when I do an olms test with my multimeter I don't get a 100% reading. It comes out at like 60% of a full open circuit. So there's clearly some resistance there, even when it's a straight circuit.