View Full Version : Calculating how long a battery will last

Translucent Daydream

02-19-2010, 02:57 PM

I am going to make a six volt lighting system for a custom flag pole that is going to run off of lantern 6v batteries. (like this http://rocky.digikey.com/weblib/Energizer/Web%20Photos/1209.jpg)

I am trying to determine how to get the bigger bang for my buck, so I would need to calculate how long different variations of lighting setups, neon, LED, etc, would last. I am pretty sure you could do it if you could calculate the total draw from the lights vs the specs on the battery. Is there a decent way of doing this mathematically or otherwise?

beowulff

02-19-2010, 03:19 PM

Do everything in the energy domain.

Your battery will have an amp-hour rating. Multiply times the battery voltage - that's an estimate of the total energy available. You don't want to discharge it completely, but you also don't want to oversize it (it's a waste of money). Pick something like 50% - 75% of the resulting energy.

Then, calculate how much power your lights will take, and multiply by the number of hours they will be on - this gives you how much energy you will need.

Remember that there are always losses - good regulators can be up to 95% efficient, but realistically you can figure on only get 75% of the energy from the battery actually delivered to the lights.

Translucent Daydream

02-19-2010, 03:23 PM

You kind of lost me a bit beowulff. Can you explain what you mean coming from the light side? I mean I follow you as far as amp/hr times voltage. But how would you covert it from milliamps to amps? And the draw of a light is going to be in watts... I can do plug in for variable math pretty well but I don't understand the conversions.

beowulff

02-19-2010, 03:47 PM

Well, a milliamp is 1/1000 of an Amp.

Watts is a unit of power. To find out how much energy you are going to use, multiply the Watts times the number of hours to get Watt-hours. So, if you have a 6V battery rated at 10 Ah, then you have 60 Wh of energy available. If you then use LEDs that run off of 4v and consume 100mA (just pulling some numbers out of the air), they use .4W (4v x .1A) so, your battery could run them for 60/.4 = 150 hours (if everything was 100% efficient, which it isn't).

Clearer?

Translucent Daydream

02-19-2010, 03:50 PM

Not really. Still lost with the watt to watt/hours. I get that you have to determine amp/hours to determine how long it will last, but I am not following you from amps to watts. Sorry, not trying to be dense on purpose. I appreciate the help here though.

beowulff

02-19-2010, 04:02 PM

OK.

Amp-hours (careful, it's not Amp/hours) is a measure of how much current can be delivered for how long. But, this is not Energy. To get that, you need to multiply by the voltage, to get Watt-hours (Amps x Volts x Hours).

So, if your 6v battery is rated at 10 Ah, it can deliver 1 Amp of current (at 6v) for 10 hours, or 10A for 1 hour (of course, this is in an ideal world - usually batteries can deliver more of their potential energy at low discharge rates, but the idea holds). In either case, you are delivering 60WH, just at different rates.

Think of it this way - assuming that you are not on some fancy cost schedule, you will pay the same amount if you use (10) 100W lights for an hour, or (1) 100W light for 10 hours, right?

Translucent Daydream

02-19-2010, 04:04 PM

Okay! I see!

Apex Rogers

02-19-2010, 05:10 PM

Not really. Still lost with the watt to watt/hours. I get that you have to determine amp/hours to determine how long it will last, but I am not following you from amps to watts. Sorry, not trying to be dense on purpose. I appreciate the help here though.

To get from amps to watts, he is using the equation: Power(watts) = Current(amps) * Voltage(volts). The same calculation can be done with amp-hours to give you watt-hours. It sounds like you have a good grasp on how to use the end numbers.

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