I 'm trying to figure out the battery life for my lighting project, and I 'm hoping you guys ca...

[G+_52Degrees]

I'm trying to figure out the battery life for my lighting project, and I'm hoping you guys can help me out...

 

I have a Lohas 100 watt LED chip, which is being powered by a 5200 mAh 3C lipo.

 

I used an LM2577 power converter to get from ~12v to 35v.

 

While simply knowing the answer would be nice, I'd really like to know how the math works, so I can do it myself in the future.

[G+_Carlton Dodd]

I don't know all the math, but Volts * Amps = Watts, so your (nominally) 100W LED array will be drawing 100W/35V= roughly 3A.  Without measuring several things, you're going to be estimating.

After that, you need to know the efficiency of your power converter to figure out the Amps in for that Amperage out.

Once you have the Amps drain, you can figure out the (again, estimated) battery life.  5200mAh means it can supply 5200mA (5.2A) for an hour, or 2600mA for 2 hours, or 10400mA for half an hour, etc. 

Let's assume your converter is drawing 5A on the input side; then your battery would last about an hour.

Look up Ohm's Law for more info.  It's very useful surprisingly often.

[G+_Dan Hockey]

The math behind it is called ohms law. E=IxR.

In simple terms volt x amps = watts. If you hook up an amp meter to your circuit and measure the amperage while it is running you can calculate the wattage.

You can then calculate how long your battery will last.

5200mah = 5.2Ahr.

You will find out that batteries do not always put out the exact ratings the label says it will.

Get the basics down first before trying to understand how Lipo's behave.

[G+_Steve Martin]

You want to be sure of your batteries max output amperage for sure. At ~3A you are going to be generating a lot of heat. If the Battery is not rated for that much output you may burn it out fast, and have a fire hazard.

 

Note also that the LM2577 is rated for 2.5 Amps Maximum. So you are going to be pushing it to the limit if not beyond.

[G+_52Degrees]

Ok... I generally have a pretty good grasp of "the basics" as you so eloquently put it, but it's been a minute since I had to think about this...

 

And I'm grumpy, so I may think you were being a jerk when you weren't. Lol.

 

So, here's the real question: excluding losses due to inefficiency, are the watts on the low side of the converter equal to the watts on the high side? It seems to me they would be, but I'm just not remembering clearly.

[G+_52Degrees]

Yeah... I know I'm pushing it, but it was cheap and small, and worked very well for my proof of concept.

 

The battery is a 5200mAh 3S 30C. I guess I screwed that up, too. Haha.

[G+_Carlton Dodd]

52Degrees

A 30C battery means that you can pull at 30x its rated hourly capacity...supposedly.  But, even being conservative, and taking it at 10C, you'd be okay drawing 52A (for 6 minutes).

I know my quads can draw 5-10A steady, and 20A or so peak, from my 1500mAh 30C battery.

[G+_Jeff Brand]

The battery is fine for the purpose, if a little short lived.

 

Watts should be the same on both sides minus the loss of the power conversion.

 

Also, you might be OK running the LEDs at 2A. Smaller LEDs have a wide sweet spot for their brightest operation. The big boys might be the same.

[G+_52Degrees]

Thanks for the help, everyone.

 

I know it works... I've been showing it off for a week. However, I'm showing it to a friend today who has an actual real-world application for a complete solution and I'd like to give him a ballpark figure for actual runtime.

 

I know it's not a lot of load for the battery. I understand that the power converter is small for longevity.

 

What I'm trying to figure out is how long this thing will provide light with a 5200mAh battery so he will know if he wants to go to the trouble of building several finished units to suit his need.

[G+_Jeff Brand]

Carlton's first post gives you the breakdown. Put another way, 100 watts at 12 volts is close to 8 amps. That battery will optimally last 5200/8000 of an hour.

[G+_Jeff Brand]

True. If you're forgetful, use one of the battery meter/alarms on the balance plug. (http://www.banggood.com/Wholesale-1S-8S-Li-po-Battery-Voltage-Tester-Checker-Indicator-Monitor-Buzzer-Alarm-p-26049.html)

 

They'll start to beep at 3.3V/cell (configurable) so you can turn it off before doing damage.

[G+_Bill Boelke]

as mentioned above, you can only run LiPo to half voltage before ruining the cells, but here are some standard calculations regarding amp-hours and the items you've listed:

power = volts x amps (watts = volts x amps)

100watts = 35v x amps (now solve for amps)

100W/35V = 2.857A = 2857mA

1A = 1000 mA (a conversion to milliamp from amp)

If your battery puts out 5200mah or 5200 milliamp hours then 5200mAh / 2857 mA = 1.82h (hours)

This does not take into consideration the device you've used to convert the battery from 12v to 35v. You'll have to check what it's input vs output power requirements to get a true picture of what you can expect. You may want to investigate using a speed controller to provide the power from your battery so that when it gets to half power it automatically shuts down.