# Battery question

Discussion in 'Power Electronics' started by drcarnine, Feb 19, 2013.

1. ### drcarnine

35
0
Mar 11, 2012
I have an LED light that uses 30 LEds. I am replacing them with 5-chip LEDs and they use 100ma. The battery pack is 3.6 volt 2000mah. How long will this battery last with these LEDs? I may connect a 6 volt 4.5ah battery and regulate it down to 3.6 volts instead of the 3AA batteries. . How long can I expect this battery to last? I am also considering only putting in half of the LEDs, Will this make the times above double? Any help will be greatly appreciated.

2. ### CocaCola

3,635
5
Apr 7, 2012
In a perfect world, 2000/100 = 20 hours...

In the real world, probably 12ish hours different batteries will have different drain curves...

3. ### Harald KappModeratorModerator

10,592
2,362
Nov 17, 2011
What voltage are the 5-chip LEDs rated?
Do you need current limiting?
How are you going to regulate from the 6V battery? A linear regulator would waste lots of power, a switching regulator shopuld be used to extend the lifetime of the battery.

For any load (not just LEDs): Time=(capacity of the batttery) / (load current)

In case of a higher voltaeg battery regulated down by a switch mode regulator:
This is due to the fact that a switch mode regulator has less average input current than output current in the same ratio as output voltaeg/input voltage. The factor 0.8 accounts for the losses within the regulator (assuming a typical 80% efficient regulator).

Minus some derating because:
- batteries cannot be drained to 0 capacity
- batteries never have the same capacity as printed on them (usually less)

4. ### drcarnine

35
0
Mar 11, 2012
multiple LEDs

Each LED says it has a max current of 100 ma.. So do I multiply all of the 30 LEDs by the 100 and get 3000mah of current used? If so, would the hours be 3000/2000 or 40 minutes? The voltage of the LEDs are 3.1 volts. I am using low-ripple buck dc-dc step down converters>

Last edited: Feb 19, 2013
5. ### drcarnine

35
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Mar 11, 2012
I didn't see your post before about the 6 volts. I am using low-ripple dc/dc step down converters.

6. ### CocaCola

3,635
5
Apr 7, 2012
That wold be correct, I was unaware you were replacing all 30 LED...

But only use about half the perfect value, 25ish minutes in the real world...

With the regulator you need to factor in the lose of the regulator, and it's quicker to use Watt/hours when you are doing a conversion...

So using your [email protected] you get 27000
LEDs are 3.6*100*30 or 10800

Now factor in your regulator at 60 or 85 efficiency depending on linear or switching... 16200 or 22950 respective...

So you get about 16200/10800 = 1.5 hours
or 22950/10800 = 2.13 hours

And then take about 75% of that perfect value because the 6V buffer will give you more run time before it falls below operational voltage...

Approx...

1.13 hours with a linear regulator
1.60 hours with a switching regulator

FYI just estimates, real world testing will give more accurate results...

Also it's a little late at night and I'm watching TV so my math might be a little off...

7. ### drcarnine

35
0
Mar 11, 2012
So if I read your post right, the 6 volts have no bearing on the capacity. So I just figure the 4.5 ah capacity which is 4500 mah. and divide that by the 3000 mah of the LEDs which gives me 1.5 hours, then take that times 0.8 which equals 1.2 hours. Is this correct?

8. ### drcarnine

35
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Mar 11, 2012
I really appreciateyour help. i think I need to only put in half of the LEDs to start with

9. ### CocaCola

3,635
5
Apr 7, 2012
Read the math again, the 6 Volts does have a bearing...

6V @ 4500mAh = 27,000 Watt/Hours
4.5V @ 4500mAh = 20,2500 Watt/Hours

10. ### Harald KappModeratorModerator

10,592
2,362
Nov 17, 2011
And you need to limit the current. An LED alone will quickly draw too much current if connected to a voltage source.
Unless a current limiter is built into the LED, that is.

11. ### drcarnine

35
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Mar 11, 2012
I was writing my reply when you posetd the math, i do understand now. Where do I get a current limiting device? If I could limit the current to the LEDs, I think could get them to the correct brightness.

12. ### drcarnine

35
0
Mar 11, 2012
Do I just need to install a resistor between the battery and the circuit board?

13. ### drcarnine

35
0
Mar 11, 2012
Can you give me a link and instructions on a current limiting device for my purposes?

14. ### Harald KappModeratorModerator

10,592
2,362
Nov 17, 2011
Last edited: Feb 19, 2013