1111 vs 5761 Runtimes Question

[scottaw]

Sorry if some of this has been asked before, i looked for an hour or so and couldn't find my answer.

I currently own a Mag11 powered by 6x eneloop, 6x powerex 2700, or 6x titanium 1800. All 3 sets are in FM carriers. I'm also running AW's D-Driver.

How can I calculate the runtimes with each of these setups? I know there's some kind of formula rather than just running each set down.

Now if i switch to the 5761 i know it's a lot more lumens, how does this affect runtimes? I'm assuming they'll be much shorter, but could this be an even better setup?

Thanks in advance guys, i'm new to hotwires and really trying to learn.

 

[soupdragon]

Short answer

with the eneloops about 25 mins with the WA1111 and about 15 mins with the 5761 but you might wanna know how I figured this see below

Long answer

The way I tend to figure runtimes in with Watt Hours

1 Watt Hour will run a 1 Watt bulb for 1 Hour or a 60watt bulb for 1 min

Battery Capacity

To get the Watt hours from a battery use the following

P(Power in watts) = I (current) x V (voltage)
If you substitute I for the batteries rating which is in maH (miliamp hours) you can get PH or Watt Hours

So for the eneloops 2000maH = 2 aH (amp hours)

2ah x 7.2V = 14.4 WH

Bulb wattage

To get bulb wattage we use the same formula

P(Power in watts) = I (current) x V (voltage)

WA1111 which at 7.2V draws 3.8 amps (taken from lux luthors testing)

3.8a x 7.2V = 27.4 Watts

The 5761 which at 7.2V draws 5.5amps gives

5.5a x 7.2V = 39.6 Watts

Runtime

If we divide Watt hours by Watts we should be left with hours

WA1111: 14.4 Watt Hours / 27.4 Watts = 0.53 Hours so 31 minutes

5761: 14.4 Watt Hours / 39.6 Watts = 0.36 Hours so 21 minutes

This is an idealised way of looking at things. In reality the battery will have different caracteristics at different currents. Bulb voltage and current will vary depending on the resistance of the light. So either find a runtime test for the battery and measure bulb voltage and current or take about 80% of the idealised figures.

A quicker way with less sums (as you can eliminate most of the figures) if you assume bulb voltage is the same as battery voltage then just divide battery capacity by bulb current but this is less accurate

IE WA1111 2aH/3.8a = 0.53 Hours

Not quite sure why I went to all the math lesson trouble I think its because my back is hurting and it takes my mind of things

 

[FredM]

This is an idealised way of looking at things. In reality the battery will have different caracteristics at different currents. Bulb voltage and current will vary depending on the resistance of the light. So either find a runtime test for the battery and measure bulb voltage and current or take about 80% of the idealised figures.

This problem becomes a larger factor the higher the current draw is.

for instance up to 3 amps the eneloops do pretty well but then they start to fall off.

 

[scottaw]

Thank you so much soup, that's the most math i've looked at since college. It may be idealised, but it gives me a good handle on things.

btw...i have back troubles as well and I highly recommend a chiropractor...but that's off topic.

Ok so with these things answered, my 2700mah cells should give me some better runtime, is there anything higher, or coming out soon? 3500mah etc..?

 

[FredM]

Actually I think your eneloops might do the best. Check out threads of interest in the batteries forum. There is a NiMH test that shows all the batteries at different current rates.

 

[FILIPPO]

if you are interested in another type of cells:

http://kaidomain.com/WEBUI/ProductDetail.aspx?TranID=3069

I suggest you to read some thread (here on CPF too) and learn how to charge and prperly use this batteries becouse thay can be dangerous...:thumbsup:

 

[soupdragon]

Yep I'd also be tempted to say that the eneloops would be best

On paper using the information I posted the 2700maH batteries would be best but in reality its quite different

If you have a look at this thread http://www.candlepowerforums.com/vb/showthread.php?t=79302 then you can see how different batteries perform at different current rates.

The best 2700maH batteries will probably only give 2000maH at 5amps and the voltage can tend to sag a fair amount. Whereas the eneloops are good performers at high current drains even so you realisicly only get about 1800maH out of them.

But at the moment thats about as good as it gets for AA nimh. Li-ions do offer more capacity but there are drawbacks. Then there are the safer technology Li-ions such as A123 and E-moli cells which are somewhere in between nimh and normal li-ions in capacity but with fewer safety concerns.

Incidently how does the WA1111 perform on medium and low settings with AW's D driver does it retain a nice white beam?

 

[scottaw]

Sounds like I should just stay with the tried and true eneloop, a favorite for a reason around here.

Funny you should ask that about the 1111 whiteness, I took my dogs out for a walk last evening and grabbed the 11, haven't really used it all that much since I put the D driver in. Man was i ever dissapointed by the low beam. It looked like it was shining through a brown filter. Not noticeable indoors, but it looked terrible outdoors. But medium had a nice "incan-ish" (just made up a word) light yellow with a decent color rendition. And high of course was a perfect white as expected from the 1111.