Need help figuring runtime

I have a 2d host with a 6aa to 3d holder. I would like to try a 64633 bulb using five aa Li-Ion's and one dummy cell. The cells are 3.6 volts and 900 mAh. What kind of runtime can I expect? Is there a formula for this?

Hello Sid,

The basic formula is capacity in mAh divided by the current in mA will give you hours of run time. This is a bit on the high side because the label often does not reflect the real world, and there is some loss due to heat.

Tom

So the 64633 is a 15 V 150 W bulb. What kind of AA size Li-ions do you have in mind, and what kind of battery holder?

Right now, my estimate of run time is about 0 seconds

Run time is about 15 minutes. That's how long it will take the blown up parts of the light to cool down.

So the 64633 is a 15 V 150 W bulb. What kind of AA size Li-ions do you have in mind, and what kind of battery holder?

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The batteries will be UltraFire 14500 AA sized 3.6V Li-Ions as seen here..
http://www.batteryjunction.com/14500.html

The holder was made by a very well know member here.

Run time is about 15 minutes. That's how long it will take the blown up parts of the light to cool down.

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I"m fairly new at this, is this tongue in cheek or could this happen? If so, please explain. Thanks....Sid

I assume you are using a 2D holder and not a 3D holder as mentioned.
What configuration is the holder? Like 6S, 3S2P, etc, 6 in series will yield 22.2v minimum, 3 series 2 parallel will yield 11.1 minimum but with double the capacity.

Your bulb pulls 10amps, if the cells can handle that (I really doubt) and the bulb doesn't instaflash you might get 5min tops.

sid2 said:

I"m fairly new at this, is this tongue in cheek or could this happen? If so, please explain. Thanks....Sid

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It's a gentle hint that your design is not feasible, for several reasons.

Firstly, lithium cobalt ion cells (the regular kind) are good for a 1C or 2C discharge rate. So maybe you could get 2 amps from a 900 mAh cell at most. If you tried to draw 10 amps from it the cell would not like it and its working life would be short.

Secondly, when you construct a 10 amp hotwire, you will need every resistance mod possible. Each additional ohm of resistance in the circuit path is going to drop 10 V from the available voltage and dissipate 100 W as heat. 100 W being generated in the non-light-emitting parts of a light can be very entertaining while it lasts, but the entertainment would not last long. You will want copper or aluminium conductors everywhere, no steel parts in the current path, and good solid low resistance electrical contacts.

In general, to drive a 150 W bulb you would want bigger and different batteries and probably a bigger host. A higher bulb voltage to keep the current down would be beneficial too.

I assume you are using a 2D holder and not a 3D holder as mentioned

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It is a 2d holder my mistake there.

I really appreciate the answers I've gotten here. I realize there's more to it then just throwing some batteries and bulbs together. My education here on the forums is just beginning. I thank you for your patience...Sid

Try to get some Li-polymer batteries. They can handle currents of up to 10C or more, so a current of 10 Amps is no longer a problem. They are used to power RC helicopters and cars, the brushless DC-motors in them can draw currents that are easily in excess of 100 Amps!

Then, consider a softstart circuit using monstrous MOSFETs (Rds on < 4milli-Ohms) and, when in doubt, take 2 in parallel.
These FETs can handle currents of more than 30 Amps continuous easily, spikes of several hundred amps don't even destroy them :huh:

This assumes some knowledge of electronics though.

For an easy start, you might look for a real big host, and then put in a cheap lead-acid battery of 12V / 7,2 Amps. The standard-one for alarm-systems.

With these enormous currents, you'll get half of the capacity at the very best, so SWITCH OFF the lamp after 15 minutes, although it might seem that the battery is not dead at all! Recharge it as soon as possible after use.

Want more runtime? Simply take two of those batteries in parallel (EQUALLY CHARGED before you do this!!) and you'll have more than doubled the runtime (because of the current per battery cut to half - this reveals more capacity per battery!)

OR:

Take a 24V bulb of the same Wattage, and connect the two batteries in series.


However you plan to do it: have fun :thumbsup:

Timmo.

LiPo is not a reasonable recommendation at this point. Yes they are rated for higher discharge rates and with proper precautions can be implemented for use in a flashlight... but they have a history of exciting explosive tendencies in the RC community.

Analogy time..

Someone asks if they could use this bottle of hairspray to get the charcoal going for their grill. Some people pitch in and point out that this isn't a very good idea as it could be dangerous for several reasons, and really wouldn't work for various reasons also. Then someone comes along and suggests just picking up a convenient gallon of gasoline for this endeavor... Besides! It's gasoline after-all... it's designed for making fire happen eh?

Eric

Pound for pound. (speaking on a standard consumer cell level excluding A123 cells) Nickel Cadmium batteries will give you the most current bang for your buck. NiCd cells have Extremely high discharge ability. Much higher than NiMH. I'm not sure though that you would get the required power out of such a small size.

Personally I would try starting out with something closer to a Mag85 rather than a multi hundred watt HID killer. Those large bulbs are better suited for large custom lights, large reflector lantern sized lights and the like not 2D maglights.