Bulb discharger - a few questions???

[technics100]

I going to build a bulb discharger and have a few questions. I have read that you should discharge at 20 amps

A tail bulb uses 2 amps, so I would need 10 bulbs..

What wattage of bulb do I use. In America they use a 1157 bulb which is 27/8w (i think) what is the uk equivalent? is it 21/5W?

will this draw the same amount of amps?

does this equation apply

amps = watts x voltage

so

27 watt bulb / 12 volts = 2.25 amps

but we are discharging to get the battery down to 0.9 volts per cell = 5.4 volts for a 6 cell battery

27 watt bulb / 5.4 volts = 5 amps

is this bulb now using 5 amps of current???

then we would only need 4 bulbs???

Am I just wrong or missing something?

Any help is appreciated...

cheers

[DJTheo]

What remains constant on a bulb is its resistance, power (watts) is dependent of the voltage.

As you calculated correctly @12V the current is 2.25A.

Resistance is R=V/I=12V/2.25A=5.33Ohm

You want to discharge at 7.2V with 20A so you need a total resistance of R=7.2V/20A=0.36Ohm.

You need a parallel connection to reduce the resistance, the total resistace for n paralllel resistors is 1/Rtot=1/R1+1/R2+...+1/Rn

if you do the calculation you will see you need approx. 15 bulbs.

Cheers

[screwbag]

I think you might be worrying about the technical side of things a bit too much here. When I used to race I just used the 1157 automotive bulb with a tamiya connector and hooked it up until the light went out. I did this for years and my batteries didn't suffer one bit they all worked great for a long time, hope this helps a little I would recomend it, it's a great idea.

[Jozza(2)]

See http://www.rccaraction.com/rc/articles/htB...ischarger_1.asp for a good article on building a bulb discharger.

HTH,

John

[WillyChang]

quote:Originally posted by DJTheo

What remains constant on a bulb is its resistance...

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Not quite, tunsten filament's resistance changes with heat

ie voltage difference & how bright it glows...?

To keep things simple, put an ammeter in circuit and just keep adding

more bulbs until you reach the desired current. []

Use both filaments too, if there's 2 (brake bulb).

[DJTheo]

The thermal resistance change coefficient of metals is pretty small, so it can be neglected, especially as with 7.2V the bulbs dont heat up much. Why does he need an ammeter if I already calculated him a good approach to 20A? Thats why we have science, do you think Airbus or Boing starts adding turbines till an airplane flies? []

[Twmaster]

quote:Originally posted by Jozza

See http://www.rccaraction.com/rc/articles/htB...ischarger_1.asp for a good article on building a bulb discharger.

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Nice little article. I only saw one problem. They quoted the parts list with 10 x #1157 (2 filament) bulbs. The bulbs shown are #1156 (single filament) bulbs.

I also would have used brass strip instead of tube and staggered the bulbs making it easier to swap a bad bulb out (you know they will fail over time from use or rough handling). But that is just my opinion.

[terry.sc]

quote:Originally posted by DJTheo

What remains constant on a bulb is its resistance

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id="quote">id="quote">

When you plug the battery to the bulb the resistance is zero, and the high current then warms up the filament until it glows. The hotter it gets the resistance goes up, until it stabilises. When the bulbs go out as the battery flattens the resistance drops, until it shorts out the battery when it is flat. This is why you should NEVER let the bulbs go out while discharging as it can damage the battery pack.

[terry.sc]

Ideally you should also use a discharge cut off device which disconnects the battery when the voltage drops to 5.5v (0.9v per cell)

[DJTheo]

quote:When you plug the battery to the bulb the resistance is zero, and the high current then warms up the filament until it glows. The hotter it gets the resistance goes up, until it stabilises. When the bulbs go out as the battery flattens the resistance drops, until it shorts out the battery when it is flat. This is why you should NEVER let the bulbs go out while discharging as it can damage the battery pack.

id="quote">id="quote">

You are right that the resistance increases with increasing temperature, but its never zero, except at absolute zero (-273C), it would be great if it was, as people need to cool down metals to that tempearutes to make supraconductors. Also if it was zero, the filament wouldnt heat up, as resistance generates heat.

The above calculations make the assumption that the filament temperature when heated with 7.2V isnt too different than with 12V, if we would know both temperatures we could correct them with a factor.

A empty battery would get shorted even with a high resistance, this isnt bulb specific, you should in each case disconnect the battery as you said at 0.9V per cell.