12v battery load testing

I wish to rig up a meter to test our launch controller batteries. follow with me and tell me where i am wrong...
12V battery(both nicad and sealed lead acid)
2 amp load assumed (estes igniter) 6 ohm resistor to be placed in parallel with meter meter has its own drop resistor to be put in series to match voltage scale 6 ohm resistor needs to be 24 watt power resistor(or larger).
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Tater
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tater schuld wrote:

Don't know quite what your 'question' is, but a couple of comments:
A charged gel-cell will be about 14 volts. Use that in your maximum power dissipation calculations (i.e., use 14V with your 6 ohm resistor as a worse-case).
Power dissipation of resistors is typically given at room temperature. However, when your resistor is dissipating that much power, it won't be at room temperature (<G>). In a fan-cooled environment, it is 'normal' to derate by a factor of two. In a non-fan-cooled environment, derate by 3 or 4.
David Erbas-White
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Buy a 24 watt 12v light bulb.
The meter will impose negligible current draw when measuring voltage.
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If you want to load test batteries, you could buy something like:
http://www.harborfreight.com/cpi/ctaf/Category.taf?CategoryIDD4&pricetype Which has a test load and volt meter integrated in one handy unit.
tater schuld wrote:

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tater schuld wrote:

That's fine Tater. A 25 watt resistor will start to get warm quite fast, but you are only going to load it for a few seconds.
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Dave Lyle wrote:

Yeah, but...
You may intend it to only be loaded a few seconds, but there's always:
1) Holding the button down too long to 'really' load the battery, or 2) Accidentally leaving it connected, or 3) Someone else not 'understanding' that it's for momentary use only, or 4) Using it briefly, but burning one's fingers on it because you don't realize it's so d**n hot, or 5) Using it for several seconds, then throwing it in your field box on top of some pyrogen, or 6) (list your favorite 'mistake' here)...
I've learned/discovered the hard way that making something simple/foolproof is the best way to go, and derating the load is just plain common sense (IMHO).
David Erbas-White
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was plannin on having it on a toggle. now it WILL be on a pushbutton

unit will be connected only to test batteries. since 25watt (or more) resitors are bulky, I dont think i'll incorporate this into our launch controlers

i'll be the oply one using it. ever. I have a tendency to make things that could be harmfull to ones health, so most people stay well away from my test equipment.

hence the wooden enclosure it will be in.

I agree, but what a nightmare it will be to get a 50-100-200 watt resistor just to load test a couple of launch batteries(note: money i spend on test equipment is money i can't spend on motors).
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Make your own! I used to wind load coils from discarded deflection coil wire from old TVs. Since you're dealing with DC, coil size/shape are not important, only resistance. The wire should be free for the asking.
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I've made a dummy load using a 2 amp 24w taillight bulb, with leads to a dual banana plug. It just so hahppens that my launch systems all use dual baanana plugs for the clip leads. And my DMM uses it for test leeads. So I just plug the bulb into the DMM leads, and plug the whole thing into the DMM. Instant load test.
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You might want to consider loading it up considerably more. 2A is good for an estes igniter, but as I understand it, Copperheads are more like 5-10A.
While copperheads may not be quite that much, you still do want to load more than 2A.
280W (assuming 14V fully charged) is a large and expensive resistor.
A couple of high amp 12V lightbulbs may be more convenient, albeit more fragile. Think 50W 12V MR15 halogens (in the low voltage lighting section of your big box retailer. HD _does_ carry bare sockets for these things.).
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followup

....
actually, one two element bulb did it suficiently enought for me to be happy, except for the fact that ti looks like half our batteries are dead. mostly used NiCads, so we'll be investing more into gell cels
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