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Homemade Discharger

Old 10-15-2004, 03:36 PM
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mustangous
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Default Homemade Discharger

I seen homemade battery dischargers made with 10 or so automotive 1157 bulbs for 6 cell packs. I was wondering if anyone has any different ideas. Can different type of bulbs be used, or resistors and what kind?
Old 10-15-2004, 03:50 PM
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Default RE: Homemade Discharger

I made one with 12 removeable bulbs , It works good I'll post a pic when I have time. It's very simple, It's for sale in the market.
Old 10-16-2004, 10:53 AM
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mustangous
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Default RE: Homemade Discharger

Is there another alternative other than the 1157 bulbs. Or is there a certain wattage the bulb has to be?

Does anyone know what rate to discharge batteries at?
Old 10-16-2004, 03:34 PM
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Default RE: Homemade Discharger

I think the idea is to discharge packs at the same rate as you'll use them (if your a car racer) If you have nimh batts you don't need to discharge at all, actyually with nicads you probly don't need to discharge either. Maybe once every 12-20 runs.

As far as making a discharger, you can use any bulb you want or can get, but the 3057 and 3357 I used are about 2 amps each that's at 12 volts. That's why I made them so they can be removed by just pulling them out.

Most stock truck will discharge at maybe 20 amps , a standard rate for sub c mod is usually 30 amps. The dean's and other bulb dischargers are usually 10 bulbs and say 20 amps for the total.

Really to give a better answer we need to know exactly why you want to discharge and exactly what batteries and useage. good luck.
Old 10-16-2004, 09:30 PM
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Default RE: Homemade Discharger

#921 bulbs are another option. They're smaller, draw about 1A or so with 7-cell packs (1.4A at 12V), and have folded wire leads you can pull away from the bulb to help solder them together. They're 12.8V bulbs (I believe) so they'll last forever with 7-cell packs if they're not dropped. I don't think building a discharging load with these bulbs will be any less expensive than with #1157's. But, they do offer a different size/shape that might be preferable.

You don't have to worry about the wattage rating of the bulbs, just the amount of current you want to draw and the voltage of the bulbs. OK, yes, multiply those two together and you get a wattage, but that number won't help you. :-)
As long as you use a bulb with a voltage rating higher than 8.4V, you're OK.

Typically, it's recommended that you discharge at the same rate you use your cells. But, IMHO that isn't necessary and it just heats up the pack (the worse enemy of NiCd and NIMH cells). None of our tests have consistently shown any increase in capacity or voltage-under-load when maintenance discharging is done at a 1C rate or higher. I recommend discharging at a 3C rate or so (that is, if you have 3300mAH cells, discharge at 10A) as a balance between the recommendation to discharge at what you use them at and not needlessly heating up your packs.
Old 10-17-2004, 12:32 AM
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Default RE: Homemade Discharger

I guess i have a dumb question, how does discharging at a 3C rate on a 3300 pack equal 10A? Does the 3C mean 3 amps? By the way i am getting back into R/C and i am trying to pick up on all the new technology. just recently bought a LST, and 2 Mini-t's. But the reason i am asking about discharging is that i pulled out my old school RC10's and are trying to fire up my old NiCads.
thanks guys for the help!
Old 10-17-2004, 12:45 AM
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Default RE: Homemade Discharger

Not a dumb question at all. :-)

"C" is a term for the capacity of the cell, expressed in milli-ampere-hours (mAH) or ampere-hours (AH). It is also used to espress a multiple of that capacity when referring to a charging or discharging current, expressed in amperes (A). So, with 3300mAH cells the capacity of the cell (C) = 3300mAH. A 3C charging rate is a current that is 3 times the capacity, which is 3x3300=9900mA, which I rounded up to 10A.

The units (mA, A, AH, and mAH) don't multiply or divide out properly when you do the math but just accept that a charging/discharging rate is a multiple of the capacity of the cell, but only refers to a current level. It is darn confusing and I apologize for diving into the lingo without explanation. :-)

<deep sigh> Wish I could afford a LST!
If you're charging old NiCd packs, do it slow the first couple of times to condition the cells, at about a 1/20C-1/10C charging rate. For example, if the cells have a capacity of 2400mAH, then a 1/20C charging rate is 1/20 times 2400 = 120mA. The 1/10C charging rate would then be 1/20 times 2400 = 240mA. So, you should charge at about 120mA-240mA.
Old 10-17-2004, 07:15 PM
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mustangous
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Default RE: Homemade Discharger

the guy at the LHS didnt know what i has talking about. But thanks for the lesson and greatly explained. I am trying to get all the cob webs out of my old r/c stuff. It must of been at least 10 years since I raced them. My discharger that i have has only 6 bulbs left and only 3 work cause the others are broken.
Thanks for all the great advise everybody......
Old 10-17-2004, 10:20 PM
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Default RE: Homemade Discharger

Welcome back to R/C! :-)
Instead of bulbs you can also use power resistors (to belatedly answer your original question) which are a heck of a lot more durable...we use them in the dischargers we designed. Just use Ohm's Law to work out the resistance and power ratings you need, derating the resistor's power rating by about 50% so the resistors last a lot longer and don't run hot enough to cook a steak on.

Just for reference...
If you wanted to discharge at 10A using 6-cell packs you need 7.2V divided by 10A = 0.72 ohms of resistance
The power being dissipated is 7.2V times 10A = 72W, double (the derating) and round up to 150W.

Using four 3-ohm resistors in parallel will give you 0.75 ohms, almost perfect. And if each resistor is rated at 50W, the total power handling capability of the load will be 200W, perfect. If you want, you can use 25W resistors (a lot smaller) but will definitely need a fan to keep them from cracking from the extreme temperatures as each one will be dissipating over 35W of heat.

Digi-Key (www.digikey.com) and Mouser (www.mouser.com) carry Vishay and/or Huntington tubular wirewound power resistors that work great for this and they're fairly inexpensive...about $3-$4 each. Add a cutoff switch and you have a complete discharger!
Old 10-18-2004, 12:09 AM
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ZM2000
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Default RE: Homemade Discharger

Mustangous

If you visit Tony's website then you can make your own discharger, cycler etc etc.

Just go to www.uoguelph.ca/~antoon/

-ZM
Old 10-19-2004, 07:52 PM
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mustangous
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Default RE: Homemade Discharger

Could you tell me if this is correct.

A 12V bulb rated @ 60W will discharge 7.2V pack @ 3A. The reason i keep talking about the wattage is that all the bulbs i find does not tell me the amperage. Just the watts.
How i got this is: A 12V source @ 60W will discharge @ 5A, therefore a 7.2V pack will draw 3A. Am i calculating correct?
Old 10-19-2004, 08:03 PM
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Default RE: Homemade Discharger

Your math is perfect but bulbs don't have a linear relationship between voltage and current (like resistors do). The bulbs will probably draw more than 3A, but there's no way to tell without testing.

For example, the #921 bulb we often use here is rated at 1.4A @ 12.8V. Normally, you'd think that they would draw approx. 0.79A by Ohm's Law calculations, but they actually draw about 1.04A in our tests.

I consider this to be a great "feature" of bulbs...you don't need as many of them for lower voltage packs as you think you do. Almost makes up for how delicate they are. :-)

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