Where can I buy a 4 Ohm 40 Watt resistor?

[FutureTrackStar]

I'm trying to charge my dead car battery (6.8 V) with a laptop charger (output is 19.5 V DC at 3.34 A, max power is 65 W). I figure I need a limiting resistor of about 4 ohms and 40 Watts. Where can I buy one of these?

 

[MrAl]

Hi,


You should be able to charge it a little with less current than that and then later once the voltage is up to 11v or more you can use a lower power resistor.
You can also charge with 120vac and a high current high voltage diode with a 120vac light bulb in series with it, but you have to be very very careful because one side of the 12v battery is connected to the car chassis unless you remove the battery from the car. Current depends on the wattage of the light bulb.

 

[Reloadron]

Finding a 4 Ohm 40 watt power resistor is easy. Ohmite and Vishay come to mind as manufacturers something like this. Mouser is another distributor also.

This is not a good idea. Laptop chargers are designed and built to charge laptop batteries and not SLA (Sealed Lead Acid) batteries. Many laptop chargers even monitor battery charge rate based on feedback from the battery. You stand a chance of destroying your laptop charger or really bad ugly things like cooking or blowing up your SLA battery. Really not a good idea.

Just My Take
Ron

 

[Jaguarjoe]

Your resistor is dissipating 40 watts. You can not just plug in a 40 watt resistor because it will get ungodly hot. You must derate it by at least a factor of two or better for it to survive. So you'll need a 4 ohm 80 watt resistor or two 8 ohm 40 watters or two 2 ohm 40 watters.

 

[#12]

Not considering the wisdom of your idea, Radio Shack has 1Ω, 10 watt resistors in (2) packs if you're in a hurry to get something locally.

 

[Jaguarjoe]

This will not waste as much power but it's not 3 amps:

9V, 700mA PLUG-IN WALL SUPPLY-MPJA, Inc.

 

[cr0sh]

I'm trying to charge my dead car battery (6.8 V) with a laptop charger

Wait - STOP. Is this a 12 V car battery - as in a real car battery, with lead plates and liquid sulfuric acid? Meant for starting a car?

If it is, its very likely that battery is toast; possibly the plates are sulphated. The best thing you can do -first- is to put a real car battery charger/conditioner on it, and leave it on it for 24 hours. Then check the voltage; it should be around 14 volts. If it isn't, ditch the battery (take it to a place that will recycle it - you don't say where you are, but if you are in the USA, take it down to Checker/Oreilly/AutoZone and use it as a core) and buy a new one.

Don't continue to mess around with this one; it isn't worth it, and the way you are attempting to go about it might end up in a fire and/or injury...

 

[Triode]

most of those places can test a battery for you too.

 

[Reloadron]

Wait - STOP. Is this a 12 V car battery - as in a real car battery, with lead plates and liquid sulfuric acid? Meant for starting a car?

If it is, its very likely that battery is toast; possibly the plates are sulphated. The best thing you can do -first- is to put a real car battery charger/conditioner on it, and leave it on it for 24 hours. Then check the voltage; it should be around 14 volts. If it isn't, ditch the battery (take it to a place that will recycle it - you don't say where you are, but if you are in the USA, take it down to Checker/Oreilly/AutoZone and use it as a core) and buy a new one.

Don't continue to mess around with this one; it isn't worth it, and the way you are attempting to go about it might end up in a fire and/or injury...

Since the OP mentioned 6.6 V I was wondering the same thing. As in a real car battery?

Also, a basic charger can be had relatively inexpensive from just about any automotive parts house / supplier. I have seen plenty for under $50 USD and those offer multiple charge rates up to 10 Amps. Most also charge 6 / 12 volt batteries.

I agree with cr0sh also in that considering the cost of a new battery it likely isn't worth screwing with but if you must, then consider a tool made for the job. Replacing a laptop charger can end up costing much more.

Ron