Best way to Dump?

[(unknown)]

Russ,

I set the diversion load up with diode, capacitor and resistor as you suggested after determining that the resistance of the relay coil was 70 ohms - thus, I got a 220 ohm, 1/4 watt resistor. When I energized the circuit, the resistor started to smoke so I quickly disconnected. Any suggestions?

Josh

[TomW]

Josh;

Smoking resistor means you need a bigger wattage rated resistor. From your post I cant tell you how big without the voltage across coil and resistor. A 1/4 watt is pretty tiny just a few milliamps or so of current will overheat that.

Cheers.

TomW

[RP]

Josh,  That relay coil has a lot lower resistance than I expected it would.  Tom is right, the resistor will need to have a much larger wattage rating.  See if you can get a 5 watt or so resistor.

Just curious, what kind of relay is this?  I suspect it's not an automotive type.

[(unknown)]

Russ and Tom,

It's an automotive relay - "5 pin relay with resistor 30-40 amp single pole double throw." I measured the resitance a number of times. It measures between 67 and 75.

[RP]

Shoot, it must have a big honking coil in it.  

Well we still need a heavier duty resistor.  It looks like the resistor will be dissipating about 5.8 watts so you may even want to go to a 10 watt resistor.  Radio Shack or other electronic stores will have them.

If they don't have the exact one you want then you can connect multiple resistors to get there.

I'm a little concerned that with the relay and resistor drawing this much current, the capacitor I suggested may be on the small side.  If so, I suspect the symptom you'll get will be a singing or chattering relay at low levels of power dumping.  If this happens just add one or more capacitors in parrallel to the original one to fix it.  Be sure all of the capacitors have at least a 100volt rating.

[(unknown)]

It occurs to me that the 12v, 40 amp relay contacts may not be up to passing the 1kw+ of current to the heater elements. Before going too much further, is that a concern?    

[RP]

Remember 1KW at 48 volts is only 20.8 amps.  

If you'd like to do some testing, you could temporarily disconnect one of the heat elements from the controller and feed it directly from the battery bank through the relay contacts.  Then use a small 12 volt battery like from a cordless drill or a "wall-wart" power supply to switch (energize) the relay on and off several times to see how it behaves.