Hey guys,

So I bought a Resistor bank discharger for my Icharger and I have had it go out twice on me. I don't think the fan is providing adequate cooling and the resistors are burning up. I was going to order new resistors to replace them and add a 2nd fan. Reading it looks like I need 5 100w 5ohm resistors to get proper discharge (40A). Could someone confirm this for me before I order?

Thanks for the help,

Aaron

Wasn't it 5x 1Ohm resistors needed?

It's simple ohm's law to figure out the resistance you need, V/I=R. You would need .21 ohm's @8.4 volts to get 40 amps. 5 5 ohm resistors in parallel will only get you 1 ohm or 8.4 amps @8.4 volts.

So I was right - for 8.4V @ 40A you'd need 5x 1Ohm resistors. (Or for 16V @40A it would be 5x 2Ohm)

The icharger steps up the voltage to the load resistor, so you don't need low ohm resistors. But my resistor bank has an RT(total resistance) of 0.5ohms, so I suspect they are 5 100w 3ohm resistors.



5 10ohm resistors in parallel will give a total resistance of 2ohms, which would only put 4.2amp load on a 8.4v battery.

The formula to calculate RT of a parallel circuit is;

1/RT=1/R1+1/R2+1/R3+1/R4+1/R5

So for 10ohm resistors;

1/RT= 1/10+1/10+1/10+1/10+1/10;

1/RT=0.1+0.1+0.1+0.1+0.1;

1/RT=0.5;

RT=1/0.5;

RT=2ohms


Now we use Ohm's law to figure out current;

8.4v/2ohm=4.2amps

Here are some load resistor bank values;

5 10ohm resistors=2ohm
5 5ohm resistors=1ohm
5 4ohm resistors=0.8ohm
5 3ohm resistors=0.6ohm

Hope this helps

Again?

If you only put resistors of equal size in parallel, you can simplify that quite a bit:

For equal(!) resisitors in parallel, it's just the resistors' single rating divided by the number of resistors.

10/5 = 2.0 Ohm
5/5 = 1.0 Ohm
4/5 = 0.8 Ohm
3/5 = 0.6 Ohm

The iChargers like a load of about 1ohm. The real key to the whole thing, and the one thing that seemingly nobody seems to mention is the wattage rating of the resistors. You need enough wattage to create the right amount of load for the battery to discharge at the current you want to discharge.

At 8.4V and 40 amps its 336W (8.4Vx40a=336W) if you have 5 5ohm resistors, but they are only 10W resistors you will never have enough wattage to do a 40A discharge on a 2s pack. My resistor bank has 4 4ohm 100W resistors and I have no problem doing a 40a discharge.

Since he was already talking about 100W resistors and just asked about Ohms, I figured it did not need mentioning.

Thank you guys! They were 1 Ohm resistors not 5 Ohm now that I think back. I am currently in China on business and wanted to order the resistors now so they will be waiting for me when I get home . My resistor bank decided to go out on me on the last day of practice for the Jconcept Stock Nats.... What convenient timing...

No, they don't like a load of 1 ohm. That's completely wrong, and massively higher than what the directions in the back of the icharger manual specify, and completely defies the rules of OHM's law.

The issue is never wattage, I've never seen a design that didn't use at least 5, 100 watt resistors. This is an overall available load of 500 watts, and I've never seen a system draw over 400 watts, as indicated by your math.

Great name, I'm flattered you chose my real name to post under.

I make them with 4 100w, 1ohm resistors wired in parallel. This is the key - series vs parallel. Then I charger can go up to 24v and 40 a output. So lots of ways to get there. . No problem discharging at 40a.

That math doesn't work out. 40a X 24v is 960 watts. Your resistors can only dissipate 400 watts and that's assuming you can provide adequate cooling to keep them from burning up at their 100watt Max each.

If your feeding 24v into a 1/4 ohm equivalent load you are drawing 96 amps.

V = IR

Is everybody using and discussing the same resistor hook up method? Channel mode with the resistor bank on the other channel, or expanding mode with the bank in series on the same channel? The channel mode has more flexibility for resistance.

I prefer channel mode for the flexibility you mention.