Needing some help with this beast

[Flukethought]

Okay I have been building boxes on and off for a while but now I want to step it up a notch.

Product purcased
Turnigy nano-tech lipo
Capacity: 4600mAh
Voltage: 2S2P / 2S Cell / 7.4V
Discharge: 90C Constant
Weight: 237g (including wire, plug & case)
Dimensions: 96 x 47 x 25.1mm
Balance Plug: JST-XH
Discharge Plug: 5mm Bullet-Connector

Fatdaddy 510 connector
No explanation need most of you know what this is.

ModMeter
ModMeter will display the battery level. If your mod is a single or a double cell, it will know and will display the battery level accordingly. It will display the voltage under load so will display the real voltage of your mod when it is in use.

It will also display your resistance, wattage and amperage by using Ohm's law to calculate the values using voltage under load and resistance and will also stay on for 10 seconds after firing.

So this issue I'm seeing is that I want to wire my box up to a mosfet that handles around 450-500 amps. Well the only thing I can find in this area is the N-channels. But I wont be able to use a n-channel as it will auto fire with the modmeter installed. So I was looking for some advice on how to handle this situation and idea on what I should do.

 

[Mooch]

500A?
Even the IRLB3034 N-MOSFET will only handle at most a bit over 25A before overheating if you don't heat sink it. You can parallel 20 of them but that's going to get interesting.

What is the maximum current that you've calculated that you will be drawing from your Turnigy pack based on your coils? There are P-MOSFETs with fairly low on-state resistances that you might be able to use. If you post a drawing of your desired wiring it would make it a lot easier to figure out what's needed to make it work.

Are you using this diagram?
Series Battery P-Channel Mosfet Wiring Diagram With ModMeter

 

[Flukethought]

well see there are n-channels that go up to 700 amps but i can't find any p channels that reach a high amp load.

 

[Mooch]

Those very, very abused and misunderstood amperage numbers are only for theoretical situations where the heat sink is kept at 25°C...an impossible task...or for mIlliseconds pulses or shorter, or they are the numbers where the silicon itself starts falling apart. They are there merely for comparisons of certain mosfet characteristics, not for DC current operation without a monstrous heat sink. The wires going from the chip itself to the legs of the mosfet melt instantly at 195A for the IRLB3034. And the mosfet would self destruct way before that.

A TO-220 case mosfet can dissipate around 2W of heat before exceeding its maximum temperature rating (without a heat sink). Using the popular IRLB3034 we see that it has a 1.7mOhm worst case resistance that approximately doubles when the mosfet is hot...3.4mOhms. That mosfet has a junction-to-air thermal resistance of 62°C/W and a max temp rating of 175°C. Assuming 30°C room temp max that gives us 175°C - 30°C = 145°C of temp rise before the mosfet could fail. At 62°C of temp rise per watt, that gives us a max of 145 / 62 = 2.34W before the mosfet overheats.

With an on-state resistance of 3.4mOhm, that gives us 2.34W = max-amps-squared * 0.0034
max-amps-squared = 688.24
max-amps = 26.2A per IRLB3034 mosfet

If these mosfets could handle hundreds of amps each we would never have to parallel them for high amp builds...but we do.

What's the lowest coil resistance you want to use? Or, what's the max you will be drawing from that pack? I can help you find a P mosfet that can work for your setup.

 

[Flukethought]

understood so what would the option be in wiring the mod meter up to a unregulated device?

 

[Flukethought]

 

[Flukethought]

I prefer to be able to hit the higher amp limits of the battery. I would like to include the modmeter if at all possible.

 

[Mooch]

I prefer to be able to hit the higher amp limits of the battery. I would like to include the modmeter if at all possible.

That battery says it can do 405A continuous but that would melt the wires thay connect to the battery. What's the lowest resistance you plan to use with this mod? We can work out the max current you'll draw. And we should be able to find a P-MOSFET that lets you use the Modmeter.

If I remember correcty about the best we can do with a TO-220 P-MOSFET, the shape most used in a mod, is get down to 7 milliohms resistance. That should be good for about 15A each. That means you'll probably need four or more in parallel for a mod that can deliver 60A. That's over 250W with a fresh battery and about 200W as you continue. That's a huge amount of power.

But I'm worried that you'll never be able to use a Keystone sled without it practically melting from the heat generated in the contacts unless your draws are very short. How are you connecting to the batteries?

 

[Flukethought]

well thats the thing I want to build a better box then the current one I have in my hands.
The current box I'm using is 504 watts max.
I don't plan on using a keystone sled at all seeing as Im using the lipo battery.
the current box im using I think uses a relay.

 

[Mooch]

OMG...I'm an idiot. LiPo, I forgot. Thanks.
What resistances are the coils you are building?

 

[Flukethought]

The lowest I would like to go would prob be .10 if possible so that would equal to around 90 amps. I think 90 amps would be a good limit.

 

[Flukethought]

IPP80P03P4L-04
Doesn't look to bad at 80 amps? what would your suggestion be?

 

[Flukethought]

Wire that will be used is 12AWG turnigy 200 degree c

 

[Mooch]

90A at 7.4V = 666W.
That is an unbelievably high power level. You're able to build a coil and set up wicking that won't burn at those levels? 0.1 ohms is usually an absolute minimum to be even considered for a a 4.2/3.7V setup. With your LiPo batteries you'll be running a 0.1 ohm coil as if it were a 0.05 ohm setup.

Good mosfet choice! That was one of the ones I was going to suggest.
But, remember, those current ratings are for special circumstances not applicable to vaping. You have to use a bit of ohm's law math to figure out how much current it can handle.

Its resistance is 4.7 milliohms when it's hot. Since that size mosfet can handle about 2W of heat before getting damaged then the equation is Power = (Amps^2) * (Resistance)

That becomes 2W = (Amps^2) * (0.0047)
Amps = 20.6

Assume 18A per mosfet since you will be operating them close to each other and encasing them in shrink wrap. This makes them heat up more.

At 90A you'll need 90 / 18 = 5 mosfets in parallel, at a minimum.

If this was my build I wouldn't do it with fewer than seven mosfets in parallel so I have a safety margin. But, that many mosfets would be crazy. I would just switch to n-mosfets and find another way to display the voltage/current info I needed.

Or, start with a much lower power level that makes it possible to wick well and not get dry hits or burn the cotton instantly.

 

[Flukethought]

well I been vaping at 500 watts daily for about three weeks now it would be nice to step it up.

 

[Mooch]

Could you describe your setup? I'm fascinated...
Battery? Coil resistance? Atty?

 

[Flukethought]

coil is .14 battery is lipo 8.4 mod is mech mod gen 2 version 1. when looking at the internal specs i see the battery is is pushing at 60 amps. I think they use a relay.

 

[Flukethought]

CP1A-12V-X relay

 

[Mooch]

coil is .14 battery is lipo 8.4 mod is mech mod gen 2 version 1. when looking at the internal specs i see the battery is is pushing at 60 amps. I think they use a relay.

Thanks!
Ok, yea, you're up there in wattage. The voltage instantly drops down so the power is a lot lower than 500W but using 4.2V as your voltage number is safer for calculations.

Something I forgot is that mosfets don't share current equally so you'll need at least seven p-mosfets to survive hit 90A. Can you fit all that?

 

[Mooch]

CP1A-12V-X relay

Whoa...that's a 20A relay!
Its carrying current rating is higher but the switching current rating is 20A and the contacts will quickly wear out when connecting current levels a lot higher than that. And it's very possible that the contacts will stick together at one point, effectively locking your mod on.