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What are , if you know any, some good ways to get rid of the heat that builds up from arcing?

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That's a good question. I've heard all kinds of ideas, ranging from air venting to water pumping. The light-weight approach is to just scoop air into your body and make a vent in the back so you have a constant flow of air taking heat away.

You possibly fried both FET's and/or their respective driver transistors. More specifically.....

Contrary to popular belief, there's no one FET for forward and no one for reverse. I understand that since there's 2 directions(forward and reverse), it must be intuitive to think that there is an FET designated for each direction. Actually both FET's are used in either direction. Each of those FET IC's(integrated circuit) has 2 FET's inside of it....an N channel FET and a P channel FET. All together you have 4 FET's. These are configured in an H-bridge circuit like you see in aftermarket versions like Rob's V1 board. When you go forward, the N-channel of one turns on, and so does the P-channel of the other. In reverse, they switch. So both FET's are actually being used in either direction.

That being said, I'd put money on it that you fried not only the FET that blistered up, but the OTHER one as well. The damage took place, probably at the exact moment you noticed the erratic behavior. What caused it could be a number of reasons(heat, short, too much current, etc.)

Now, as far as any OTHER part being damaged, without being able to test it I couldn't tell you. Replacing the FET's and seeing if it works would be a clear indicator, but could also blow the NEW FET's if you're not careful when applying power, because if the driver transistors fried in one state(on/off) you could just be putting the new FET's in the same situation that fried your old ones. Whether your xmod/mini-z's power switch is on or off, the FET's are connected to the cells, so turning it off would not save an FET from frying if something had gone wrong.

What I'm guessing you experienced was too much current draw from a semi-stalled hot motor and the FET's just couldn't transfer the current from the li-ions. Sounds like you need either a stack, a V1, or one of my FET boards.

All things set aside, bull, I'd be happy to walk you through fixing it and getting it running again.

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do you need an h bridhe to control the motor with a 5v regulator?

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No, your xmod already has an H-bridge inside of it. Look at the FET's and H-BRIDGE's section.

The 'external' h-bridge is what you're referring to, and all that does is allow for a way more current to your motor. You only need the regulator if you're going to exceed 8.4V. You can build a killer motor to run on 8.4V, and use the h-bridge to drive more current to your hot motor. You'll need neo-dymium magnets to maintain torque, but you can take the turns down quite considereably if you have a nice h-bridge. The net result is more power to the ground and more speed.

So think of the regulator as higher voltage potential, and the h-bridge as higher current potential.

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the only difference is that I used the wire that comes with 9v batteries connectors

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And how exactly did you have this rigged? 9V connector on car, and the other on cells? When you connected the cells to the car, you just connected the 9V connectors together?

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.until it fried my reverse it keep running forward ok with no reverse

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Ok, what fried your reverse? The connector? What was the car doing right before you noticed the erratic behavior?

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so I bought a another xmod to change the board and used the rest as parts, but the fun just lasted for 5 days and then no reverse again, followed by no power at all.

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Hmmm....was it connected in the same way as the first car? Was it doing the same thing when it fried?

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When I connected the batteries to the charger it recognized the batteries, they charge ok but when I reconnect them to the car the power just last for one minute and then shuts down

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The cells get completely discharged after only one minute?? Does it shut down for a few seconds and then get a burst of speed again? Do the cells need to be recharged after that one minute? If so then it sounds to me that you AT LEAST have a couple bad cells. They could have discharged lower than the cell's critical low voltage(@3V). In that case it's time to give Robert a call. He'll either replace the cells or sell you new ones, of course.

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I have a new board and dont want to mass up again

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I don't blame you. Sounds like you should have upgraded your FET's before messing with li-ions. This is common with new li-ion users. If you have too much cell and motor, then you fry the FET's. If you have too much FET, then you are fine. FET's first, then cells and motors.

What are you going to do with the fried boards? I could use them. Name your price.