Brushless upgrade on four trucks means new ESC, motor, transmitter, receiver, metal gears on 4 trucks. That is a lot. If you do it, check my signature for the DumboRC radio on Banggood and you can get a cheap motor/ESC combo as well. But that is a lot of work. You will also need to change the gearing so you’ll need to test and find the right pinion and do that x4. Even then these trucks will be stupidly fast. Insanely fast. You will break stuff on brushless even on 7.2 v NiMH. Then your batteries will last maybe 7 minutes a piece.
I know you want to avoid it but the cheapest, best and most useful upgrade is a decent charger and 2S LiPo. In your case a charger that can charge 4 at once. You won’t need to change gearing or buy metal gears. Your run time per pack will be half an hour. You’ll need a 5 pack of voltage alarm/monitors ($20 for 5) and zip tie one to each battery. Toss the battery hold down and get a 5 pack of Velcro battery straps for $10 - the slots to run the strap is already there. The trucks will be faster but not insanely fast.You won’t have motor heat problems and you won’t be likely to obliterate a couple trucks because they now go 60mph.
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Or you can go hard case LiPo. The batteries will last long enough that you will only need one each. If you want a little more safety go with a hard case. Banggood can’t ship lipo to Canada so I have bought a Gens Ace hard case 2S that is 5,000 mAh and a 7,000 mAh HRB hard case from Amazon. With 7,000 mAh it will last even longer.
I think you have a bigger safety risk from getting taken out by one of 4 60mph Amps running at once than from bursting a LiPo. ? Just zip tie a voltage meter/alarm to each one and set the alarm at 3.4 and you will be fine. Set up your charger somewhere non flammable and charge with the batteries inside a LiPo safe bag. I do mine in the middle of the concrete floor. Store the batteries in the garage in a steel toolbox or something else non flammable inside the lipo bag and keep them at storage voltage. You can be the one to check the charger setup you can charge all 4 in an hour or two with a quad charger.