Easyjumper(TM) Multi-function Portable Car Jump Starter

[Skimask]

You never met a tow truck driver? That is a common thing they do to jump start a car very quickly.

^^^ +1 ^^^
Although I just asked a friend of mine that has a tow truck business.  Said he uses some off the wall 18v system rather than a 24v system.  Says he still gets roughly the same kick, but less chance of it blowing up something on the 'jump-ee' car.  I mean you really gotta wonder if those cheap won-hung-lo branded car stereos could handle a solid 24v anyways...

[abbotsmike]

Bit of a bump, my dad was asking me about one of these for work (he's a mechanic) specifically wanting one that can start large turbodiesels (3l V6 or more). Most of the commercial ones seem to struggle with that, and the one he was looking at getting cost £180!

I'm planning to build one, the way they should have been built. I think for £100 or less I can source a high drain 4S Li-Po, a BIG diode to do the reverse charging protection, a box with a lipo bag inside to contain the lot, and a balance charger. I have enough plugs, sockets and cable lying around to connect the lot up. Then hopefully, one monster compact jump starter!

[eas]

Bit of a bump, my dad was asking me about one of these for work (he's a mechanic) specifically wanting one that can start large turbodiesels (3l V6 or more). Most of the commercial ones seem to struggle with that, and the one he was looking at getting cost £180!

I'm planning to build one, the way they should have been built. I think for £100 or less I can source a high drain 4S Li-Po, a BIG diode to do the reverse charging protection, a box with a lipo bag inside to contain the lot, and a balance charger. I have enough plugs, sockets and cable lying around to connect the lot up. Then hopefully, one monster compact jump starter!

You've basically described the constructor of the commercial unit I acquired, with a bag instead of a case, and, of course, the larger capacity.

I'm guessing that using a big diode for isn't going to add anything but expense and voltage drop compared to the paralleled power transistors the commercial packs use for reverse current protection.

[max666]

...
I'm guessing that using a big diode for isn't going to add anything but expense and voltage drop compared to the paralleled power transistors the commercial packs use for reverse current protection.

I was going to say that at this high currents a p-n junction might actually be superior to the resistive character of a FET, since the voltage drop of a p-n junction primarily increases only logarithmically, while the voltage drop of a FET increases linearly. However a quick components check had me find MOSFET's with only 6 mOhm drain-source-resistance, which at 100 A for example would give 0.6 V voltage drop, while a comparable Schottky-diode might have 0.77 V and a normal diode might have 1 V voltage drop.

Pondering about it for a moment has me come to the conclusion that this must be because we only have bugger all reverse voltage requirement. FET's that can handle reverse voltage of 100 V and more have dramatically higher drain-source-resistance, while p-n junction diodes can easily handle 600 V and still only show 1 V forward voltage drop.
For example the MOSFET's I found, that can handle 600 V reverse voltage, don't have a drain-source-resistance of less than 50 mOhm, which at 100 A gives us 5 V voltage drop.

[Stonent]

Maybe IGBT in it?