Its very simple, don't exceed the max continuous current rating on the battery(ies) you are using. Say for example, I want to build a parallel battery box mod. Just a switch, a BIG switch, and 2 LG HE2 batteries. They are reputable 20 amp max batteries. While drawing 20 amps per battery they will not supply 4.2 volts. Maybe like ~3.7 a second or 2 into the first vape. After that a little less. If I plug these values into Ohms law, I come up with a ridiculous .0925 ohms. This would be worst case. The problem with this is there would be voltage drop through the mod, and the switch. So you would'nt be getting quite as much power as we would expect, but it would still be lots. The biggest issue would be shorting the output. This is where some get into trouble with these Mechs. Also running a super low sub ohm build on a single battery that cant keep up. Like one of those overly inflated batteries we see all over. I think you'd be better off running a .2 to a .3 ohm build on a Siegelli 150.