Instead of damaging your friends batteries why don't you buy or borrow a watt meter .connect it up and see how many watts,volts,amps,your set-up is drawing no-load and under-load?:
As mentioned it may be over-propped drawing too many amps, the ESC,motor connectors may be faulty causing higher resistance and heat.
Is the ESC set-up properly? Even if it is , you really shouldn't be running Lixx packs down to low voltage cut-off on a regular basis. All that does is overheat ,raise the internal resistance and puff the packs shortening their life/performance. Better to set a timer and come in before the packs reach low voltage.