ksoc is on the money: the larger capacity battery cannot kill your HL (batteries only supply whats needed), but by being able to run your tank for much longer periods than stock, heat will gradually buildup within it because its fairly sealed plastic "box" - With stock you get 30-45mins then goes flat, then it has a chance to dissipate the heat (once its off its no longer creating heat).

5000mah might last 3+ hours, so a better heatsink or fan to give additional cooling becomes necessary - basically they were never build to run for such extended periods.