How to add more juice from the battery?

Hi. I have a 2006 CR-V and it has 2 12vdc outlets. I use my suv for work and I need to connect a lot of equipment like GPS, 2 power inverter, a box like an amp function and laptops. Currently I have a hard wire connected directly to the battery's terminal ( + & - ), it has a switch and a 25A fuse. But still the power inverter/s makes this whining alarm sound that indicates that the input voltage is below the regulated 12Vdc+. Without changing the alternator, what other options can i do? Like capacitors? And, is it right to just connect the negative wire to the batt's neg terminal or I should connect it to the car' ground terminal... Any suggestion will be appreciated. Thank you.

Ok. Thank you for the input.