What you're experiencing when one light affects the brilliance of the other is "voltage drop". Even when wires are technically large enough to carry the current that your lights are using, the resistance of those wires contributes to the total voltage drop of the circuit. The weak link in the system you've drawn is that you only ran one negative for circuits that have individually supplied positive wires. Just for giggles, after your battery is fully charged, while the lights are on, try adding a second negative wire to show what effect this could be having. Also, as a battery ages, its internal resistance also rises, adding internal voltage drop and affecting performance even when fully charged. - Grandad