if u have to end up installing extra batteries to power amps for a sound system what is a normal life expectency? i know it depends but what have u guys experienced?

 

Recently in my MX-3 had an Optima 850 spiral cells batt. It was 7 years old when "died". Now in my RX-8 I have an Exide Maxxima 900A Deep Cycle spiral.cells batt., my expectancy about it's life is 8 years.
There are Maxxima 900A and Maxxima 900A Deep Cycle. The Deep Cycle type costs more, but it's strongly recommended for car's HiFi. It doesn't damage if it's got deep cycle.
If you want the biggest power in the smallest size, buy spiral cells battery!