45 or 70 AH means the capacity of the battery. that means charging or discharging current ( in amperes) into time (in hours). in my opinion to use high capacity batteries the engines alternator and it's charging circuit should be designed for high current outputs ( design for high power output). if we discharge high capacity battery ( by any reason) the cars alternator and charging circuit have to supply more current for long period to restore all lost energy which might burn ur alternator. so as my opinion it is better to stay with the manufacturers to avoid stressing ur alternator, easier jump starting, easy recharging, better mileage etc..