I think my batteries are mfg in 2015. I will replace all 4 with flooded and know I will be good for the next 3-5 years. I would think that if you were going to replace on a schedule, then you could buy a cheaper, maint free battery and be pretty confident that you will get 3 years. If you were going to run your batteries to failure, then buying a better quality open cell would be better. That way you could check cells often and do maint as required? I talked to the Sentry Charger tech today. Sentry chargers have independent circuits for each battery bank. He said always replace all the batteries in one bank (batts wired together). You do not have to replace both banks as the charger will handle the difference in loads.