Careful.. 1) You should never let a lead-acid battery go "completely down", not even a so-called "deep-cycle" one, if you want it to last a long time. Look at the manufacturer's specs and build a voltage-based cut-off switch that will shut down the output before they go below the manufacturer's recommended discharge voltage. You could get fancy and include some kind of over-ride for a "real" emergency, but at that point you really should have a generator and someone to fuel it.
2) Direct charging of even that big of a bank with a 50A charger could be above the manufacturer's upper limits on charge rate, and you'll likely boil off water, reducing the life of the batteries, venting gas, and generally wearing out the system. You need to limit the charge rate to their recommendations, if you want the batteries to survive a long time. (Number 1 should help with this. Maybe the 5A charger will be fine, if you do #1. It won't take 80 hours then...) Most of the time, the better option is a generator. Yes, the repeater system goes down while it takes over, but you can run for a very long time for a lot less headache than a battery system. You could design the battery system just to hold-over the system during generator cut-in, but even that's overly complex. Most generators take less than a minute to start and decide that their output is sane enough to switch in the load... Are they designing for long-term outages or smoothing out short-term ones? That's the main design question to ask... Nate WY0X
