one option is to run a string of 6volt batteries for the 48volts, but keep another 6volt in rotation, much like a spare tire is rotated out of the trunk/boot and put on the front of the car, while one of the rears go into the trunk,, er something like that.
while the spare battery is out of the string, it can be properly serviced, equalized well, logged for specific gravity and all that.
then if sometime in the future you have one fail in the string you will have the spare that is effectively the same age as the rest of the string and you get more mileage out of the string as a result.
i plan on doing this with my string for 48volts, and use anderson connectors to make the interconnects so that i can easily switch out any battery within the string and put the spare in rotation.
if i were to be planning on using batteries deeply, and was starting out, i would go with 8 of the 6volt golf cart batteries to cut my teeth on... well maintained they will return reasonable service and even though not last anywhere near as long as a set of rolls/surrette (my preference)
they work out to be a fair value in $/kw/hr delivered.
fwiw
there are issues with parallel strings of batteries too, most especially if you don't monitor your bank properly and keep them watered, keep the specific gravity by equalization etc.
i have seen older batteries being the weaker sister coming up to what the regulator/controller sees as fully changed long before the others get there. this of course causes the newer batteries to not break down their sulfation and it gets hard, reducing capacity to near that of the weaker sister.
it also seems like if you have one battery go bad, it usually isn't long before another starts to fail, then another...
having said that, series strings can be a holy nightmare if you let them get out of whack, if you aren't monitoring them regularly you will find one getting lower in specific gravity, and thus lower in capacity, and if you don't have means to correct the one battery in the string ... well it gets to be a problem.
i had a set of agm 6volts get out of whack, it took many hours of careful equalization (yes you can do it to agm's if you follow the manufactures guidelines carefully), followed by deep cycling, followed by more equalization, wash rinse repeat over about 5 days!
i won't let that happen again!
friends don't let friends let their battery strings get out of whack!
bob g
bob g