Most battery's discharge capabilities are determined using a C-20 rate. This means the specifications given for a battery's performance are determined when the battery is discharged over a 20 hour period. SLA batteries boast between 300-500 charge cycles, but this is when they are discharged at a C-20 rate. Using them in an electric bike kit discharges them 10 times (or more), faster than the C-20 rate. So this means you will not get anywhere near 300-500 charge cycles out of your batteries. With normal use, never running your batteries completely dead, and being very vigilant in charging your batteries back up IMMEDIATELY after each use, you can realistically expect between 100-200 charge cycles out of them.
The amount, or depth of discharge also has a lot to do with the life-expectancy of your batteries also. SLA batteries should never be run completely dead; since this will damage them and reduce their life expectancy. Automobiles use SLA batteries to start the car's gasoline engine. So why do car batteries last five years then, you ask? Because when the car battery is used to power the engine's starter motor, it is only used for a few seconds and is discharged only 10 % of it's total capacity (if that); then it is IMMEDIATELY recharged when the gasoline engine starts. This is why a car battery can last five or more years.
