I really do not believe they need to be discharged at all, call the batt tecks at batterystuff.com they will take the time to go into detail what I am going to say, all you need to do with any baterrys U are putting in series is charge them up fully and independently and wait 12 hours and do a hydrometer test , u need to waut 12 hours to ensure U are not testing just a surface charge, a wait in this time frame will be enough to do that, if theyare the same put em in, if they are not U have the option of useing a desulphater charger to try to bring the low one to life, and do the test again, even when using a desuphator u need to have a full charged batt before they will do there job, the only true way to test a batt is this way, U do not ever need to discharge a battery, that is not good for them... after charging them seperatly and they are the same per test hook em up and then U can charge em with a 24 volt charger maintainer , but U should once a year switch em front one to back and charge em up individual again If U want to get the max life out of em, the back batt is always a lazy batt after years of being charged in series, because the first batt takes the charge first and the secound bett may always be a little low, most batt chargers only charge to 75 to 80 percent, U need a good charger desuphater maintainer that says clearly what it will do, If U only charge em in series the back batt will never fully charge period. Randy