Before giving up on 'battery chargers' and going the adjustable voltage power supply route, and never looking back, I had, and still have a transformer based 2 and 12 amp schumacher, and a schumacher sc2500a bought in 2007.
The transformer based charger maxes out at 0.62 amps on the 2 amp setting, and 4.6 amps on the 12 amp setting.
The sc2500a will take a battery up into the 17 volt range, whenever it decides to do so. It cannot be trusted. I ONLY use it when it can grow legs or be rained on, and only on other's vehicles.
Even on its AGM setting, it will gladly go well above 15 volts on the 12 or 25 amp setting and doing this to an AGM time after time is very bad for it.
When I put a snap on ferrite onto its DC output, the ferrite buzzes in my fingers, and my wattmeters on its output go batcrap crazy with or without it.
I've replaced its 40mm fan with a larger 60mm fan and opened up the opposite end with a drill, to improve airflow through it. I used to use it in conjunction with my 40 amp power supply when I wanted more than 40 amps charging, but now I have a 100 amp adjustable voltage power supply, and sometimes combine that with my 40 amp one.
If one actually measures amperage the battery decides to accept, at the voltage the charger holds, or happens to use a good hydrometer on flooded non maintenance free batteries, they will realize most smart chargers revert to a float voltage well before the battery is truly fully charged. This might be OK for occassional use as a garage charger, on a starter battery, but if this done to a deeply cycling battery, it will lose capacity quickly and go belly up, far too quickly.
Most so called 'smart' chargers, switch from absorption voltage, to float voltage, in the 88 to 92% charged range, and it could take 12+ more hours at float voltage, on a healthy battery, for it to be in the 99% charged range, and an older battery will never reach full charge at float voltage, no matter how long the charger is plugged in or how long that green light glows.
A true 100% state of charge rarely occurs with smart chargers, but for most, 94% is more than good enough, especially with that green light telling them their genitals are not undersized.
Getting from 80% charged to 100% charged is no less than a 3.5 hour proposition, with ANY lead acid battery or ANY charging source, and that assumes mid 14 volts is being held that 3.5 hours, and the battery is still relatively healthy. Lower voltages and less healthy batteries can easily double and triple that time to a true 100% state of charge.
When one insists on returning their lead acid battery to a the ideal true 100% State Of Charge, and can actually determine when that point is reached, with a temperature compensating hydrometer, or an ammeter on a battery held at absorption voltage, using ANY smart charger is a lesson in frustration, as all will switch to float voltage prematurely, with the exception of some schumachers which will goto 17v and make the battery fizz like a newly opened soda and stay there for hours.
Much easier to just remain proudly ignorant, and put your faith in green lights and the marketing literature, especially when the battery is only used for starting a fuel injected vehicle that never sits unused for extended periods of time, and uses a battery purveyor which does not fight tooth and nail when the warranty replacement is attempted.
