I believe I have found why my minivan is taking so long to re-charge. The battery was installed 2018 and it was a 7 year Toyota battery. It was totally dead, from a door slightly open. Time for a new battery, hope I can get to the dealer. In the past the Centech charger has worked fine to charge the totally dead battery, and keep others topped off. I just thought I may get a new charger. Whichever I may get must be uncomplicated, not a state-of-the-art. Explain desulfation and do I need to be concerned with that. All vehicles are in a garage, the zero-turn and tractor are in a covered/sided building and hopefully not being used when very cold.
Letting a flooded lead acid battery (whether sealed or the older style with the filler caps) go totally dead is damaging to the battery. The more deeply it is discharged, and the longer it sits in that state, the worse the damage. If your battery were brand new, and it discharged as you described, it most likely could be recovered if it were fully recharged as soon as possible. Chances for recovery improve if you use a desulfating battery charger. In a battery at or near the end of it's projected life span, it's not surprising that draining it to zero might kill it.
Sulfation is the build-up of lead sulfate crystals on the plates of a lead-acid battery. It interferes with a battery's ability to hold a charge and to deliver current to a load. Sulfation occurs very slowly in a full charged battery. The speed at which it happens increases if the battery is discharged, even if just partially discharged. All batteries self-discharge over time, even if there is no load on them. What's worse: in modern vehicles (including some tractors) there is also a "phantom load" on the battery even when the vehicle is turned off - this is keeping various things such as clocks and computers "alive". It can become a real problem, especially on vehicles which are not used regularly.
Here is one of the better write ups on
battery sulfation I've come across. There is a bit of marketing-speak on the part of the manufacturer, but it's still a good overview.
I can understand the desired for a simple charger, however, the older style "dumb" chargers were not designed to be left connected to a battery long term. They can overcharge the battery and boil off electrolyte, doing permanent damage. This means occasionally monitoring the charger and remembering to disconnect it once the battery is charged.
Among other features, smart chargers protect the by carefully controlling the charging voltage at various stages of the charging process and by continually monitoring the state of the battery: they'll discontinue active charging and drop into "maintenance mode" once the battery is fully charged. They are still very easy to use: just connect them to the battery, plug them in and leave them. (Some chargers also allow you to select a charging profile optimized for the type of battery: e.g. flooded lead acid, AGM, Lithium. For most of this that I have used, they remember the setting, so you don't have to reset it unless you move the charger to a different type of battery.)
