Another reviewer mentioned this charger puts out 15.0V in AGM mode and that's too many volts for some AGM batteries and it caused his to outgas (reducing the battery's life).  My Optima yellow top AGM battery is rated for 15V charging so I thought this charger would be okay for it, but the charger I got puts out 15.5V!  I did find specs that say 15.6V "regulated voltage" is okay for "rapid charging" on my battery but they don't define what rapid charging means.  They do say with 15.6V to "charge until current drops below 1 amp" which I think means you can do 15.6V until the battery gets closer to being charged and stops accepting high amperage.  Unfortunately this charger does the exact opposite, starting at lower voltage and increasing to 15.5V around when it decides the battery is at 90% charge (which seems to equate to about 12.6 or 12.7V on my battery).  The charger keeps outputting 15.5V until the battery is fully charged, which may take hours, so it seems unlikely that's good for most batteries and I'm pretty sure it's against the charging specs on my battery.  Note that the charger shows 15.4V on its readout but my volt meter shows it's actually 15.5V.Nevertheless I let it keep doing 15.5V and checked periodically to make sure the battery wasn't hot (it never even felt warm and an IR temperature gun read 86F max) and wasn't hissing (hissing would mean it was overcharged and releasing gas, permanently reducing its life).  It took almost exactly 12 hours for the charger to decide it was 100% charged and go into maintenance mode, but when it did, it picked 12.4V!  AGM batteries are supposed to be charged at around 13.6V for maintenance (float) charging, so I have no idea why it picked 12.4V.  I left it on for another hour and it actually reduced the voltage to 12.3, at which point I took it off the charger.  Volt meter said the battery was at 12.31V.I can't decide if this charger is broken, has a weird charging algorithm, or is actually very smart.  I say that because my battery was discharged to about 3.5V a few weeks ago, well below the voltage it's designed to be discharged to (it's a "deep cycle" but even deep cycles don't tolerate discharging below 50%, and even below 10.5V may hurt them), so it's now damaged and won't hold the full 13.6V it's supposed to.  Instead, if I leave it on the charger at 15.5V for an hour, it hits 12.8V and then quickly loses charge to 12.5V in about 30 minutes.  So maybe the charger somehow figured that out and decided that 12.4V was "full charge" on my damaged battery so that's what it used as its float charge.  I don't see how it could determine that if the battery was 12.8V or higher after the long 15.5V charge, but who knows.I also don't know if 15.4V is really bad for my battery, or for the "average AGM".  I spent awhile researching but couldn't find any real authoritative information, just a bunch of people theorizing.  Since it's above the 15V standard charge on the battery spec sheet I don't think it's a good thing, but then again I also found research (nrel dot gov/vehiclesandfuels/energystorage/pdfs/evs17pres.pdf) showing that seriously overvolting a yellow top battery to 17V under certain conditions actually increases its life.  So I don't know what to think, but I'm leaning towards thinking it's a bad thing to go against the manufacturer's spec sheet unless you're in a lab using specialized chargers.What I do know is that the desulfination feature on this charger didn't do a thing for my damaged battery.  In fact I don't think it even kicked in.  I connected the charger to a small sealed lead acid battery that I left unused for too long and it went into desulfination mode (I think - the manual doesn't say how you're supposed to know) for over 8 hours pumping 16.8V into the battery until it said "bad bat" on its screen and stopped charging.  In all that time the battery never showed over 15% charged on the charger, though it did end up holding about 10.9V.  In my online research it was mentioned in a few places that there are no studies showing desulfination actually works so I'm pretty dubious about it.  It definitely didn't work on either of my batteries.It was also interesting to note that the sealed lead acid battery never got warm, which I find rather curious considering how long it was putting in such a high voltage.  I had the charger connected to a kill-a-watt meter and it showed the charger was pulling only 0.15amps and 9watts, and this is in 2amp charging mode, so maybe the charger is smart enough to use low amperage when it's using high voltage and that prevents the battery from getting damaged.  When it was pushing 15.4V to my AGM battery it was only drawing 0.35amps and it actually reduced the amps to 0.29 as it got closer to 100% charged.  So maybe that protects the battery from overvoltage damage, but I don't know enough to be sure.  Maybe a battery expert will chime in in the comments.Another important thing to consider is that desulfination mode could damage your car's electronics if you don't remove the battery from the car to charge it.  16.8V is more than most cars were designed to handle, and I found someone mentioning that BMW warns not to use chargers that do things like that, so it's definitely a concern.  I don't know if I feel comfortable using this charger again without removing the battery from the car.  Some chargers claim to somehow magically detect the presence of car electronics and limit their voltage, but this charger says nothing about that in its manual.Other than all that, the design of the charger is nice.  Wires fit into a pocket beneath the retractable handle.  It will show you the battery voltage on its readout until you tell it to start charging (although it reads 0.1 to 0.2V lower than my volt meter, which is annoying).  A button lets you toggle between display of voltage or battery percent.  It lets you choose standard, AGM, and gel cell, and choose from three rates of charge.  The slower 2A charge rate is supposed to be better for many batteries, so use 2A or 6A if you aren't in a hurry.  I tried both those modes and it still puts out 15.5V in 6A mode but it puts out 15.3V in 2A mode.Since this is a dedicated 12V charger you don't have to worry about it "autodetecting" (actually, auto-misdetecting) your discharged 12V battery as a 6V and never charging it past 6V, which is a problem with far too many chargers these days.Despite its nice controls and smart form factor, I give this charger a low rating because I think it tries to push some batteries beyond their limits in the name of charging them as quickly or as fully as possible.  One reviewer said it caused his AGM to outgas and another said it burnt out his battery after a few months of trickle charge mode.  If you have the right brand of battery that can handle this charger's output, and your battery isn't damaged, it's probably fine, but the fact that it can damage things and may even damage car electronics in desulfination mode strikes me as very bad design, especially given that desulfination seems like more of a marketing ploy than a valuable feature.  I also don't like that its volt meter reads 0.1 to 0.2V under actual voltage.