Craftsman/Diehard "quality"

 Because I've never owned anything but used vehicles, I've had an automotive battery charger since about 1985.  Recently the timer stopped working, which is sorta bad, but I never trust mechanical timers so I was monitoring it and nothing bad happened.

 But I needed to replace that timer. 

(it was readily available on Amazon, probably turns up in a hundred different products including bathroom light timers)

Aside from a heaping helping of spider webs, this is what I found inside...

 The small (about 1in x 1in) circuit board hanging loose from the wires it is soldered to - that's the regulator board for the fan.  There is no regulation or filter provided for the battery being charged.  The fan exhausts the cabinet from the bottom, and thus also fails to blow on the rectifier "heat sink".   Boy was I young and stupid when I bought this thing.

 Changing out the timer was trivially easy, even tho it was held into the panel by some low-profile, tiny-head hex-head screws.  The case was held together by about twenty tek screws.  I wonder where it was made, they sure hadn't heard of Fastener Reduction!

 I was disappointed to find no inductor inside.  Professional grade battery chargers have a big fat inductor (AKA "choke" because it chokes current flow) in series with the output as a filter.

Note that Google's AI knows abso-fucking-lutely NOTHING about this topic.  It will lie to you instead of saying "I don't know".  AI will NEVER say, "I don't know" and that's a huge problem.  Stop trying to use AI!

 I am making an effort, but since Google made AI responses the default behavior, my choices are A) an engine with no depth, like DDG, or B) an engine that wastes tremendous quantities of electricity and water only to lie to me.  I dislike this choice.

 ANYWAY, the battery acts like a capacitive load - you don't filter voltage peaks when your load is capacitive, you filter the current peaks.  Passing current peaks to the battery shortens the plate life!

 Now that I've seen the inside, and that it has a mounting place for an inductor, I may track down said inductor and install it.  Sears sold three versions of this charger; Silver, Gold, and Platinum.

 This was the mid-grade "Gold" unit.  Presumably the pro-grade unit had the choke.  And based on what I'm seeing here, presumably the "Silver" unit just connected your battery directly to the power line. ;)

 edit: So far, I can't find the OEM inductor - if there was one - nor a suitable alternative that I can afford.  There are some perfectly serviceable units for welding supplies, but they cost more than a new, much fancier charger.

 This charger line is so old, there aren't even videos on YouTube about people servicing theirs - it's as if this product line never existed.

 In cheap units like this one, there is only a step-down transformer and a full-wave rectifier.  The so-called charging rates on the selector switch simply change taps to different maximum voltages.  There is no temperature sensor either.  Charge tapering is not managed, it is only the product of the battery's charge coming up to meet the charger's fixed output voltage.  In fact, it doesn't even monitor state-of-charge (battery voltage on an expanded scale).

 It has no trickle-charge or "battery maintainer" circuitry.  It is strictly a brute force machine, and it assumes a more-or-less healthy battery that just needs its charge brought back up.  It will not limit its rate-of-charge if the state of charge is low.

 High charging currents should only be used to equalize a fully charged battery with unequal cell voltages.  This typically happens when the battery has been severely depleted (you left the lights on, would not start the car) and then recharged.  After that, the battery (total series) voltage may be 13V or so, but individual cells can be quite a lot lower or higher.  Charging an unequalized battery slowly kills the plates of the under-charged cells, and of course the total amount of energy it stores is much less than if all cells are working and charged properly.

 So my typical process is to use the 2A rate until it stops charging, then switch to 15A rate until it seems fully charged, then give it a very short equalization charge at 50A for only a few minutes.

 Did I mention you should always disconnect the battery from the vehicle before using an outboard charger?  Yeah, that's a thing. If you go to a high current setting while the vehicle is connected, resulting in an excessively high voltage (16V or more) across the battery and vehicle power bus, you can zorch your Engine Control Module, bricking your car.  So don't do that.

 And the charging current meter is useless due to low readability; I need to replace it with something equally cheap but with a digital display.  Accuracy isn't all that important in this case, it's only used to tell when charging rate is dropping (meaning battery voltage is coming up).

Battery University's excellent page for lead-acid batteries: https://www.batteryuniversity.com/article/bu-403-charging-lead-acid/

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