An archaic DieHard device has seemingly died hard; is hacking it to resurrect a portion of its original function a worthwhile endeavor?
A decade-plus ago, shortly after moving (part-time, at the time) to Colorado, I came across a smoking (no pun intended…keep reading) deal at Sears: a 12V vehicle battery charger supporting both standard SLA (sealed lead acid) and AGM (absorbed glass mat) cells, along with 2A, 10A and 50A (!!!) charging current options, for $32.99. I bought two, one for me and the other for my then-girlfriend (and now-wife), since we had separate residences at the time.
I’ve held onto both—in spite of the fact that I also now own several newer microprocessor-controlled (versus this transformer-based model) chargers, not only significantly more compact but offering enhanced features such as desulfication support—primarily due to the 50A jump-start capability that only the old-school DieHard charger seemingly delivers.
Geriatric degradation
When I fired one of them up a few months back after not using either of them for a while, though, I noticed that it was making a loud humming sound—incrementally louder at the 2A, then 10A, and finally 50A settings, as I’d recollected from the past—but much louder at each output option than I’d remembered. To confirm, I pulled the other charger out of its box, which also hummed but at the noticeably lower din that I’d recalled. Plus, the first charger didn’t seem to be doing anything charging-wise, whereas the second still seemingly worked fine.
Here’s the first (loud humming) charger, which I re-hooked up just yesterday to my 2001 Volkswagen Eurovan Camper (which uses a standard SLA, not AGM, battery), at the 2A setting:
10A setting:
and 50A setting:
The gauge readings don’t seem to make sense in any of these cases. As background, I top off the charge (normally using one of my more modern chargers) on the battery in the in-storage van once a month at the beginning of the month. I took those photos a bit more than halfway through the month, after a small amount of leakage discharge had inevitably occurred (less than at the end of the month, but still not nothing). So, the full-charge indication doesn’t seemingly reflect reality. Compared to them, the 2A- and 10A-setting displays when using the second (lower humming) charger are more in line with my expectations:
as is the second charger’s 50A-setting display, which I’ve shot as a video because this time, unlike previously, the LED is rapid-blinking as expected:
A 0V output isn’t always bad news
Just prior to taking the prior photos yesterday, I’d actually begun my investigation by hooking both chargers up to my multimeter to see what they were outputting. Here’s the first (loud humming) charger at its 2A, 10A, and 50A settings, first configured for use with an AGM battery:
and then set for a standard SLA battery:
The output levels were, I initially (albeit incorrectly) ascertained, in the ballpark of what one would expect for a 12V battery charging target, although perhaps a bit low. Now look at what happened when I hooked the second (lower humming) charger up, again at its 2A, 10A, and 50A settings, first configured for use with an AGM battery and then a standard SLA battery:
I’ve saved you from looking at six consecutive images of the multimeter displaying the exact same thing: 0V. This initial outcome actually had me wondering whether the second (lower humming) charger was the one that had “gone south”, until I did a bit of online research and learned that this behavior is to be expected. Unless the charger detects that it’s connected to a correct-polarity battery that isn’t already drained (hold that thought), it will disable its output, among other reasons, to prevent sparking in the presence of hydrogen and other off-gassing.
Some amount of transformer hum is to be expected, of course, as many folks reading this already realize; the root-cause phenomenon is known as magnetostriction and results in a generated tone at twice the mains AC frequency (i.e., at 120 Hz in the U.S., for example):
Additional hum sources, quoting Wikipedia, are “stray magnetic fields causing the enclosure and accessories to vibrate.” And it’s also normal for the hum volume to increase somewhat under higher load. Abnormally loud hum and other noise, however, is the result of other, degradation-induced factors, such as progressive disintegration of the transformer’s core adhesive, resulting in separation of the laminated layers, or a rattle caused by loose component mounting bolts.
(Sorta-) twin sons of different mothers
At this point, I’ll point out something else interesting (at least to me) that my research uncovered: there were (at least) two different internal designs that reached production for this particular DieHard charger. It’s the model 71222; as you can see from this closeup of the outer box, mine’s specifically a model 28.71222 (here’s a link to the user manual):
But in searching around, I also came across references to another version, the model 200.71222, including another user manual link (this time even including a parts list and wiring diagram!). The two variants seem functionally identical from a high-level description standpoint and look similar from the outside, too, aside from a multicolor front panel motif in the model 200.71222:
versus my more monochrome model 28.71222. But the insides are a different matter…
At this point, I’ll point out another “information” (I’m using the term somewhat loosely) source that I came across during my research: this video:
Bonus points to Jason Hemphill, the video creator, for knowing (for example) the difference between the transformer’s primary and secondary sides, as well as for (sorta) explaining the purpose of two diodes connected to the transformer’s center tap secondary. But when, in pointing out what he called the “little smart board”, he voiced the following elucidation:
These wires over here…they’re just control…they don’t do anything…
I admittedly started shaking my head. And when, with the charger still powered up, he then yanked the “little smart board’s” fourth (black) wire out of what it was plugged into at its other end (item 7, the 35A circuit breaker, if you’ve already cross-referenced the parts list and wiring diagram in the user manual I pointed out to you earlier), I about fell out of my chair. And then I realized that although his charger was also a DieHard model 71222, it didn’t look like mine on the inside; I hadn’t yet noticed the front-panel motif variance between the two.
Looking “under the hood”
At this point, I’ll transition to the teardown portion of my write-up, before returning and concluding. Beginning with the obligatory outer box shots:
Can’t forget this all-important one…😂
I next opened it up:
and then pulled out the contents (I later found the paper user manual in my filing cabinet):
Convenient carry handle:
Only after connecting the charger to the battery and selecting the desired settings should you, and I quote, “Plug the charger into a live AC power outlet”. Further, “Unplug the AC cord before disconnecting the battery clips”….as well as prior to unplugging internal cabling, yes?
Back off my soapbox…
and back to the backside to uncoil the power cord:
Don’t worry, I won’t ascend the soapbox again. That said…
And now to dive inside. You may have noticed the four screw heads on the sides, two per. Guess what comes next?
That got me partway there:
Oh yeah, there’s another screw head on the underside:
But grandma, what a big transformer you have!
At this point, I was still clinging to the delusion that this charger might be working (I hadn’t yet found Jason Hemphill’s video), so I didn’t disassemble it further. Still, I hope the photos of the internals of my model 28.71222 will be educational for you, not only standalone but also in comparison to Hemphill’s presumed model 200.71222. Here, first off, is the rear-located internal PCB, both much larger than the one in Hemphill’s charger and with an integrated circuit breaker (more accurately stated: fuse pair):
Check out the sizeable SCRs (silicon-controlled rectifiers) and discrete transistors bolted to metal heat-transfer plates on either side of the PCB!
An inner view of the front panel, with the charging current switch at lower left, the gauge at upper right and the AGM-vs-standard SLA switch below it:
And, last but definitely not least, the predominant contributor to the unit’s ~11 lb weight, the transformer. Here’s the primary winding:
And the secondary:
and finally, accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes, perspectives of the top (primary at left, secondary at right):
and one side (ditto):
Old-school pros, cons and conclusions
Note that the output voltage Jason Hemphill was getting out of his charger prior to his “hack” is a close approximation of what I’m seeing with mine at its 50A setting. He indicated in his video that he exclusively uses his “fixed” unit at 50A, so I’m assuming his entire video was also shot with it configured that way (I couldn’t find a sufficiently clear video frame of the front panel to confirm). And by the way, in scrolling through the comments and his responses, I realized I owed him more credit than the little I’d initially allocated (with minor grammar tweaks by yours truly):
You are exactly right. It is not fixed. And you’re also right; it’s likely to be a 10-cent transistor. But most people will not be able to fix the transistor issue. They won’t spend the time to find it, order it and replace it. The solution I’m offering is to turn it into an old school charger. It takes out the safety technology. This solution is an option for people who are old school and are used to working with things that way. As I stated in the video, this isn’t an option to hook up to a battery and walk away. So, if you’re someone who can’t hook up a battery right or doesn’t understand the idea of overcharging and needs idiot-proof technology to do that for you, this isn’t your option: go buy a new one. But if you’re old school, this will do the job.
Further perusing the 100+ comments (resulting from 115,000+ views to date!) of Jason Hemphill’s video was not only educational but also entertaining. I learned, for example, that the DieHard model 200.71222 is internally identical to the Schumacher Electric (the original developer, I’m assuming) SE5212A charger. No idea who originally developed my DieHard model 28.71222, however. And even if I did, I’m not going to try to resurrect this one, no matter that plenty of other folks seemingly prefer ones of a fully manual fashion.
Sure, by bypassing the “little smart board,” the now-manual charger might attempt to resurrect a fully drained battery, but my more modern chargers already do the same thing. They, plus the still-working sibling to my malfunctioning model 28.71222, will also automatically shut off at the end of the charging cycle, versus overcharging and potentially ruining the battery (not to mention causing other potential broader problems). And they’ll also save me from calamity should I distractingly hook up the charger to the battery in a reverse polarity state.
Thoughts on the topics discussed and internal circuitry revealed in today’s piece? Let me know in the comments! If you’re interested in inheriting this charger and converting it to a manual version yourself (note that I take no financial or other responsibility for any subsequent calamities), send me an email! And by the way, if you’re interested in finding out more about how car battery testers work, head here!
—Brian Dipert is the Principal at Sierra Media and a former technical editor at EDN Magazine, where he still regularly contributes as a freelancer.
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