I immediately used this trickle charger to charge my battery I knew that was dead. It came back to life!!

I purchased this to keep my shelved replacement UPS batteries maintained and in top condition.Last December I purchased 2 replacement UPS batteries for an incredible price. The OG ones were more than 5 years old at the time (nearing the end of their useful life, or so I thought) and the UPS self tests seemingly supported this. The next significant power failure, however, the originals lasted as long as they had when they were new. They are still performing well, now approaching 6 years old.Unfortunately, that means that the batteries I purchased 8 months ago have been on the shelf longer than they should. Making things even more complicated, I still have 2 other batteries from the retired UPS units that I have been “topping off” every few months ever since, using the old units as chargers. Those batteries are supposedly still viableI, but have not been properly tested since.This is spinning out of control and something needs to be done. Now!My solution, after careful consideration, this unit!My initial testing has been on smaller UPS replacement batteries, separately and in series; all of them 12 volts. The batteries I’ve been talking about are 24 volts (2x12v in series), but very similar otherwise.The first four batteries tested were removed and replaced (after nearly 5 years of service) from a supplemental battery pack when that system’s expected run time had become significantly shorter than it had been previously. All four tested OK. The bad battery was in the main UPS, it tested bad when isolated from the external battery pack.Next I tested a known failed battery removed from a rechargeable portable 12v power supply and self contained vehicle jump starter. This battery was replaced a few years ago and has been sitting in the new one’s box ever since.This charger valiantly attempted to resurrect this battery, starting with SAVE mode, but in the end the battery couldn’t hold a charge and subsequently tested bad. It was always cool to the touch (you’ll understand the significance of this later.) This was the expected outcome. The charger passed this test by failing this battery!After that, a “Hail Mary!” I unexpectedly found a brand new, sealed replacement UPS battery that had been completely forgotten about for more than 8 years. Just for kicks I hooked it up, expecting nothing; it should be completely dead and unrecoverable!To my surprise the charger engaged it in SAVE mode. Over the next few hours, the battery gradually became quite warm (some might even say hot), but never to the point where I had to pull my hands away for fear of being burned.The user’s manual described this situation quite well and clearly explained what conditions warranted aborting the process immediately; those conditions were never met. The manual did indicate that desulfating the battery might be required as determined by the charger’s initial evaluation. It doesn’t indicate this process is being performed externally, but I believe the substantial heating was an indication that it was being conducted. Once it exited SAVE mode and started charging the battery in earnest, it cooled down to slightly warm. Charging took a few more hours. It eventually finished and entered a 12 hour charge retention test, which it passed with flying colors. It’s not under load, so maybe not as good as new, but it’s holding a full charge! Miraculous success in my book!Now, to the reason I purchased this unit in the first place. Maintenance on the “shelved” 24 volt UPS back up batteries.The first one didn’t go well. It started in SAVE mode (i.e. dead) and stayed mostly cool to the touch throughout. I did notice that it eventually became slightly warmer in one corner. Once it exited SAVE mode, it started charging and gradually began to heat up. By three hours in, it had become very hot. Following the recommendations for a “severely neglected battery” in the instruction manual, I aborted the process immediately. Based on the information in the manual, it had likely developed a bad cell while in storage that couldn’t be recovered.Another battery, essentially the “twin” of the first, and treated identically, was next. When connected to the charger it started up in the first charging phase (50% to 75% charged), quickly progressing to charging phase 2 (75% to 100%) and within a half hour charging was complete. It was “topped off”, tested good and was retaining its charge. Excellent!The remaining 2 batteries were the ones purchased last December. They both should have been installed in a UPS months ago. At this point they need a maintenance charge, and hopefully that’s all!Recommendations for these batteries is to install them in a UPS within 6 months of production. When purchased new from a reputable dealer, they’re usually about four months old. If not installed by 18 months, risk of permanent damage becomes a factor. Once I opened the boxes, they were both tested by the factory on 8/6/22, well within the 18 month window.Everything went well with charging for both, pretty much the same as the second battery.In the end: lost one, another slightly used but OK, and the last 2 are essentially “new old stock”. All 3 “good” ones are “topped off” and ready when needed.This charger has performed admirably! Although no one has used it for this specific purpose before now (that I know of), it’s exactly what it was designed for. It has allowed me to test and maintain my shelved UPS batteries, as well as evaluate those retired from service.Everything under control now!UPDATE (mid January):Topped off all batteries currently in storage (standard maintenance.) Of the four 5+ year old 12 volts removed from the supplemental battery pack, 2 just needed to be topped off, but the other 2 needed to be desulfated. This required quite a lot of additional time (mostly to perform a 12 hour charge retention test rather than the standard 30 minute one.) Both batteries were ultimately recovered, but my decision to replace them seems to have been a good one.The 3 24v batteries had no issues and topped off quickly. The OG ones in the 2 UPS units are giving no indication of failure. They are now approaching 6.5 years old.So far, so good, as OEM replacements have become much more expensive recently.