Citizen Eco-Drive Titanium H500 S021260 HST Battery MT621 Changed 12/30/2022

This ultra-low mass solar PV charged Lithium Magnesium Titanate MT621 battery powdered eco-drive quartz titanium chronograph by Citizen features light blue lume on the hands & hours, making it easy to read in the dark after the phosphorescent europium doped alumina glow powder charged up by the same light that produces DC current in the solar PV module under the dial that charges the battery to power the quartz H500 eco-drive movement by Citizen :) 

The low contrast white on silver / silver titanium color scheme low contrast & hard to quickly read at-a-glance in regular visible light. The dual-tone LED flash from the iPhone camera here illuminates the dial much more, making it far easier to read in this photo than IRL. 

Its ultra-low mass 82.6 grams almost makes it feel cheap, but its actually made of a titanium alloy that's stronger than steel, with compact mechanical strength ideal for a low mass wrist watch case & band. This makes it wear or feel almost invisible by contrast to my much heavier daily Invicta Pro Diver 3044 automatic NH35A movement watch with a mass of 210 grams respectively. The Pentagon funded SR71 Blackbird Stealth Fighter Jet was also made with titanium ^^ I love titanium metal, its super dope!

Opening the case by removing the case back was a completely annoying frustrating event because of how overly tightly torqued down the screw back stainless steel back plate was fastened into the titanium case from the factory. At such high torque settings the water-resistance gasket produces resistance friction, making the initial loosening of the case back extra difficult. 

I have a special watch repair kit & had to take the band off the watch, mount the watch into a special glass-fiber nylon watch vise then mount the watch vise into a 4 inch steel vise & apply over 80 fl-lbs of toque to undo the case which had not been opened since it was manufactured back in 2006. 

The OEM battery MT621 was fading & providing poor run-time per charge relative to my newer ECO-Drive watches, so I got a replacement battery for $13.20 online (tax & shipped) via Amazon.com & did the DIY style replacement at home :) 

After removing the band & case back, it was just a matter of pushing the small battery retaining leaver out of the way with the end of the tool in the right side of the image & popping or gently prying the battery out. 

Aligning the new battery with the gold tab to connect to the contact in the bottom of the battery holder the key to making it connect electrically correctly. Thankfully there is a cutout in the movement that shows you exactly how to insert the battery. You can see the golden tab on the old battery to the left of the watch in the image.

Having a closer look at this cheap movement, it features plastic gearing & no jewel bearings, typical of a low-cost optimized quartz movement. Depending on what kind of plastics CITIZEN used, the operating life of this movement limited to the life of the plastic gearing. Metal gears mounted in jewel bearing cups lubricated with synthetic automatic watch lubricants can operate in such mechanisms for 100+ years, where plastic movements like this have perhaps 10-40 years of operating life. This unit from 2006 already about 17 years old. 

That means the OEM rechargeable lithium manganese titanate battery had a long useful life as a rechargeable battery, especially since common lithium ion batteries used in smartphones & similar IoT devices start to fade or lose capacity after just a couple or few years of daily operation. Sure the energy levels here are the nano-watt range, so such a comparison very apples to oranges, but there are many different lithium ion battery chemistries available, titanate having one of the longest operating lives in both cycle life & calendar life, but a lower cell voltage & lower energy density & higher cost per unit of energy storage, so LFP or lithium Iron Phosphate offers better real world feasibility & thus more common & more widely applied as a lithium ion battery chemistry type :) 

The H500 movement around $82 online, mostly because of its high technology solar photovoltaic charging dial & battery charging circuit & controller & dual motor chronograph functionality. 

Now its charging under a low intensity warm white LED indoor lamp bulb for 8+ days, or if any sunlight from the sky becomes available (unlikely, its winter, grey, overcast, western Washington state near Seattle dim ambient outdoor light levels), then I can park it in a window sill to pick up a stronger source of light for 5x faster PV charging :) 

The replacement battery came partially charged with instructions saying it must be fully charged initially by parking the watch in a source of bright light for 24+ hours of light exposure or longer if lower intensity light used to charge the solar movement battery :) 

Eco-Drive movements claim they never need a battery & the OEM MT621 from the factory could have been milked for longer, but the runtime was down to just a few days before the low battery warning double second jump of the second hand would appear, even after saturate charging it in direct sunlight for many hours in the window this summer. So I knew it was time for a new battery & had the tools & knowledge & skills to order the right battery, to take the watch apart, to change the secondary battery & put it all back together correctly :) 

I am curious if the watch will last during my lifetime or if the plastic gears will give up the ghost. I have a collection of about 10 watches with 3-eco-drives & one other solar digital watch, so 4 solar powered. The rest are automatic mechanical or fancy quartz & digital from my late father Ken's collection. I started becoming interested in automatic watches when shopping on overstock.com back in 2004, when I found a 7S26 powdered Seiko 5, 34mm with white high contrast dial & low mass stainless case & band for $50 :) I paid $200 for this Titanium Citizen Eco-Drive Chronograph by comparison. 

Apple Watch's have become the fitness tracker alternative to Fitbits we used to use, at much higher costs, they also have terrible 18 hour battery life & required daily charging on a magnetic puck, though the Series 8 Apple Watch I rock daily with a bright orange nylon band, charges way faster. Speaking of batteries, I just changed the CR123 battery in my tiny Fenix E16 flashlight this morning, the last day of 2022, December 31st. Happy New Year friends!