The soul of a Citizen Eco-Drive watch lies within an inconspicuous yet vital energy storage component. Enthusiasts often refer to it simply as the "watch capacitor." In models powered by the H240 movement, once this tiny energy capsule ages, the watch begins to suffer from intermittent operation and a drastic drop in power reserve. Today, we will take a closer look at this "capacitor" used in the H240 movement—explaining what it actually is, how to replace it, and how to breathe new life into your timepiece.

The Origin of the Name "Capacitor": Actually a Rechargeable Battery

First, let’s clear up a common misconception. In the context of Citizen Eco-Drive watches, the component widely known as the "capacitor" is—in most modern movements, including the H240—actually a rechargeable secondary battery, not a capacitor in the technical sense. True capacitors (such as the Panasonic 295-29 or 295-51 "gold capacitors" used in early Eco-Drive models) store charge directly on their plates, resulting in high self-discharge rates and low capacity. In contrast, the energy storage component used in the H240 movement is a lithium-ion rechargeable battery. However, because it is button-shaped and closely resembles a capacitor—and because supercapacitors were indeed used when Eco-Drive technology first launched—the term "capacitor" has persisted.

Therefore, when you are ready to replace the "capacitor" in your H240, what you are actually looking for is a rechargeable button-cell battery.

H240 Movement Energy Storage Component Model

The Citizen H240 is a classic Eco-Drive quartz movement featuring functions such as a perpetual calendar, alarm, and world time; it is commonly found in early multi-function Eco-Drive chronographs. Its standard energy storage unit model is:

CTL621F (or marked as MT621; the two are equivalent)

This battery has a diameter of 6.8mm, a thickness of 2.1mm, a nominal voltage of 3V, and a nominal capacity of approximately 5mAh. Despite its tiny size, when paired with the solar panel beneath the dial, it can power the watch for months on a full charge (the H240 movement offers a runtime of approximately six months, or even longer in power-saving mode).

How to verify if you have a CTL621F?

After opening the case back, you will find a button-shaped component tucked beneath the metal retaining clip on the movement. It is usually marked with "621" or "CTL621" on the side; the negative terminal faces upward, while the positive terminal makes contact with the metal contact point at the bottom. If you see a model number starting with "295-", that indicates a true capacitor; these are typically found in older movements and are rarely used in the H240.

Typical symptoms of capacitor aging

If you notice the following issues, it is highly likely that the energy storage capacitor needs replacing:

Drastically reduced runtime after a full charge. A watch that once ran for months after a day of sun exposure now runs for only a few days—or even just hours—on a full charge.

Frequent "two-second jump" of the second hand (a low-charge warning) that persists even after prolonged exposure to light, preventing the watch from returning to its normal rhythm.

Stopping at night. The watch runs normally while worn during the day but stops a few hours after being taken off at night, with the hands resetting to the zero position by the next morning.

Resetting or erratic function. Low voltage can cause the perpetual calendar to lose power and reset, reverting the date to its baseline and requiring reconfiguration.

Complete inability to store energy. Regardless of how long it is exposed to light, the second hand only performs the "two-second jump," and the watch stops immediately once the light source is removed.

These symptoms indicate that the internal battery's internal resistance has risen sharply, or its capacity has degraded to the point where it can no longer store energy; this process is irreversible, and replacement is the only solution.

Replacement process and precautions

Replacing the CTL621F is a delicate task. It is recommended for enthusiasts with the necessary tools and experience; beginners should ideally entrust the job to a professional watchmaker. Tools Required

Case-opening ball or wrench (depending on the case back type)

Toothpick or insulated tweezers

Anti-static plastic tweezers (for handling the battery)

New CTL621F (ensure you buy the watch-specific version; pre-soldered tab versions are unsuitable, as the original uses direct contact)

Alcohol swab, air blower

Water-resistant grease (if restoring water resistance)

Brief Procedure

Remove the watch strap and open the case back. For screw-down backs, use the appropriate tool, taking care to protect the threads.

Remove the battery retaining clip. Loosen the spring clip or screw securing the energy storage unit, noting the position of any insulating spacers.

Remove the old capacitor. Gently pry it out with insulated tweezers to avoid short-circuiting the positive and negative terminals. Observe the orientation of the old unit; the negative terminal usually faces upward, matching the new unit.

Install the new CTL621F. Place the new battery into the slot using anti-static tweezers or a bamboo pick, ensuring good contact between the positive terminal and the gold contact point below, and that the negative terminal faces up against the retaining clip.

Reset the movement. Replacing the battery in the H240 movement often triggers a full reset, causing the hands to spin erratically; an "AC Reset" and "Zero-Position Calibration" are required. Typically, there is a reset hole marked "AC" on the movement; briefly short-circuit it with tweezers (or short the battery's positive terminal to the marked point on the movement). Then, follow the manual to recalibrate the reference positions and reset the perpetual calendar.

Test charging. Before closing the case, expose the dial to strong light and observe whether the second hand moves normally (one tick per second) and if functions are restored.

Clean and lubricate the water-resistant gasket, screw the case back on tightly, and finish.

Key Reminders

Do not reverse polarity. Installing it backward won't burn out the movement, but the watch will not operate.

Avoid short circuits. Do not touch both the positive and negative terminals simultaneously during installation to prevent instantaneous discharge that could reduce the battery's lifespan.

Resetting is mandatory. Power loss causes the H240 to lose perpetual calendar data; without performing a reset and zero-position calibration, the date hand may be inaccurate or functions may malfunction. You can search online for tutorials on "H240 movement reference position adjustment." Water resistance check. If the watch will be exposed to water, it is best to perform an airtightness test after the replacement.

Genuine vs. counterfeit capacitors: choose carefully.

The quality of CTL621F capacitors on the market varies widely. Try to select products clearly marked as "Citizen OEM" or manufactured by Maxell/Seiko; some cheap aftermarket parts suffer from high internal resistance and severe self-discharge, meaning you’ll be back to square one within six months. A genuine CTL621F capacitor features a neatly applied bottom insulating gasket, a smooth surface, and clear markings.

If your watch is a model that uses a true supercapacitor (such as the 295-6900 or 295-51—models you might encounter in older watches), note that these are not directly interchangeable with the CTL621F. They have different voltage characteristics, and mixing them up could damage the movement or cause overheating.

Revitalizing the energy capsule: giving the watch a new lease on life.

After replacing the energy storage capacitor, an H240 watch can usually return to a state of full charge and long-lasting operation. A day of exposure to a high-intensity flashlight or sunlight is all it takes to bring it back to life, ready to accompany you faithfully for months on end. This tiny capsule is both the watch's power source and a testament to the silent, efficient operation of Eco-Drive technology. When you personally swap out the depleted old cell and watch the second hand resume its steady, confident beat, the satisfaction you feel—as a watch enthusiast—represents the simple, profound bond between human and machine.

So, if your H240 is showing signs of fatigue, there is no need to panic. Simply find the correct CTL621F capacitor, install it with care, and let time flow once more on your wrist.

CITIZEN UT621 Battery for Citizen H240 Watch capacitor