You've identified the core problem well: the built-in limitations of standard solar shed lights due to the battery charging/discharging risks and the photoelectric switch tied to the outdoor solar panel. Your idea of a specialized mechanical timer to circumvent this is clever.Let's break down the solution and explore some options

Understanding the Requirements
SPDT Functionality

You need a timer that acts as a Single-Pole, Double-Throw (SPDT) switch. In the "off" state, it connects the battery to the solar charger. When the timer is activated, it disconnects the charger and connects the battery to the light.

Mechanical Timer

You want a mechanical timer, not a digital one.

15-Minute Resolution

The timer should be able to be set in increments of 15 minutes or less.

Reliability

Mechanical timers are usually quite reliable, which is a good choice for this application.

Low Voltage DC

The timer needs to handle the low voltage DC current of the solar system (probably 3.7V or 7.4V from the battery).

Possible Solutions & Considerations1. Relay-Based Solution with a Standard Mechanical Timer

This is likely the most feasible and robust approach:

Components

A standard mechanical 15-minute timer. A low-voltage (e.g., 5V or 12V) SPDT Relay. The relay's coil voltage should be easily achievable (e.g., you might use a USB power adapter for the relay coil). A small power supply for the relay coil. (5V USB adapter)

Wiring


Timer Activation

Wire the mechanical timer to control the relay coil. When the timer is "on," it energizes the relay coil.2.

Relay Contacts

The SPDT relay contacts become your switching mechanism.

Common (COM)

Connect the battery (+) to the common terminal of the relay.

Normally Closed (NC)

Connect the normally closed terminal of the relay to the solar charge controller input. This is your "charging" state.

Normally Open (NO)

Connect the normally open terminal of the relay to the (+) input of your light. This is your "light on" state.

How it Works

When the timer is off, the relay is in its "normal" state. The battery is connected to the solar charger (NC contact). When you turn the timer on, the relay is energized, switching the battery connection from the charger to the light (NO contact).

Advantages

Standard parts are readily available and inexpensive. Relays can handle the current load without problems. Reliable solution. The timer is not directly switching the battery current.

Disadvantages

Requires some basic wiring skills. Adds complexity compared to a single component. Requires a small separate power supply for the relay coil.2.

Finding a Specialized Mechanical Timer (Highly Unlikely)

While theoretically possible, finding a mechanical timer with SPDT contacts specifically designed for low-voltage DC and with the load separation you need is extremely unlikely. Mechanical timers are generally designed for higher voltage AC circuits. Even if you did find one, it would probably be expensive and difficult to source. The relay solution is almost certainly the better path.3.

DIY Electronics Project (More Complex)

You could design a circuit using a microcontroller (like an Arduino) and a relay to achieve this. The microcontroller would control the relay based on a timer. This is a significantly more advanced project that requires electronics design, programming, and prototyping skills. It's not recommended unless you're already comfortable with these areas.4.

Modified Solar Light (Requires Disassembly & Rework)



Concept

If you're willing to take apart an existing solar light, you could potentially modify the circuit.

Challenges



Battery Protection

You'd need to ensure proper overcharge and over-discharge protection for the lithium battery if you're bypassing the built-in circuitry. Lithium batteries can be dangerous if mishandled.

PE Switch Removal/Bypass

You'd need to disconnect or bypass the photoelectric switch entirely.

Wiring in a Standard Timer

You'd still need to incorporate a relay to provide the SPDT switching function and then use the timer to energize the relay coil.

Downsides

Voids the warranty. Risk of damaging the solar light. Requires soldering and electronic skills. May not be reliable.

Recommendation

The relay-based solution with a standard mechanical timer is by far the most practical, reliable, and cost-effective approach. It gives you the functionality you need without requiring specialized or hard-to-find components.

Steps to Implement the Relay Solution1. Purchase the Components

Mechanical 15-minute timer. Look for one that is small, easy to mount, and easy to set. Low-voltage SPDT Relay (5V or 12V coil). Choose one with screw terminals for easy wiring. Small Power Supply for Relay Coil: (USB adapter) A project box to contain the relay and timer. Appropriate gauge wire.2.

Mount the Components

Mount the timer and relay inside the project box.3.

Wiring

Carefully wire the components as described above. Double-check your wiring before applying power.4.

Testing

Test the system to ensure it works as expected.5.

Integration

Connect the battery, solar panel, and light to the timer/relay box.6.

Installation

Mount the solar panel outside the shed, run the wiring to the timer box inside the shed, and connect the light.

Important Safety Considerations



Lithium Battery Safety

If you are ever working directly with lithium batteries or modifying a solar light's charging circuit, be extremely careful. Lithium batteries can overheat, catch fire, or explode if they are overcharged, over-discharged, short-circuited, or damaged. Always research and follow safety precautions.

Wiring Safety

Ensure all wiring connections are secure and insulated to prevent shorts.

Professional Help: If you are not comfortable with electrical wiring, hire a qualified electrician to perform the work. Flag for review