Estimating Save Battery Life

You can get a very rough estimate of battery life by measuring the voltage on the Game Boy boards in millivolts across the battery-series resistor. Just use Ohm's Law to find the current: I = V / R where V is the voltage in millivolts you measure and R is the resistance in ohms. If you use these units, the current will be in milliamps.

Fun fact: you can measure this voltage using TP2 and TP3 on the back of the board as your multimeter probe points.

Then, find the milliamp-hour rating of your selected battery (preferrably from a datasheet). For example, a Renata CR2032 battery is rated for 225 mAh. Take this number and divide by the milliamps calculated above, to get a rough estimate of the number of hours the battery can supply when the console is turned off (when it is on, the Game Boy powers the cart so the battery is unused). Divide by 24 and then 365 (or just divide by 8760) and you will get a number of years.

So for an overall equation to determine the very approximate number of years the battery will survive:

Years = Resistance of R1 (ohms) * Battery capacity (mAh) / Voltage (mV) / 8760

For an example: an MBC5 cartridge where R1 is 10 kΩ (or 10000 Ω), using a CR2025 rated for 165 mAh, and a voltage of 10 mV measured across the terminals of R1, yields 10000 * 165 / 10 / 8760 = 18.84 years of battery survival. This number can be different due to changing environmental conditions, self-discharge of the battery, and also actual capacity of the battery - discharging a battery at very small currents will increase its apparent capacity, and as the voltage decreases, the current required for data retention decreases as well.

Just backup your save data within a decade of making the cart and you'll be fine.