Battery Life Science

Solving the battery life issue

Can a CR2032 power a Digimon and Decent Backlight long enough to not change batteries weekly?
Here is the science.

Digimon Battery Supply

CR2032       3V (Full Ideal Battery, actual batteries usually closer to 2.8V)

Capacity: 235mAh (Current at 1mA)

Available Current: 10mA (Full Battery)

The graph shows battery life capacity vs different current loads, as high current load will reduce overall capacity of battery. Capacity ratings are taken when voltage reaches 1.6V, which is not a functional voltage for Digimon. 2.4V is much more realistic as a battery with less voltage does not give decent contrast to Digimon screen, the basic sign to change batteries.

The Digimon LED Circuit draws large spikes of current giving a CR2032 a estimated capacity of estimated under 100mAh. This is a loss of rough 50% capacity, which we will discuss mitigating slightly with a large electrolytic capacitor to store charge between pulses.

The Digimon normally has extremely low current draw, which is why batteries last a while.

Digimon Power Specs

Digimon has roughly three different current draws

~15uA - Digimon Idle with Screen Off (Current draw the majority of the time)

~500uA - Digimon Screen On (Current draw when in use)

~4mA - Digimon Screen On and Processing/Updating (very short pulses from IC)

LED Modification Power Specs

Current draw of the FX Light circuit is designed to be minimal to prolong battery life.

~0.7uA - Screen OFF, Light OFF (IC Powered Off)

~1.35mA - Screen ON, Light OFF (IC Powered On and Idling)       

~5mA - Screen ON, Light ON (Primary/Low Brightness)

~16mA - Screen ON, Light ON (Maximum Brightness)

So roughly now with Screen on and Light on we get to a decent amount of current draw, with small spikes during screen updates, such as animations, and menu changes. These spikes and increased current draw will lower the CR2032 overall battery capacity. The CR2032 battery can only output 10mA constant to a load so to provide more current fast during load spikes a capacitor can help store needed charge. Also to avoid further loss to overall battery capacity, a capacitor in parallel with battery seems logical.

A simple 100uF capacitor should be enough, to help level out current spikes on the battery. The Digimon circuit board has a location marked as if waiting for this capacitor. This capacitor was considered in the engineering of the Digimon, but was not needed due to low current spikes. As this is optional we do not need to install this capacitor but it will help extend the battery life for this modification. This reduces average current spikes by half, keeping battery current more steady and nearer to 1mA, which gives us a estimated battery capacity of 220mAh until 2.4V.

Now just to compare a regular Digimon's battery life to a Modified Digimon's battery life.

Estimated average of 2 Hour of Digimon screen time a day, with 30 minutes of light on, the rest idle time. Now calculate the average daily current draws using very safe estimates. 

Regular Digimon

22 Hours x 0.015mA = 0.33mA <-----Idle

2 Hours x 0.5mA = 1mA <-----Screen On (Sometimes Pulses as high as 4mA, but average 1mA)

1.33mA / 24 Hours = 0.055mA   <----- estimated average current through-out the day

235mAh / 0.055mA = 4272 Hours = 178 Days = 5.8 months <-----estimated average life expectance of a battery

Modified Digimon

22 Hours x (0.015mA + 0.0007mA) = 0.35mA <-----Idle

1.5 Hour x (1.54mA + .5mA) = 3.06mA  <----Screen ON

0.5 Hours x (5mA + .5mA) = 2.75mA <-----Screen ON and Light ON

6.16mA / 24 Hours = 0.257mA  <-----estimated average current through-out the day

220mAh / 0.257mA = 856Hours = 36 Days = 1.2 months <------estimated average life expectance of a battery

Conclusion: Battery lifetime expectancy is reduced to roughly 20% with this mod. Battery lifetime should be roughly close to 1 - 2 months. 

This seems an acceptable trade off for this modification, as with any addition of light, adds current draw, and a shortening of battery life.