Battery Meter Simulation

Importantly, this function sets the bits in the integer pointed to by display which may or MAY NOT BE the global display variable.
To properly set the display bits, set_display_from_batt() will need to do bit shifting along with bitwise operations to construct the correct bit pattern for the display.
A good trick to use is to create a series of bit patterns that correspond to the various digits. For example, according to the diagrams above, the bit pattern for 9 is 0b1101111. If a 9 should appear on the display somewhere, this bit pattern should be shifted and combined with the existing bits in display so that a 9 will show. Creating similar constant mask patterns for each digit, the decimal place, the positions of the Voltage/Percent indicator, and the bars for the visual level indicator is a good way to make this problem manageable.
A detailed explanation of one approach to the problem follows.

Create an array of bit masks for each of the digits 0-9. The 0th element of the array contains a bit mask like 0b0111111 which represents the bits that should be set for a 0 digit, the 1th element of this array has a mask like 0b0000011 which are the bits to be set for a 1. There should be ten entries in this array in indices 0-9.
Use modulo to determine the integer value for right, middle, and left digits for the display from volts or percent. Each digit should have a value from 0-9 and be Stored in its own variable.
Start with an integer variable of 0 (all 0 bits).
Use left digit variable to index into your array of masks to determine the bits that should be set for it and shift them over an appropriate amount combining with the existin display bits.
Combining bits here is a logical operation of setting all bits that are one in the mask to 1 in the display variable.
Repeat this process for the middle and right digits shifting left by appropriate to where the digits should line up.
There are several special cases to consider such as leading 0’s in a percent should be blanks so nothing should be drawn.
Don’t forget to set the additional bits like the decimal place, the V/% indicator, and the level bars for the visual indicator.
Importantly make code as simple as possible as the target CPU has a limited amount of RAM and longer codes will occupy more RAM. Avoid nesting conditionals very deeply: if/else if/else structures are fine but nesting a conditional within this will be penalized.