Putting capacitors in series will increase their working voltage while decreasing their capacitance. So four 25V 100uF caps in series will be similar to a 25uF 100V cap...similar because the parasitics will be a lot worse which isn't good for bypassing.

Putting caps in series combines capacitance while reducing lead inductance and keeping the same working voltage.

So you'd need to have a bunch of series chains in parallel with each other. Takes up a lot of room and adds a lot of lead inductance which defeats the purpose of bypassing.

C9 will never have more than 5V on it since it decouples the Vcc line for the 74HC00 so a 10V, or more, rated part will be OK here.
C4 will have up to whatever voltage you decide to run your motor at ( 3.6V to 36V ) so a 16V rated cap would be OK if you are only feeding 12V into the V+ lead for the motor.

Originally Posted by dknguyen

Putting capacitors in series will increase their working voltage while decreasing their capacitance. So four 25V 100uF caps in series will be similar to a 25uF 100V cap...similar because the parasitics will be a lot worse which isn't good for bypassing.

Putting caps in series combines capacitance while reducing lead inductance and keeping the same working voltage.

So you'd need to have a bunch of series chains in parallel with each other. Takes up a lot of room and adds a lot of lead inductance which defeats the purpose of bypassing.

Of course, you meant parallel.