Latching Solenoid Drive Voltage Problem
You can calculate the required capacitor from:

C= \$\frac{I \cdot T}{\Delta V}\$ 

Where I is the coil current, T is the pulse time, and \$\Delta V\$ is the allowable voltage droop.

For example, if the current is 1A, the allowable droop is 1V and time is 50ms, then a 50mF capacitor (50,000uF) is required. Naturally, a larger capacitor will take more than 10ms to charge if the battery cannot supply enough current to hold the supply up for 50ms.