I have a switching power supply (RC Esc). I am measuring the input voltage from battery using a voltage divider. The Divider is attached to a differential DAQ. I have very large transient spikes in both the voltage and current (current shunt). I can suppress a lot of the spike by placing a capacitor across Vlo and Vhi right before going into DAQ. Is this common practice and what is the math involved in the spike reduction / cutoff frequency.
Yes some low pass filter is common and essential.
Technically it's called a transient suppression filter for : Conducted load transients V=LdI/dt, radiated crosstalk and anti-alias suppression (where the rise time interferes with an error of the sampled value) .
The details of the filter need to be defined in terms of your needs for timing and accurate voltage measurement or SNR. i.e. if you are looking for a slow error recovery like low battery , fly home or slow down immediately, that affects how much you can filter the input with a low ESR cap for special low pass filter.
The most common method for measuring DC voltage accurately in the presence of a high transient spike noise is to choose the BW of noise to be suppressed and design a filter to suppress that to an acceptable SNR or the signal.
Since you have some sampling rate for data, your signal BW is likely to be DC to 1/3 of the sampling rate. You must have some anti-aliasing filter to prevent additional errors from noise spectrum above the sampling BW appearing falsely as a signal.
This is not a complete answer and depends on your specs for SNR, BW , load spike noise levels and signal integrity or acceptable error.