In order to know the "battery voltage" while actively charging, you will need to measure the charging current I
c as well as the terminal voltage V
t, and have a good idea of the "internal resistance" of the battery, R
i. Then, V
batt ≈ V
t - I
cR
i. Ideally, you'd measure R
i
yourself, but if you just want a nominal value,
a study by team 1640 a few years ago indicated that a good FRC battery has an internal resistance of about 0.02Ω. Alternately, if you use a 2A trickle charger, the difference would only be about 0.04V, even when still charging at full current.