ndf = pd.DataFrame(columns=['temps', 'voltage'])
#ndf = ndf.append({"temps": 1, "voltage":2}, ignore_index=True)
n = 1000
for i in range(0, len(df), n):
    ndf = ndf.append({"temps": df.iloc[i:i+n].temps.mean(), "voltage": df.iloc[i:i+n].voltage.mean()}, ignore_index=True)