In case you ever need to know.  If your battery ever runs low on water, the battery light comes on.

After refilling the water, you probably need to drive around a bit to allow the alternator to recharge the battery to a stable state.

It would appear that on a 2002 F-150, the optimum recharge RPM is 2,000. Below that, the battery light stays lit. Above 3,000 RPM, the same thing happens.   So, this means you need to drive around at about 65 MPH ( American ) to archive the desired recharge capabilities.