Monitor Battery During Charging

How can you accurately monitor the state of the battery when its being charged. I have a simple battery monitor right now for a Lead acid battery that works when the charge is disconnected but shows that the battery is a 100% full when the charger is connected to the battery. The circuits is a basic LED bar graph with a voltage divider.

Are the battery monitor's wire leads separate from the wire coming from the charger (ie connected directly to the battery terminals)?

Pulse the charger on/off; read the battery voltage during the off times.

Ok that sounds simple enough. Is that how they do it on device like cell phones and ipods.