I want to measure the battery voltage while it is charging, 12v li_ion battery here I am using and a source supply is used to charge a battery and providing supply to the load(like inverter whenever supply turned off, load uses the battery power ). a multimeter is used check the battery voltage that time it showing source voltage if I remove the source and checked that time it showing correct battery voltage how I measure battery voltage when the source is connected.
What your after is correcting for the battery ESR and float charge, sadly this is not documented for most batteries, but you can infer it to a limited extent
If your just after it for this one battery, it may be better to note a few points while it is charging and use that to know roughly where it is.
Lithium Ion batteries needs specialised chargers.
- You have to monitor both current and voltage to determine the charging status of the battery.
- The voltage alone will never give an idea of accurate charge. 3.
- The battery is charged with constant current source until the nominal voltage of the cell is reached
- The battery is then supplied with constant voltage source until the charging current falls down to a predetermined charge current.
Full charge occurs when the battery reaches the voltage threshold and the current drops to 3 percent of the rated current.
More helpful information from here