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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 enter image description herevoltage when the source is connected.

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    \$\begingroup\$ Wahhhh!!! You're attaching a lithium battery to a voltage source!? That's not how any of this works. Lithium-battery charging has to be done with a charge controller, unless you like fire, toxic fumes and explosions. \$\endgroup\$ – Marcus Müller Sep 1 at 11:24
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    \$\begingroup\$ Also, your question betrays a lack of the very basics of electronic circuits (nodes in a network only having a single voltage is literally the first 15 minutes of the first lecture on the first day of any electrical engineering curriculum), so I strongly recommend you do not build your own Lithium Ion charger. \$\endgroup\$ – Marcus Müller Sep 1 at 11:25
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    \$\begingroup\$ Hi, warm Welcome to EE:SE! It is very easy to confuse us with the words. A drawing or a schematics helps us to really get an overview about your set-up diagram. It helps save time and also avoid confusion. A good, clearly detailed and specifically targeted question yields a good helpful answer. \$\endgroup\$ – Umar Sep 1 at 11:43
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    \$\begingroup\$ There IS NO 12V LiIon battery. A 3S battery (3 cells in series has a 3 x 3.6 = 10.8V mean voltage and a 4.2 x 3 = 12.6V absolute maximum voltage. || The maximum voltage during charging t the battery terminals with load and source attached, or just source is 12.6V for a 3S battery. IF you charge the battery correctly then with no load and with source connected the voltage will be almost identical at the end of charging with or without source connected. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. ... \$\endgroup\$ – Russell McMahon Sep 1 at 12:46
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    \$\begingroup\$ You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | You MUST use a properly designed LiIon charger. | \$\endgroup\$ – Russell McMahon Sep 1 at 12:46
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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.

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  • \$\begingroup\$ It is not wholly clear from what he is asking what he is doing BUT it is exceedingly dangerous to infer the battery state from measured states during LiIon charging. The MAXIMUM voltage for LiIon is 4.2V/cell (or manufacturers recommended Vmax) and inferring Vreal based on calculated ESR etc may work for the suitably skilled but is more likely to see the battery killed for most people. You know that :-). \$\endgroup\$ – Russell McMahon Sep 1 at 12:50
  • \$\begingroup\$ Ok. Safety hat on, You never set your power supply higher than the voltage you want to charge to, and as he has not blown himself up yet, he must have some level of current limiting. With that out of the way, you have a battery charging, without unplugging it from the supply, how do you know if its 10%, 50% 80% etc, this is where my suggestion for empirical data came from, the ESR and float charge ratio on these things is non linear, so its going to be easiest to just measure them once, note them down and use them to cross reference later. \$\endgroup\$ – Reroute Sep 1 at 12:55
  • \$\begingroup\$ And before the obvious reply comes to haunt me, I am aware most smart chargers will output the % of charge, as he is asking these questions, he probably does not have one, It all comes down to how much you respect the dangers of what your doing, I have charged many lithium chemistry batteries with a lab supply, but I never walked away from it, or left it in a way that if things went wrong would cause damage. this is the respect part. Its how most people come to work around risks, learn what can hurt you, and treat it carefully. \$\endgroup\$ – Reroute Sep 1 at 12:59
  • \$\begingroup\$ Try charging your camera batteries (2S) in a Chinese hotel room when you relaise that your charger is still at home in New Zealand :-). || A lab supply does a fair job of charging CCCV if it has V & I set points (as I'm sure you know). Set V to say 4.1V/cell. (4.0V/cell even safer) Set I to Imax. Connect. Battery charges at Imax until V limit is reached when it will change to CV and I will tail down under battery chemistrty control. With CV set to slightly under 4.2V/cell the tailing down to ~= 0 A is "sagfe enough". Set it to 4.2V/cell and it will do nasty things if left long enough. \$\endgroup\$ – Russell McMahon Sep 2 at 2:57
  • \$\begingroup\$ Along the way under CC charge you get a good feel for where you are from the Voltgae. When V reaches Vmax you get a fair idea where you are in the CV phase from the current. \$\endgroup\$ – Russell McMahon Sep 2 at 2:58
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Lithium Ion batteries needs specialised chargers.

  1. You have to monitor both current and voltage to determine the charging status of the battery.
  2. The voltage alone will never give an idea of accurate charge. 3.
  3. The battery is charged with constant current source until the nominal voltage of the cell is reached
  4. 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

Here is a charging graph for Li-ion battery:
enter image description here

Below is again a graph of charge capacity versus voltage measured
enter image description here

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