What would happen if I charge a LiPo battery with a constant current source?
I know that properly it should be charged constant-current until 4.2V and then constant-voltage mode, but what if I just keep sourcing current to it, will it exceed the 4.2 limit or it will charge happily at 4.2V with whatever current it needs?
Charger schematics is taken from LM1084 datasheet.

In this application the LM1084 acts as a constant voltage, constant current part by sensing the voltage potential across the battery and compensating it to the current voltage. To maintain this voltage, the regulator delivers the maximum charging current required to charge the battery. As the battery approaches the fully charged state, the potential drop across the sense resistor Rs reduces and the regulator throttles back the current to maintain the float voltage of the battery

With suggested schematics you can set current to say 5A at 3.5V and only 100mA at 4.2V, but will battery stop at 4.2V by itself or not?

  • \$\begingroup\$ Li-ion batteries are slightly "leaky" in the opposite direction; I would not advise floating one. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 8 '15 at 23:23
  • 2
    \$\begingroup\$ If you charge with constant current, ignoring voltage, the battery will not stop at 4.2V. It is hard to predict exactly what would happen. But for sure the battery will heat up. The voltage will climb above 4.2, and the battery will fail prematurely. It is definitely unsafe, so if you do it, take whatever precautions are necessary in case the battery vents gas, or ignites. Do not use the battery after the test. \$\endgroup\$ – mkeith Sep 8 '15 at 23:29
  • \$\begingroup\$ If for some reason you decide to try it, make sure to record it for YouTube. It seems LiPo battery explosion videos are fairly popular. (Obviously that should not be your primary concern though) \$\endgroup\$ – user253751 Sep 9 '15 at 5:44
  • \$\begingroup\$ I think I don't completely understand the "constant-voltage" charging part. Why battery keeps draining current when it reaches 4.2V? There's no voltage difference between battery and charger! What would be the equivalent circuit of battery when charging? \$\endgroup\$ – Igor Rybalka Sep 9 '15 at 9:50

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