I just started studying about batteries and battery chargers. I have this simple query so please help me out.

I am trying to charge my 3000mah li-ion battery. The battery voltage is around 3.6v and the charger regulates the 5v from the source to 4.2v. I connect the battery to the charger. Now what is the voltage at the output should I expect? Shouldn't the output voltage be at 4.2v so that the battery gradually charges to 4.2v ? I ask this because I am using Bq2057 IC and once I connect to the battery to the charger the output voltage is dropping to 3.63v.

My assumptions were, 1. I considered the charger as voltage source 2. Battery also as a voltage source

I experimented this buy connecting two DC voltage sources(5 and 3.3v) in parallel and measured the voltage at the node and it was 5v. So I was expecting the charger output to be 4.2v even when I connected my battery to it.


  • 3
    \$\begingroup\$ "connecting two DC voltage sources(5 and 3.3v) in parallel" - please don't do this. You might get lucky, but often, one, or both or the sources will die. Imagine two 'ideal' sources; for the 1.7V difference to be 'absorbed' in the wiring, a significant amount if current is required assuming your wiring resistance is low. \$\endgroup\$ – RJR Feb 4 '16 at 7:41
  • \$\begingroup\$ This simple logic (constant voltage charging) kind of (and only "kind of") works for chemistries with huge overload tolerances, like lead-acid. Even nickel based cells are much more complicated in charging (look for "negative delta-V"), and lithium ones demand the most sophisticated logic. Otherwise they catch fire. \$\endgroup\$ – Agent_L Feb 4 '16 at 10:31

My assumptions were,

  1. I considered the charger as voltage source
  2. Battery also as a voltage source

Your 2nd assumption is more a less correct.

Your 1st assumptions is not entirely correct. Li-ion1 chargers start with constant curent charging (charger is a current source), then switch to constant voltage charging.

enter image description here
(Source of the plot. By the way, that entire site is an excellent resource on battery topics.)

Finally, I'd like to warn that you should be very careful with Li-ion batteries, because they catch fire when improperly charged (more on that here). Consider a safer battery chemistry, for your introductory experiments. Lead-acid or NiMH batteries are more abuse-tolerant and less risky.

1 Charging profile varies from one battery chemistry to another.
2 Related thread: Difference between controlled current and constant voltage charging

| improve this answer | |
  • 2
    \$\begingroup\$ ...because they can catch fire or explode when improperly charged. This really can't be stressed enough. Experimenting with Li-ion charging needs safety precautions. A fireproof blast shield, at least even a modest one, around the apparatus should be a minimum. \$\endgroup\$ – J... Feb 4 '16 at 10:06

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