Electronic – Trying to understand Battery Chargers

batteriesbattery-charging

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.

Thanks

Best Answer

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.

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(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

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