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In-depth Battery Knowledge inquiry

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New Member
Hi, I'm new to electronics and I've been starting a project of my own which is a charger (since i though i should start first at power electronics since its all about electricity before pursuing switching electronics or automation). I had previously created a charging system (using the li-ion batteries of laptops and power banks) that is used for charging mobile phones, has 4.2v output using this electronic module http://www.dx.com/p/tp4056-diy-1a-micro-usb-li-ion-battery-charging-board-charger-module-blue-373990

Since i build one but doesn't know really much what to consider (since i followed some vids and tutorials for DIY in exact manner), i find myself seeing what i had done is not efficient at all like other charging system would do like those used on industry (been pursuing my project to be on an industrial level). So to be able to fully understand the supply first will be a good milestone for me since i would apply it on all of my projects.

upon doing my own research and study (done research on trends here also) i couldn't find which would be the true nature of batteries (specifically about li-ions only first). Since I bump on an idea answered by someone. For everything i know, i thought that batteries for example my smartphone's

3.8v li-ion battery
3300 mAH

1) my thought about these specs is that the battery produces 3.8v (practically would be 3.78v or 3.82v) and will always give 3.8v. Is this practically true?
2) 3300 mAH means 3300 milliamp-hour meaning it has a capacity of 3.3 amps within an hour. Is this practically true?
3) 3.8v means this is the real volt value at it's minimum and maximum of the battery. Is this practically true?
4) for example, my supply is 3.8v and 1000mAH, i have a load of 3.8v and uses 500mAH. At what point do i know its discharged already (w/o considering the state of my load, let's say it's a resistor only), just using a multimeter?
5) is the real volt capacity of 3.8v li-ion battery is 4.2v?

answer with maths are even better as long values and variables used are indicated. I just read an answer on this forum https://electronics.stackexchange.c...d-voltage-for-charging-a-3-7-v-li-ion-battery , answered by Overmind where he quoted:

" No, the battery does constantly not give 3.7V. This is the voltage value at a way lower capacity. 3.7V does not means much. That is the value at which the battery is most stable at, but the actual value when fully charged is 4.2V, so a charger will have to provide higher than this if you want to fully charge it.

At 3.3V standard Li-Ion cells are considered discharged because they can no longer provide enough sustained current for the average applications they were designed for.

The minimum voltage for charging a standard Li-Ion is 4.201V. But considering impedances of the charger and cell, most chargers have 4.25 or even 4.3V when running blank (not connected to a cell).

Although those values have been chosen this way they are not like the 10 commandments. You can discharge a cell under 3.3V but it will provide less current and you can charge it even up to 4.3+V and it will have higher capacity, but both practices will lower its lifetime.

So in emergency cases, overcharge them to 4.3V and you'll have some extra capacity to work with and if you want to have a very long time for a cell, let it charge only up to 4V but that will only provide ~80% of the stated capacity.

Getting back to chargers, I'd design my charger like this:

Option 1: 4.0V - safe mode - for safe charging and long life and overdischarged/bad cell recovery.

Option 2: 4.21V - standard - normal charge mode (bad cells with reduced capacity will overheat if charged at standard level)

Option 3: 4.32V - overpower - over-charge mode (lower cell life, very high danger for reduced capacity cells) "

Is this practically true?


Well-Known Member
Most Helpful Member
Welcome to ETO.
BatteryUniversity, for example here, is a good source of reliable information about various battery types and their charging profiles. Be particularly careful with lithium batteries as incorrect charging or discharging can result in fire or explosion!


Well-Known Member
Have a look at the MC34671 part.
I use it and it makes charging Li Ion batteries safe and simple.
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