Author Topic: Need help with LiIon charging Idea  (Read 2173 times)

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Offline MarkoToxTopic starter

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Need help with LiIon charging Idea
« on: December 17, 2014, 04:19:39 pm »
Hello, im new here so i will introduce myself in my first post.
My name is Marko and i live in Croatia, i am a big fan of EEVblog so i came here looking for help, english is my second language so sometimes i may struggle to write so bare with me please :)

I didnt know how to explain this so i drew a simple picture/circuit to show you my problem/idea
I want to charge multiple (3) LiIon batteries in series and i dont want to use big charger every time becase i want to inscorporate it in a simple project im doing

I was gonna use 3 single LiIon charger module but im not shure if it would work and i dont wanna damage batteries or chargers

I had an idea that i could use one charger for each battery connected in series but the problem is that chargers have common/shared groud, so input and output of the chargers have same output witch means i would short everything and probaby fry them, but if i put diodes on the input grounds of the chargers like in the image maybe i could prevent that from happening

Cant wait for your responses, suggestions or solutions to my problems, thanx :)

Here are the pictres:




 

Offline mikerj

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Re: Need help with LiIon charging Idea
« Reply #1 on: December 17, 2014, 04:47:03 pm »
You can't use diodes to isolate cells in series for charging, it just won't work.

You have three choices:
1) Charge the series connected cells from a single, higher voltage charger and implement cell balancing/monitoring on individual cells.  This is how most commercial products with lithium ion batteries work (e.g. laptops), though most often the only implement over/undervoltage protection rather than full cell balancing.  This means the cells can become unbalanced after a number of charge/discharge cycles.
2) Use fully isolated chargers to charge the individual cells.  Advantages are that cells are automatically balanced.  Disadvantage is cost and complexity.
3) Implement some kind of switching scheme to either connect cells in series or in parallel (not very practical in most cases).
 

Offline MarkoToxTopic starter

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Re: Need help with LiIon charging Idea
« Reply #2 on: December 17, 2014, 04:52:58 pm »
Thanx for reply, i will read more about balancing, a have saw some modules available for couple of cells, so to use my method i would neet to have 3 power sorces that are issolated from eachother and dont have common ground? is there any circuit that would make 3 isolated outputs from one power source like fast switching between them so the chargers dont work at the same time?
 

Offline mikerj

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Re: Need help with LiIon charging Idea
« Reply #3 on: December 18, 2014, 05:51:32 pm »
to use my method i would neet to have 3 power sorces that are issolated from eachother and dont have common ground? is there any circuit that would make 3 isolated outputs from one power source like fast switching between them so the chargers dont work at the same time?

A transformer.  Either a single transformer with three isolated secondary windings (current/voltage regulation would have to be performed separately), or three separate transformers.  Driven by a high frequency switch mode regulator, the transformers could be pretty small.
 


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