Li-Ion battery charger

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DrWhoF

Joined: 06 May 2005
Posts: 334
Helped: 12

11 Nov 2006 6:00 picaxe li ion chargers

I have battery packs of 3 cells of Lithium-Ion batteries (2200mAh).
I need to buid a charger which can charge more than one pack in a sequence.
For example: I connect 3 packs to 3 different terminals and charger will start charging first pack then second and so on.
Each terminal should have an LED to signal charging. At first it shoud blink slow then faster and when a pack is fully charged it shoud be on.
Any suggestion will be highly appreciated.

IanP

Joined: 05 Oct 2004
Posts: 6494
Helped: 1542
Location: West Coast

11 Nov 2006 9:38 Re: Li-Ion battery charger

Your charger, as I can visualise it, should consist of three major blocks:
- 2-2.2A current source (fast charging, 1C current for 2200mAh battery)
- (as many channels as you need) 3-cell battery charger with ON/OFF function
- microcontroller to supervise the whole circuit

First, look at the Li-Ion battery charging characteristics ..
From the attached picture you can see that the charger should start charging battery using current-limited constamt voltage source ..
A constant voltage charger sources current into the battery in an attempt to force
the battery voltage up to a pre-set value (usually referred to as the set-point voltage or set voltage) ..
Once this voltage is reached, the charger will source only enough current to hold the voltage of the battery at this constant voltage (hence, the reason it is called constant voltage charging) ..

To build a 2A current source I suggest that you use the LM2576 .. se example below ..
The charger itself can be based on the LM3420-12.6, as the target voltage for a 3-cell Li-Ion battery is 3 x 4.2V = 12.6V ..
And the rest can be build around the PICAXE-18X microcontroller ..

Regards,
IanP

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DrWhoF

Joined: 06 May 2005
Posts: 334
Helped: 12

14 Nov 2006 9:47 Re: Li-Ion battery charger

How can 3-cell charger block be controlled by a microcontroller?

IanP

Joined: 05 Oct 2004
Posts: 6494
Helped: 1542
Location: West Coast

14 Nov 2006 10:20 Re: Li-Ion battery charger

If you look carefully at this circuit you will find out that there is the ON/OFF switch based on the Q3 MOSFET ..
It works like this:
when the power is applied a current starts to flow through ZD3 and R4 ..
This current generates voltage accross R4 and when the voltage is higher than the MOSFET's Vgson it is turned ON and the circuit starts charging a battery( block) ..

If you remove the 3.9V zener diode and R4, and connect the Gate of Q3 to any general purpose I/O pin (make sure that the MOSFET's Vgson is below 5V) you will be able to turn charging ON or OFF, as required ..

Regards,
IanP