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A lightweight, simple Li-Ion charger for running off DC.

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Johnny Kang

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Hi, I want to build my very first li-ion charger, so any help and tips are much appreciated. I've only worked with digital circuits, so my experience in this area is close to zero.

I have a 5s6p li ion ncr18650b battery pack that's going to power my portable desktop. I'm planning to use a BMS(battery management system) for 5S with overcharge/overdischarge/overcurrent/temperature/short-circuit protection with cell balancing option. I'm looking at http://www.bestechpower.com/communicationbms/BMS-D298V1.html
which also comes with a smbus comm which can be interfaced with a simple driver.

What I'm struggling with is converting my 21V(24v psu with buck circuit) DC power supply into a current limited supply that can safely charge each cell at less than 1C, so 6C for the entire pack. For the sake of simplicity and weight(would prefer to keep the charger by itself less than 200g) I'm looking for a simple method of charging slowly until the battery reaches 21V and then shutting off( a single stage charger, not like CC/CV chargers with IC).

That being said, I wouldn't mind working with li ion charger IC, but I'm having a sort of decision crisis with so many to choose from!

Here is a simple diagram to explain my system:
9CkvK.png


Again appreciate any advice!
 

I do not know your level of expertise but a common problem is making things much too complex.

"
What I'm struggling with is converting my 21V(24v psu with buck circuit) DC power supply into a current limited supply that can safely charge each cell at less than 1C, "

Now I am very confused.......................one uses a specialized Li-ion charger to charge cells.
 

A series parallel array of li ion cells that you buy off the shelf may suffer sharing issues..
Here is your lion battery…

They are series parallel in the elec cars but those batts are each made to be matched with each other.

**broken link removed**

As far as charging the batts from your 24v supply, you want to charge at 1C….so that’s 5 * 3.7 = 18.5 Amps and max 5x4.1 = 20.5Volts.

So yes a buck would be ok, maybe linear.com do a nice buck converter IC that you can use to give you that…in fact I am sure they do…

However, your max duty cycle is 85% and that’s too high for some control IC’s …so maybe pick a sepic instead. SEPIC has low side fet drive, and you can easily chuck in a high side current monitor for regulation of the output current.
But also you could use a flyback , or even an LT8705 based buck-or-booster.
 

Thanks! Exactly what I was looking for.
 

A series parallel array of li ion cells that you buy off the shelf may suffer sharing issues..
Here is your lion battery…

They are series parallel in the elec cars but those batts are each made to be matched with each other.

**broken link removed**

As far as charging the batts from your 24v supply, you want to charge at 1C….so that’s 5 * 3.7 = 18.5 Amps and max 5x4.1 = 20.5Volts.

So yes a buck would be ok, maybe linear.com do a nice buck converter IC that you can use to give you that…in fact I am sure they do…

However, your max duty cycle is 85% and that’s too high for some control IC’s …so maybe pick a sepic instead. SEPIC has low side fet drive, and you can easily chuck in a high side current monitor for regulation of the output current.
But also you could use a flyback , or even an LT8705 based buck-or-booster.

I bought cells from my supplier before and he said matched cells are possible, albeit little bit more pricy. Also, last time he sent me a test sheet which I double checked with my mulitimeter and they were pretty close, I only remember one cell which had quite high internal resistance out of 100 cells. I used them to replace the e-bike battery which was really too small for my commute(24v, 6ah).
 

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