4S2P 18650 charging current draw for DIY charger.

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Lord Marmaduke
Last seen: 5 years 7 months ago
Joined: 10/04/2013 - 17:59
Posts: 5
Location: Scotland
4S2P 18650 charging current draw for DIY charger.

Hi folks. A quick question I’m struggling to get my head around. I normally use my 18650s for modded bike lights but this is for another project.
So…as part of a portable radio I’m making, I’m building a little charging circuit.

So we start with 19VDC (4 amp) into my radio from a laptop charger.

Enter a buck regulator 19VDC => 16.8VDC (http://www.aliexpress.com/item/Free-Shipping-1pcs-LM2596-LM2596S-DC-DC-4...)

Then the 16.8VDC will enter my charging protection/balancing board (http://www.aliexpress.com/item/4S-30A-High-Current-Li-ion-Lithium-Batter...).

My cells are Panasonic protected 3400mah. I’m using the charging board above to balance the cells.

So my question is: How much current will the charging 4S2P pack draw? My little buck regulator is only rated for 2-3amp. (I think 3amp needs a heatsink).

Do commercial chargers have some means of regulating the current available to the cells? I don’t want to burn out my regulator or allow the cells to charge too fast.

Advice is much appreciated!

Last seen: 3 years 5 months ago
Joined: 03/17/2012 - 20:10
Posts: 4122
Location: Connecticut

Both links do not work
If I understand what you are trying to do, why not choose a buck driver that can also regulate your input current to say, 1A
I like the idea
Without seeing the specs you may not even have to buck the charging voltage down to 16.8
I think it might actually have to be a little higher to allow for some headroom for the BMS circuit to operate. Depends on how the BMS board operates.
What you are planning on doing can be dangerous. Be very careful. Even if you know what you are doing, it can still be dangerous

Lord Marmaduke
Last seen: 5 years 7 months ago
Joined: 10/04/2013 - 17:59
Posts: 5
Location: Scotland

Hey, thanks! Actually I just found this cute module for about USD 2 http://www.aliexpress.com/item/1pcs-5A-DC-to-DC-CC-CV-Lithium-Battery-St...

It’s a constant current and constant voltage module. I think this should make things safe. I’m actually building my radio inside a steel ammo box so if things go boom I should be OK Smile

I think I’ll stick to 1.5 amp charge current to keep things cool.

Barkuti's picture
Last seen: 7 hours 38 min ago
Joined: 02/19/2014 - 14:46
Posts: 5580
Location: Alhama de Murcia, Spain

Well, obviously you need a CC-CV supply for that purpose. In fact, lots of flashlight led drivers are sorts of custom CC-CV supplies.

There are modules which, besides Constant Current and Constant Voltage output adjustments, allow you to dial a “termination current” and then display some sort of indication (leds, displays, etc) to inform you of the device's status.

Example of an all-in-one simple solution:

5A CC CV XL4005 Step-down Buck Power Supply Module Lithium Charger for arduino


I, however, would select a two device approach: a “charging engine” plus a voltage/current meter for precise charging monitorization.

Charging engine(s):

And this cheap and good volt/ampmeter: 1pc Red Blue LED DC 0-100V 10A Dual Display Voltage Meter Digital LED Voltmeter Ammeter Panel Amp Volt Gauge which looks good also. 


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