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Ham Radio - Camera - Computer - F8HP - UV5R - Baofeng - Pofung
USB Charger for the 7.4v
3600/3800mAh Battery Pack

John 'Miklor' K3NXU
Jim  KC9HI

The Big Question
How do I charge my 7.4v 3600/3800mAh Li-ion battery using the tiny jack on the side of the battery?
Yes, there are USB cables available that will fit the side jack of the battery, but unless they are regulated 8.4V intelligent Li-ion chargers, your battery could be undercharged, or potentially damaged.
This configuration will allow you to use any 5V USB source. Whether you are out in the field, mobile or using solar, this solution works and is 100% safe for your Li-ion battery.
The example shown here is for a 3800mAh extended battery pack for a Baofeng series radio, but this direct to battery charger module is a applicable to any 7.4v Li-Ion battery pack.
Some solutions that Don't work are:
-  Charge it from your car battery
-  Use the wall wart from the charger
Both of the above will eventually toast the battery.
Here's a simple solution that I received from Jim (KC9HI) that requires no soldering, just the right components.
(1)   The first item is the heart of the project. A regulated charging module designed specifically for Li-Ion batteries. The input is a standard 5V USB source, and the output is a 3.5x1.35mm jack at 8.4V regulated.

Designed for 5000mAh or under batteries. Output max is 500mA which gradually tapers off as battery charges. Should charge a depleted battery overnight.
(2)   Next is the interconnecting cord.  5.5x2.1mm to 3.5x1.35mm
(3)   The third component is the 5.5x2.1mm  to  3.5x1.35mm adapter.
Here's what the completed assembly looks like.
There are two LED indicators on the charging module.
-  BLUE when plugged into the USB port
-  RED  when inserted into the battery
-  BLUE when battery is fully charged
So, where can the components be purchased?
A quick eBay search came up with the following using these titles:

(1)  5V USB Charger Module for 2S 7.4V Lithium Li-ion Li-Po Battery Cell 8.4V
$ 3.22
(2)  5.5x2.1mm Female to 3.5x1.35mm Male Plug DC Power Jack converter
$ .99
(3)  DC Power Adapter Cable 5.5x2.1mm Female Jack Socket to 3.5x1.35mm Female
$ .99  
Total cost:  $5.20
Assembly time:  15 seconds
Note: Some components available on Amazon, but a bit pricier.

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