3800mAh Battery USB Charger

The Big Question

Can 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 allows 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 charge 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:

-- 5V USB Charger Module for 2S 7.4V Lithium Li-ion Li-Po Battery Cell 8.4V
      eBay Link   $2.95

-- 5.5x2.1mm Female to 3.5x1.35mm Male Plug DC Power Jack converter cord.
     eBay Link    $1.59

-- DC Power Adapter 5.5x2.1mm Female to 3.5x1.35mm Female
     eBay Link    $ .99

Total cost around $5.00
Assembly time: 15 seconds

     OR

If these adapters cannot be found, and you have a soldering iron handy, you can purchase two 3.5 x 1.35mm plugs and create a jumper.

Note: Some components available on Amazon, but a bit pricier.

OR

A 3rd Alternative / BT1013 Cable

Now available are direct charging cables. These come with a regulated USB charger cable allowing you to charge a 3800mAh (BL-5L) directly through the battery side charge port.

Amazon BT1013 BTech BT1013