Field Battery Charger is an old fancy name, but nothing could be better described a portable field battery charger, by the name it can be hand carried, it can be used in field operation where no urban electricity supplied. I used the B6 charger but found it difficult to charge 3S LiPo battery because its nominal input voltage is 15V while I only have 12V source, and it would drop voltage when prolong usage.

B6 is a serial charger, it meant that the 3S battery is charged in serial, I thought if I can charge it in parallel, that is charging each cell independently, since each cell voltage is 4.2V, so input 12V should have a large buffer to prolong use. I found a module XL4015 may be fulfilled my need, it is a constant voltage and constant current output module. My initial thought was to connect there modules output in serial and the input in parallel, but once I hooked up and found I was wrong.

Because when I connected the output in serial, the positive output of one module equivalently connected to the ground of next module as the input and output are common grounded, I could use 3 input source but nonsense, so I need to isolate module ground connection I look for an isolated DC/DC transformer. I found a small module called "WSTECH DC/DC Isolation Converter", input at DC9-36V, output DC12V 0.9A, I choose it because it is marked with "Isolation", the other types may not have. My design of output current at 2.0A, the input equivalent is 0.7A at 12V, so the DC/DC module is sufficiently enough.

Step 1: Assemble the Parts

The XL4015 has one version specific used to charge battery, this version has there LED at red, green and blue colors, it is told that green one is standby/full charge, blue one is charging, red one is current limiting. However it is hard to find even if you have specified but still get the version with only there RED LED. I also deployed three AV meter to monitor the charging status, from experiment once the charging current below 0.1A it is told fully charged, with the AV meter, I can unplug the battery when the current shown 0.0A, however the price for AV meter is a bit higher than the XL4015 !

I assemble the parts as shown, it is straight forward but not for the AV meter, the AV meter has two ports, voltage and current port, voltage port has three wires, black, red and yellow, current port has two wire, red and black wires, the connection sequence as follow:

Voltage port:

Red wire connects to XL4015 positive input for power

Yellow wire connects to XL4015 positive output and each battery positive terminal

Black wire is unused

Current port:

Black wire connects to XL4015 negative output

Red wire connects to XL4015 output and to each battery negative terminal

The battery output terminal connect to a 4 pin socket similar to the B6 charger, it is said the balance charging ports, but it is still need to connect the main leads, while in this charger no need to connect the battery main leads still get the balance charging function.

Step 2: Setting the Module

The setting up is simple, use a trusted digital meter measure the output voltage and turning the left side potential meter to exactly 4.2V, change the digital meter to current mode, direct short the output and turning the right side potential meter to around 2.0A, The AV meter may also need adjusted, there are trim ports for current and voltage individually.

Step 3: Building the Enclosure

The enclosure is as simple as nothing, I used scrape document plastic folder so that I can view the AV meter and no need to break a hole, the output socket fitting most LiPo battery pack, 18650 diy pack, you can even charge a single cell, and used to check the voltage even no input supplied, as long as the battery pack still have 4V voltage.