A 12V mobile charger for the Baofeng BL-5L 3800mAH battery


I bought my first UV-5R to use as a scanner. With the limited number of frequencies I wanted to monitor, even with its slow 3 channels per second scan rate, it was a cost effective option to a "real" scanner that would run close to $100. It came with a CH-5 charging stand, but since this UV-5R was going to remain at the QTH connected to the 12VDC supply that powers the other rigs using one of the 12V Battery Eliminator/Adapters, the charging stand wasn't really needed.

My next two UV-5Rs (bought to actually use as HTs) came with their own chargers, so now I had three CH-5 stands. Along with a caution not to operate the transciever while charging, the Miklor website FAQ for the UV-5R mentions a TYT branded regulated 12V charger safe for use on the BL-5L's side mounted coaxial jack, but apparently you can only get one when you buy the TYT transciever. Apparently the majority of the 3800mAH 12V charging cables available are simply automotive cigarette lighter plugs wired direct to a coaxial plug that fits the BL-5L, but feeding 13.8V or more directly into the battery could have catastrophic results. I got to thinking my extra CH-5 charging stand might be better served safely repurposed as a mobile charger. Being EmCom oriented, I always think of keeping things going with 12V when commercial AC power is down.

The CH-5 is powered from an AC wall-wart adapter labeled as having a 10V DC output. I measured it unloaded at close to that, and when powered down the voltage almost immediately dropped to zero and the LED on the adapter went out. Since most simple linear adapters will slowly drop in voltage as the filter capacitor discharges, I assume the AC adapter that comes with the CH-5 is a regulated switching type.

To the left is my DMM reading the unloaded DC output of the CH-5's wall adapter. The DMM on the right is reading the unloaded output of an automotive cigarette lighter plug cable, sold as a 3800mAH Baofeng battery charging cable by the same Chinese eBay vendor I purchased a UV-5R with BL-5L from, plugged into a 13.8V 3A bench supply. Caveat emptor.

I opened the plug on that cable and found, aside from a bad solder job, there truly was no regulation and charge monitoring necessary for a Li-Ion battery in there, just a current limiting resistor for the LED. To add to a potentially explosive situation, the plug of the cable came with a 3A fuse in it. I cleaned up the solder joints and, since the CH-5's AC adapter is labeled as only having a 500mA capability, installed a bit more conservative 1A fuse.

To get the actual 13.8VDC automotive power down to 10V was going to take a step-down regulator of some sort. I had both a LM2596 buck converter module and a LM7810 3-terminal voltage regulator on hand. Albeit larger in size I chose the former because of its cooler operation so I could enjoy the economy of buying a plastic enclosure. The LM7810 could potentially dissapate close to 1/3 of its input voltage as heat, but if I had already had an aluminum case I might have considered using it and heat sink instead. The LM2596 has two mounting holes for #4-40 hardware.

The next order of business was to get inside the CH-5 and see what I had to work with there. At the front of the bottom are two indentations, using a pocket screwdriver to push down and in on those got the case open.

Inside I found more iffy soldering. All of that cleaned up after I removed the unneeded input jack, replacing both output and input leads with Philmore 64-2210 red/black 22 AWG zip cord, mounted the electrolytic capacitor flush with the board and removed the LED for relocation, using the yellow and green wires from 26AWG modular telephone cord. There are two tiny mounting holes on the PCB, although only one is used in the charging base. I enlarged both to fit over #4-40 nylon screws and trimed the PCB up to the 10V+ trace.

Once I knew what I'd be working with I decided a Hammond 1591XXMSBK would fit the bill, and ordered one from Mouser Electronics. The enclosure has stand-offs for #4 x 1/4" self-tapping screws molded into both the lid and the bottom, so a piece of perf-board could be cut to size and mounted inside. By mounting the CH-5's PCB and LM2596 to that, the only holes necessary in the enclosure are for the power in and out plus the 3mm status LED.

For the 12V input, I already had the cord I'd purchased, so I used a Mouser 163-4012-EX jack on the case, soldering a short length of the zip cord on it before mounting. For the 8.4V output to the BL-5L, I used a Mouser 171-PA35135-1-E plug on around a foot of the zip cord, running the cord through the snug-fit of a rubber grommet using a wire tie as a strain relief inside. The only drill bit I used was a 1/8" in the Dremel tool, I used a hand reamer and eyeball engineering for the larger holes.

Here's how it went together, with both modules mounted so the solder pads were accessable. Since it's simply wiring both modules together, I soldered everything except the red wire from the LM2596 OUT+ that would go to the 10V+ input of the CH-5. I attached a DMM to that wire and ground, powered up the LM2596 through the lighter cable attached to the 13.8V bench supply and set the LM2596 to the same output voltage I got when I tested the CH-5's AC power supply. Shut power down, disconnect DMM, solder wire to CH-5 10V+. Power up and verify the LED was blinking. Connect a BL-5L and see if the LED turns red, then after a while green.

Here it is with the lid on doing its job.

And done for the day.





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