I recently received a “TOP 5A CC CV LED Drive Lithium charger Power Step-down Module W/ USB Voltmeter” which I purchased for shy of $7 from ebay seller alice1101983 who operates the TxHang Electronic store on eBay. My order was shipped from Shenzen China promptly, and arrived via international post in just over a week.
From the listing I knew it was a switch mode DC-DC power supply built around a buck converter. Also:
- 5-36VDC input range
- 1.25-32VDC output range, adjustable
- 0-5A output current, also adjustable, with output short protection
- Max output of 75W with thermal shutdown
- Up to 96% efficiency
- A display that can be switched between showing: input voltage; output voltage, current and power.
The listing also included detailed application notes for use as a step-down, current limited power supply, a constant-current LED driver, and battery charger.
I purchased the device because the adjustable constant voltage and constant current features, and the way it transitioned between the two were enough to make it useful to do supervised, one-off charging of lithium ion battery packs that had multiple cells per series, something I needed to further investigate getting data from old laptop battery packs.
The device has three LEDs to facilitate use as a battery charger. The LEDs indicate whether it is voltage or current limited, whether it is “charging” or whether the battery is full.
I knew that it determined that the battery was full the way most lithium chargers do, by monitoring the current during the constant voltage phase and noting when it dropped to 1/10th of what it was during constant current phase. Real chargers stop charging at this point. It wasn’t clear whether this one did, or whether it just lit the light.
I can now report that this isn’t suitable for unsupervised charging of lithium ion batteries, because it doesn’t terminate the current once it determines the battery is full. This is bad for the health and safety of lithium ion batteries.
I’ve noticed another quirk as well. If the output of the device is connected to a battery and the power is removed from the input, the display, and voltmeter end up being powered by the battery, which seems to me to be less than ideal.
There are also issues with the USB output connector on the board. First, it appears that while there are resistor pads for indicating available, output current to USB devices, they are all unpopulated. Second, and more concerning, the USB output voltage isn’t fixed at 5V, but instead varies with the output voltage setting. As a result, there is a very real possibility of frying a USB device if one doesn’t know about this quirk, or you do, but don’t remember to adjust the output voltage. I’ll probably fill in the socket with hot glue, or desolder it and remove it from the board all together.
- Quick shipping
- Constant Voltage and Constant Current options
- Built in volt and amp meter
- Useful for supervised one-off battery charging
- Doesn’t automatically terminate battery charging. Dangerous for un-supervised charging.
- No safeguards against non-standard voltage on USB port.
- Drains battery when power input is removed.
All in all, not a bad module to have on hand, if you know its limitations.
A brief look at some of the key components on this module
Buck Converter IC
The heart of this module is an XLSemi XL4015, which is described as a 5A 180KHz 36V Buck DC to DC Converter. It is marked “XLSEMI, XL4015EI, 4032R.”
One thing stands out immediately when I look at the english language datasheet:
Wide 8V to 36V Input Voltage Range
I’m not sure what to make of the fact that the seller claims that this device has a 5-36V input voltage range. Its possible that the input has enough resistance to cause a 3V drop, but that seems rather unlikely. It may also be that the datasheet is out of date. More likely though, I think someone along the way either made a mistake in drafting or copying the spec for the module, or the converter is intentionally being used outside of its specification.
Speaking of intentionally using the IC out-of-spec, Julian Ilett reviewed a similar module using a different XLSemi buck converter IC, the XL4005, and noted that when he set the output voltage to 0, it no longer limited current properly. Not entirely surprising, since that IC has a minimum output voltage of 0.8v. I haven’t tried the same test with this module, but I suspect that it will also allow you to set the output voltage below spec, and that similar behavior will result.
Bottom line is: Check the specs! Don’t count on the designers of these modules to put in safeguards.