3pcs Type C BMS 2S 3S 4S 1A 2A 4A 18650 Lithium Battery Charger Board USB C Step-up Boost Module for Li-Po Polymer Power Bank(3pcs 4S 2A)

Very small and useful

Glad I hooked up a voltmeter to the output. Unit reports 18V on the battery circuit on all 3 boards I received.Test before you hook up your battery.I can still use this for other batteries but not my 4.2v, which is entirely why I bought it.

Mine were 4s lithium ion. Perfect solution. Weirdly needed a correct wire. If it doesn’t work try a different wire and block. No idea.

Alex H. is right. You can only use this device with dumb 5V USB power. Basically, it will work with USB-A to USB-C charging cable that comes with some devices.If you connect it to a real USB-C power adapter (like one from a laptop, MacBook, or other high-power demand devices), you will get no power -- for the reasons Alex H. mentions.This is the 4 amp 3S version that appears to draw far too many amps from a typical USB-A power supply for it to be able to charge 3S 18650 cells.I will try the 1 amp 3S version to see whether the lower amperage will keep my USB-A power adapter from overloading.[UPDATE: The 1 amp 3S version worked fine for recharging three 18650 cells in series (3S). It isn't fast charging by any means -- it took about 12 hours to charge the cells -- but it stopped charging at the correct voltage level. The 1 amp version got a little warm, but not worryingly so. Now that I'm getting over the disappointment that these devices don't work with "real" USB-C voltage auto-detecting power supplies, I'm finding them adequate for the price. I really wanted something that would negotiate a higher voltage, but apparently, this feature isn't available anywhere for dedicated charging modules yet.]-

I have a love/hate relationship with this product. First, it should not be offered in a 4A model. Can it output 4.2v 4A from a 5V supply.....technically yes. But that inductor get SO incredibly hot that there's not way it's going to last for any reasonable length of time without active cooling. At most, I wouldn't use these for more than 2A. Maybe in China it's easy to find a non PD (power delivery) USB-C adapter that can output more than 2.4A but those don't really exist as a quality product. The best I've been able to find is 5V 3A which isn't a normal standard for USB. Sustaining 4A from a 2.4A supply with only .8V of drop is a recipe for trouble. But I feel like you would need a 5V 5A USB-C (non QC or PD) adapter to make that work well. Ideally, they'd put a PD controller on this, charge $5 more and then you'd have a product that is actually useful.TL:DR; buy the 1A or 2A and stay away from the 4A. Use the best and highest amperage 5V only charger you can get.

They work great and arrived very fast, and are exactly as advertised!!!

I have this wired to my 3s battery pack, and everytime I tried to charge my battery pack, it would cause my USB charger to cut off and restart. It did work for a brief second but will stop working all together.

The charger works with basic 5v usb ports where there is always power. Basically it is expecting a direct DC input.It does not work with any smart chargers that do USB PD. Since this device cannot negotiate a USB PDO it won't turn on.Also worth noting that it gets quite hot while charging. Still works as advertised, just be aware of the limitation mentioned above.