Is this Li-Ion charger just a resistor or am I dumb? - Page 1
Forum section: electronics / beginners

author: John_doe
author_rank: Regular Contributor
timestamp: March 26, 2022, 11:49:07 am
content: I bought an RC toy car from Amazon for $12 (USD) shipped. Specifically this one:https://www.amazon.com/dp/B09CQ54X7P/It comes with a Li-Ion 14500 battery (two of them actually) that has a protection circuit inside the PVC heat shrink tube. The protection circuit is a standard DW01 + 3 x 8205A.The said charger connects to USB on one side and to the battery on the other. There is a red LED which lights when it's charging and shuts off when it's done.When the red LED is off and charging is done, I measure 5V (the USB voltage) on the output of the charger (should be maximum 4.2V for Li-Ion). I took a photo of the charger circuit:This is what I believe is happening just by looking at the circuit: The USB positive goes straight to the output. The USB negative goes through a 3.9 ohm resistor to the output. This resistor is between the gate and source of an N-Channel MOSFET with a very low Vgs(th), when charging, the MOSFET is on and lights the LED. When the output voltage reaches 4.250±0.050 volts, the DW01 that is attached to the battery enters overcharge protection mode and shuts off the 8205A MOSFETs. Because of the high impedance, virtually no current goes through the 3R9 resistor and thus the MOSFET shuts off the LED.So my question is: Am I wrong?

author: mikerj
author_rank: Super Contributor
timestamp: March 26, 2022, 12:21:41 pm
content: You are correct. They are relying on the protection circuit stopping the charging, which obviously isn't the best idea but doesn't surprise me at all. You could snip the wire off and solder it onto one of those little TP4056 modules, they are very cheap and do use the proper li-ion CC/CV charging profile.

author: John_doe
author_rank: Regular Contributor
timestamp: March 26, 2022, 01:21:13 pm
content: Thanks, that's what I plan on doing. I just wanted to make sure I wasn't making a silly mistake.I think I'll put a 4.7K for Rprog on the TP4056. That'll give me about 0.5C charging current.

author: strawberry
author_rank: Super Contributor
timestamp: March 26, 2022, 02:19:16 pm
content: 1W for 1206 SMD resistor

author: magic
author_rank: Super Contributor
timestamp: March 26, 2022, 03:46:08 pm
content: That's what it looks like and the transistor is probably NPN.

author: John_doe
author_rank: Regular Contributor
timestamp: March 26, 2022, 04:01:41 pm
content: Quote from: magic on March 26, 2022, 03:46:08 pmWouldn't putting that much voltage on Vbe without a base resistor fry an NPN ?

author: magic
author_rank: Super Contributor
timestamp: March 26, 2022, 04:04:27 pm
content: I guess you're right.The BE junction would conduct crazy current if you put it between a discharged battery and 5V.

author: strawberry
author_rank: Super Contributor
timestamp: March 26, 2022, 04:41:12 pm
content: digital BJTmost likely C945 as this device is designed for garbage , when you have a most sophisticated farmer lying around for free and not enough time to complete all marvelous electronic devices that world ever seen before

author: amyk
author_rank: Super Contributor
timestamp: March 26, 2022, 09:37:58 pm
content: It'll work reasonably well, plenty of manufacturers have figured out that big complicated charger ICs are not necessary. That said, stopping the charge at 4.25V will wear out the cells much more quickly than the usual 4.2V, although there are cells specifically designed for 4.25V use.