I can't see why my voltmeter isn't measuring 12v when it's directly connected... - Page 1
Forum section: electronics / beginners

author: slikvik
author_rank: Contributor
timestamp: March 10, 2024, 07:51:29 pm
content: My daughter has a light up mirror with a 12v DC adapter, but also appears to be battery operated.It broke, so I've opened it up and saw what I think is very strange wiring!It has a standard 3.7v li-ion battery with its own DWO1 protection circuit, however it's directly wired to the 12v barrel jack (!?!)Putting aside 12v going into the 3.7v li-ion for one moment, the wiring of the circuit seemed weird to me as both the 12v DCandthe batteryoutputthat powers the little light control PCB are both wired to the same P- and P+ points on the protection circuit. I thought I'd measure the volts directly on the P- and P+ points ,while 12v DC is connected, to see what was being output, but it would only read the battery voltage of around 3.2v!As you can see from the photos, I'm essentially putting my multimeter on the red and black wires that come directly from the DC barrel jack so where has the 12v gone?I hope that makes sense! Please help, this is driving me crazy!(Spoiler, I've removed the battery and 12v dc, and converted the mirror to 5v USB)

author: amyk
author_rank: Super Contributor
timestamp: March 10, 2024, 08:03:25 pm
content: Are you sure that's the right adapter for it? There are 4.2V current-limited ones for which the schematic of the device you drew makes sense.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 10, 2024, 08:07:57 pm
content: Possibly the "12 volt" adaptor is actually a charger specifically for that lithium cell.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 10, 2024, 08:26:43 pm
content: Because it's probably not a plain 12V adapter, but battery charger, so it limits the output current, thus the voltage will rise as the battery charges up.After a while it will reach about 4.2V and this will be the fully charged state.I hope the charger detects this and stops, otherwise it's a concerning issue, to say the least.

author: slikvik
author_rank: Contributor
timestamp: March 10, 2024, 08:33:57 pm
content: So the DC adapter is actually mine. They didn't send one through so when I contacted them via Amazon they said they don't supply one but it's a standard 12v/1A dc input??! The description on Amazon also says "12v 1A DC"It's a very cheap Chinese mirror.The adapter is definitely outputting 12v when I place my meterinthe barrel, but I soon as it's connected and I read from the P terminals (again soldered straight to the barrel jack), it shows 3.2v...exactly what it also reads when I unplug the adapter and just leave the battery.So while I'm probably made a boo boo using any old 12v adapter, given I AM, my question remains and I'm still confused I'm not reading 12v

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 10, 2024, 09:47:35 pm
content: Stop doing that inmediately, likely you're putting uncontrolled current and voltage into the battery.With some luck the battery management circuit will cut-off things before getting dangerous but this is definitely a no-no.Get a proper charging circuit, they're cheap and simple to interface:www.aliexpress.com/item/1005004616088520.htm

author: Jwillis
author_rank: Super Contributor
timestamp: March 11, 2024, 04:48:21 pm
content: I would say that the BMS board is burned out and shorting the supply. That would explain the voltage drop since your measuring across a resistive short.When the BMS is working properly the input voltage should measure the same as supply and the output to the battery should read around 4.1V to 4.2V. When battery goes down to around 3.2V the BMS cuts output power to load. When charged the BMS allows battery power to the load. For a S1 BMS the input voltage should be no more than Around 5V maximum. To much voltage and the BMS will over heat and burn out. You can get S1 battery managment boards really cheap. Since these small boards are really dumb they don't control input voltage . So never use a S2 BMS on a single cell. An S2 will try to charge at 8.4V which is bad for a single cell.

author: kjr18
author_rank: Regular Contributor
timestamp: March 11, 2024, 05:16:17 pm
content: Wow, that's nuts. Are you sure that there is no resistor in between 12v and battery? Anyway if you want to charge battery more safely, you should get some cheap TP4056 boards, some are populate with DW01 and mosfet package, just like that board you got there. You can power those with any usb charger.

author: IanB
author_rank: Super Contributor
timestamp: March 11, 2024, 07:55:45 pm
content: Quote from: slikvik on March 10, 2024, 08:33:57 pmBear in mind that an ideal battery is a short circuit (zero resistance) with a voltage appearing across it. Therefore, an ideal 3.2 V battery will always read 3.2 V, regardless of what you connect to it.Now a real battery is not ideal, but a lithium ion battery comes fairly close. In particular, it has a low internal resistance, which makes it close to a short circuit. So when you connect your 12 V supply to it, the 12 V supply would get overloaded and short circuited, such that it can no longer output 12 V. It may, perhaps, go into short circuit protection mode, and limit the output current to prevent damage. In effect, you have created a battle between the 12 V supply and the 3.2 V battery, and in this case it seems like the 3.2 V battery is winning.

author: radiolistener
author_rank: Super Contributor
timestamp: March 11, 2024, 09:08:59 pm
content: Probably they expect that battery protection circuit will stop the charge when battery voltage is rising above 4.2V. And charger uses some ballast resistor for charge current limiting.This is dangerous charger, it can damage your battery and even produce fire.

author: Jwillis
author_rank: Super Contributor
timestamp: March 12, 2024, 12:44:40 am
content: Quote from: radiolistener on March 11, 2024, 09:08:59 pmThey do stop charging when the battery reaches 4.1V to 4,2 volts, when they work. What I find strange is that a 12V power supply is used when only 5V is needed.

author: slikvik
author_rank: Contributor
timestamp: March 12, 2024, 07:49:27 pm
content: Thanks for the responses everyone, some of that is very useful.As I said at the start, I knew that was dangerous when I opened it up, which is why I've now converted to a lovely 5v USB with a couple of resistors to extend life and a 3 way switch for white light, yellow light and OFF.When I saw how it was wired - with what is essentially a direct line from 12v to the little LED control panel but I wanted to check it with my volt meter, while plugged in. Hence my original question of why am I not reading 12v from a + and - point soldered straight to the barrel jack wires, when I was reading +12V from the supply when unconnected. I hope that all makes sense now.Very dodgy bit of kit sold by Amazon.co.uk and sitting in my 13 year old daughters bedroom plugged in 24/7!!Already have some TP4056's that I use for other projects but honestly I just didn't see the point of a permanently placed vanity mirror being battery based anyway!